--- /dev/null
+%
+% (c) The University of Glasgow 2006
+% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
+%
+
+TcSimplify
+
+\begin{code}
+module TcSimplify (
+ tcSimplifyInfer, tcSimplifyInferCheck,
+ tcSimplifyCheck, tcSimplifyRestricted,
+ tcSimplifyRuleLhs, tcSimplifyIPs,
+ tcSimplifySuperClasses,
+ tcSimplifyTop, tcSimplifyInteractive,
+ tcSimplifyBracket, tcSimplifyCheckPat,
+
+ tcSimplifyDeriv, tcSimplifyDefault,
+ bindInstsOfLocalFuns,
+
+ misMatchMsg
+ ) where
+
+#include "HsVersions.h"
+
+import {-# SOURCE #-} TcUnify( unifyType )
+import HsSyn
+
+import TcRnMonad
+import TcHsSyn ( hsLPatType )
+import Inst
+import TcEnv
+import InstEnv
+import TcType
+import TcMType
+import TcIface
+import TcTyFuns
+import DsUtils -- Big-tuple functions
+import Var
+import Id
+import Name
+import NameSet
+import Class
+import FunDeps
+import PrelInfo
+import PrelNames
+import TysWiredIn
+import ErrUtils
+import BasicTypes
+import VarSet
+import VarEnv
+import FiniteMap
+import Bag
+import Outputable
+import ListSetOps
+import Util
+import SrcLoc
+import DynFlags
+import FastString
+
+import Control.Monad
+import Data.List
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{NOTES}
+%* *
+%************************************************************************
+
+ --------------------------------------
+ Notes on functional dependencies (a bug)
+ --------------------------------------
+
+Consider this:
+
+ class C a b | a -> b
+ class D a b | a -> b
+
+ instance D a b => C a b -- Undecidable
+ -- (Not sure if it's crucial to this eg)
+ f :: C a b => a -> Bool
+ f _ = True
+
+ g :: C a b => a -> Bool
+ g = f
+
+Here f typechecks, but g does not!! Reason: before doing improvement,
+we reduce the (C a b1) constraint from the call of f to (D a b1).
+
+Here is a more complicated example:
+
+@
+ > class Foo a b | a->b
+ >
+ > class Bar a b | a->b
+ >
+ > data Obj = Obj
+ >
+ > instance Bar Obj Obj
+ >
+ > instance (Bar a b) => Foo a b
+ >
+ > foo:: (Foo a b) => a -> String
+ > foo _ = "works"
+ >
+ > runFoo:: (forall a b. (Foo a b) => a -> w) -> w
+ > runFoo f = f Obj
+
+ *Test> runFoo foo
+
+ <interactive>:1:
+ Could not deduce (Bar a b) from the context (Foo a b)
+ arising from use of `foo' at <interactive>:1
+ Probable fix:
+ Add (Bar a b) to the expected type of an expression
+ In the first argument of `runFoo', namely `foo'
+ In the definition of `it': it = runFoo foo
+
+ Why all of the sudden does GHC need the constraint Bar a b? The
+ function foo didn't ask for that...
+@
+
+The trouble is that to type (runFoo foo), GHC has to solve the problem:
+
+ Given constraint Foo a b
+ Solve constraint Foo a b'
+
+Notice that b and b' aren't the same. To solve this, just do
+improvement and then they are the same. But GHC currently does
+ simplify constraints
+ apply improvement
+ and loop
+
+That is usually fine, but it isn't here, because it sees that Foo a b is
+not the same as Foo a b', and so instead applies the instance decl for
+instance Bar a b => Foo a b. And that's where the Bar constraint comes
+from.
+
+The Right Thing is to improve whenever the constraint set changes at
+all. Not hard in principle, but it'll take a bit of fiddling to do.
+
+Note [Choosing which variables to quantify]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Suppose we are about to do a generalisation step. We have in our hand
+
+ G the environment
+ T the type of the RHS
+ C the constraints from that RHS
+
+The game is to figure out
+
+ Q the set of type variables over which to quantify
+ Ct the constraints we will *not* quantify over
+ Cq the constraints we will quantify over
+
+So we're going to infer the type
+
+ forall Q. Cq => T
+
+and float the constraints Ct further outwards.
+
+Here are the things that *must* be true:
+
+ (A) Q intersect fv(G) = EMPTY limits how big Q can be
+ (B) Q superset fv(Cq union T) \ oclose(fv(G),C) limits how small Q can be
+
+ (A) says we can't quantify over a variable that's free in the environment.
+ (B) says we must quantify over all the truly free variables in T, else
+ we won't get a sufficiently general type.
+
+We do not *need* to quantify over any variable that is fixed by the
+free vars of the environment G.
+
+ BETWEEN THESE TWO BOUNDS, ANY Q WILL DO!
+
+Example: class H x y | x->y where ...
+
+ fv(G) = {a} C = {H a b, H c d}
+ T = c -> b
+
+ (A) Q intersect {a} is empty
+ (B) Q superset {a,b,c,d} \ oclose({a}, C) = {a,b,c,d} \ {a,b} = {c,d}
+
+ So Q can be {c,d}, {b,c,d}
+
+In particular, it's perfectly OK to quantify over more type variables
+than strictly necessary; there is no need to quantify over 'b', since
+it is determined by 'a' which is free in the envt, but it's perfectly
+OK to do so. However we must not quantify over 'a' itself.
+
+Other things being equal, however, we'd like to quantify over as few
+variables as possible: smaller types, fewer type applications, more
+constraints can get into Ct instead of Cq. Here's a good way to
+choose Q:
+
+ Q = grow( fv(T), C ) \ oclose( fv(G), C )
+
+That is, quantify over all variable that that MIGHT be fixed by the
+call site (which influences T), but which aren't DEFINITELY fixed by
+G. This choice definitely quantifies over enough type variables,
+albeit perhaps too many.
+
+Why grow( fv(T), C ) rather than fv(T)? Consider
+
+ class H x y | x->y where ...
+
+ T = c->c
+ C = (H c d)
+
+ If we used fv(T) = {c} we'd get the type
+
+ forall c. H c d => c -> b
+
+ And then if the fn was called at several different c's, each of
+ which fixed d differently, we'd get a unification error, because
+ d isn't quantified. Solution: quantify d. So we must quantify
+ everything that might be influenced by c.
+
+Why not oclose( fv(T), C )? Because we might not be able to see
+all the functional dependencies yet:
+
+ class H x y | x->y where ...
+ instance H x y => Eq (T x y) where ...
+
+ T = c->c
+ C = (Eq (T c d))
+
+Now oclose(fv(T),C) = {c}, because the functional dependency isn't
+apparent yet, and that's wrong. We must really quantify over d too.
+
+There really isn't any point in quantifying over any more than
+grow( fv(T), C ), because the call sites can't possibly influence
+any other type variables.
+
+
+
+-------------------------------------
+ Note [Ambiguity]
+-------------------------------------
+
+It's very hard to be certain when a type is ambiguous. Consider
+
+ class K x
+ class H x y | x -> y
+ instance H x y => K (x,y)
+
+Is this type ambiguous?
+ forall a b. (K (a,b), Eq b) => a -> a
+
+Looks like it! But if we simplify (K (a,b)) we get (H a b) and
+now we see that a fixes b. So we can't tell about ambiguity for sure
+without doing a full simplification. And even that isn't possible if
+the context has some free vars that may get unified. Urgle!
+
+Here's another example: is this ambiguous?
+ forall a b. Eq (T b) => a -> a
+Not if there's an insance decl (with no context)
+ instance Eq (T b) where ...
+
+You may say of this example that we should use the instance decl right
+away, but you can't always do that:
+
+ class J a b where ...
+ instance J Int b where ...
+
+ f :: forall a b. J a b => a -> a
+
+(Notice: no functional dependency in J's class decl.)
+Here f's type is perfectly fine, provided f is only called at Int.
+It's premature to complain when meeting f's signature, or even
+when inferring a type for f.
+
+
+
+However, we don't *need* to report ambiguity right away. It'll always
+show up at the call site.... and eventually at main, which needs special
+treatment. Nevertheless, reporting ambiguity promptly is an excellent thing.
+
+So here's the plan. We WARN about probable ambiguity if
+
+ fv(Cq) is not a subset of oclose(fv(T) union fv(G), C)
+
+(all tested before quantification).
+That is, all the type variables in Cq must be fixed by the the variables
+in the environment, or by the variables in the type.
+
+Notice that we union before calling oclose. Here's an example:
+
+ class J a b c | a b -> c
+ fv(G) = {a}
+
+Is this ambiguous?
+ forall b c. (J a b c) => b -> b
+
+Only if we union {a} from G with {b} from T before using oclose,
+do we see that c is fixed.
+
+It's a bit vague exactly which C we should use for this oclose call. If we
+don't fix enough variables we might complain when we shouldn't (see
+the above nasty example). Nothing will be perfect. That's why we can
+only issue a warning.
+
+
+Can we ever be *certain* about ambiguity? Yes: if there's a constraint
+
+ c in C such that fv(c) intersect (fv(G) union fv(T)) = EMPTY
+
+then c is a "bubble"; there's no way it can ever improve, and it's
+certainly ambiguous. UNLESS it is a constant (sigh). And what about
+the nasty example?
+
+ class K x
+ class H x y | x -> y
+ instance H x y => K (x,y)
+
+Is this type ambiguous?
+ forall a b. (K (a,b), Eq b) => a -> a
+
+Urk. The (Eq b) looks "definitely ambiguous" but it isn't. What we are after
+is a "bubble" that's a set of constraints
+
+ Cq = Ca union Cq' st fv(Ca) intersect (fv(Cq') union fv(T) union fv(G)) = EMPTY
+
+Hence another idea. To decide Q start with fv(T) and grow it
+by transitive closure in Cq (no functional dependencies involved).
+Now partition Cq using Q, leaving the definitely-ambiguous and probably-ok.
+The definitely-ambiguous can then float out, and get smashed at top level
+(which squashes out the constants, like Eq (T a) above)
+
+
+ --------------------------------------
+ Notes on principal types
+ --------------------------------------
+
+ class C a where
+ op :: a -> a
+
+ f x = let g y = op (y::Int) in True
+
+Here the principal type of f is (forall a. a->a)
+but we'll produce the non-principal type
+ f :: forall a. C Int => a -> a
+
+
+ --------------------------------------
+ The need for forall's in constraints
+ --------------------------------------
+
+[Exchange on Haskell Cafe 5/6 Dec 2000]
+
+ class C t where op :: t -> Bool
+ instance C [t] where op x = True
+
+ p y = (let f :: c -> Bool; f x = op (y >> return x) in f, y ++ [])
+ q y = (y ++ [], let f :: c -> Bool; f x = op (y >> return x) in f)
+
+The definitions of p and q differ only in the order of the components in
+the pair on their right-hand sides. And yet:
+
+ ghc and "Typing Haskell in Haskell" reject p, but accept q;
+ Hugs rejects q, but accepts p;
+ hbc rejects both p and q;
+ nhc98 ... (Malcolm, can you fill in the blank for us!).
+
+The type signature for f forces context reduction to take place, and
+the results of this depend on whether or not the type of y is known,
+which in turn depends on which component of the pair the type checker
+analyzes first.
+
+Solution: if y::m a, float out the constraints
+ Monad m, forall c. C (m c)
+When m is later unified with [], we can solve both constraints.
+
+
+ --------------------------------------
+ Notes on implicit parameters
+ --------------------------------------
+
+Note [Inheriting implicit parameters]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider this:
+
+ f x = (x::Int) + ?y
+
+where f is *not* a top-level binding.
+From the RHS of f we'll get the constraint (?y::Int).
+There are two types we might infer for f:
+
+ f :: Int -> Int
+
+(so we get ?y from the context of f's definition), or
+
+ f :: (?y::Int) => Int -> Int
+
+At first you might think the first was better, becuase then
+?y behaves like a free variable of the definition, rather than
+having to be passed at each call site. But of course, the WHOLE
+IDEA is that ?y should be passed at each call site (that's what
+dynamic binding means) so we'd better infer the second.
+
+BOTTOM LINE: when *inferring types* you *must* quantify
+over implicit parameters. See the predicate isFreeWhenInferring.
+
+
+Note [Implicit parameters and ambiguity]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Only a *class* predicate can give rise to ambiguity
+An *implicit parameter* cannot. For example:
+ foo :: (?x :: [a]) => Int
+ foo = length ?x
+is fine. The call site will suppply a particular 'x'
+
+Furthermore, the type variables fixed by an implicit parameter
+propagate to the others. E.g.
+ foo :: (Show a, ?x::[a]) => Int
+ foo = show (?x++?x)
+The type of foo looks ambiguous. But it isn't, because at a call site
+we might have
+ let ?x = 5::Int in foo
+and all is well. In effect, implicit parameters are, well, parameters,
+so we can take their type variables into account as part of the
+"tau-tvs" stuff. This is done in the function 'FunDeps.grow'.
+
+
+Question 2: type signatures
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+BUT WATCH OUT: When you supply a type signature, we can't force you
+to quantify over implicit parameters. For example:
+
+ (?x + 1) :: Int
+
+This is perfectly reasonable. We do not want to insist on
+
+ (?x + 1) :: (?x::Int => Int)
+
+That would be silly. Here, the definition site *is* the occurrence site,
+so the above strictures don't apply. Hence the difference between
+tcSimplifyCheck (which *does* allow implicit paramters to be inherited)
+and tcSimplifyCheckBind (which does not).
+
+What about when you supply a type signature for a binding?
+Is it legal to give the following explicit, user type
+signature to f, thus:
+
+ f :: Int -> Int
+ f x = (x::Int) + ?y
+
+At first sight this seems reasonable, but it has the nasty property
+that adding a type signature changes the dynamic semantics.
+Consider this:
+
+ (let f x = (x::Int) + ?y
+ in (f 3, f 3 with ?y=5)) with ?y = 6
+
+ returns (3+6, 3+5)
+vs
+ (let f :: Int -> Int
+ f x = x + ?y
+ in (f 3, f 3 with ?y=5)) with ?y = 6
+
+ returns (3+6, 3+6)
+
+Indeed, simply inlining f (at the Haskell source level) would change the
+dynamic semantics.
+
+Nevertheless, as Launchbury says (email Oct 01) we can't really give the
+semantics for a Haskell program without knowing its typing, so if you
+change the typing you may change the semantics.
+
+To make things consistent in all cases where we are *checking* against
+a supplied signature (as opposed to inferring a type), we adopt the
+rule:
+
+ a signature does not need to quantify over implicit params.
+
+[This represents a (rather marginal) change of policy since GHC 5.02,
+which *required* an explicit signature to quantify over all implicit
+params for the reasons mentioned above.]
+
+But that raises a new question. Consider
+
+ Given (signature) ?x::Int
+ Wanted (inferred) ?x::Int, ?y::Bool
+
+Clearly we want to discharge the ?x and float the ?y out. But
+what is the criterion that distinguishes them? Clearly it isn't
+what free type variables they have. The Right Thing seems to be
+to float a constraint that
+ neither mentions any of the quantified type variables
+ nor any of the quantified implicit parameters
+
+See the predicate isFreeWhenChecking.
+
+
+Question 3: monomorphism
+~~~~~~~~~~~~~~~~~~~~~~~~
+There's a nasty corner case when the monomorphism restriction bites:
+
+ z = (x::Int) + ?y
+
+The argument above suggests that we *must* generalise
+over the ?y parameter, to get
+ z :: (?y::Int) => Int,
+but the monomorphism restriction says that we *must not*, giving
+ z :: Int.
+Why does the momomorphism restriction say this? Because if you have
+
+ let z = x + ?y in z+z
+
+you might not expect the addition to be done twice --- but it will if
+we follow the argument of Question 2 and generalise over ?y.
+
+
+Question 4: top level
+~~~~~~~~~~~~~~~~~~~~~
+At the top level, monomorhism makes no sense at all.
+
+ module Main where
+ main = let ?x = 5 in print foo
+
+ foo = woggle 3
+
+ woggle :: (?x :: Int) => Int -> Int
+ woggle y = ?x + y
+
+We definitely don't want (foo :: Int) with a top-level implicit parameter
+(?x::Int) becuase there is no way to bind it.
+
+
+Possible choices
+~~~~~~~~~~~~~~~~
+(A) Always generalise over implicit parameters
+ Bindings that fall under the monomorphism restriction can't
+ be generalised
+
+ Consequences:
+ * Inlining remains valid
+ * No unexpected loss of sharing
+ * But simple bindings like
+ z = ?y + 1
+ will be rejected, unless you add an explicit type signature
+ (to avoid the monomorphism restriction)
+ z :: (?y::Int) => Int
+ z = ?y + 1
+ This seems unacceptable
+
+(B) Monomorphism restriction "wins"
+ Bindings that fall under the monomorphism restriction can't
+ be generalised
+ Always generalise over implicit parameters *except* for bindings
+ that fall under the monomorphism restriction
+
+ Consequences
+ * Inlining isn't valid in general
+ * No unexpected loss of sharing
+ * Simple bindings like
+ z = ?y + 1
+ accepted (get value of ?y from binding site)
+
+(C) Always generalise over implicit parameters
+ Bindings that fall under the monomorphism restriction can't
+ be generalised, EXCEPT for implicit parameters
+ Consequences
+ * Inlining remains valid
+ * Unexpected loss of sharing (from the extra generalisation)
+ * Simple bindings like
+ z = ?y + 1
+ accepted (get value of ?y from occurrence sites)
+
+
+Discussion
+~~~~~~~~~~
+None of these choices seems very satisfactory. But at least we should
+decide which we want to do.
+
+It's really not clear what is the Right Thing To Do. If you see
+
+ z = (x::Int) + ?y
+
+would you expect the value of ?y to be got from the *occurrence sites*
+of 'z', or from the valuue of ?y at the *definition* of 'z'? In the
+case of function definitions, the answer is clearly the former, but
+less so in the case of non-fucntion definitions. On the other hand,
+if we say that we get the value of ?y from the definition site of 'z',
+then inlining 'z' might change the semantics of the program.
+
+Choice (C) really says "the monomorphism restriction doesn't apply
+to implicit parameters". Which is fine, but remember that every
+innocent binding 'x = ...' that mentions an implicit parameter in
+the RHS becomes a *function* of that parameter, called at each
+use of 'x'. Now, the chances are that there are no intervening 'with'
+clauses that bind ?y, so a decent compiler should common up all
+those function calls. So I think I strongly favour (C). Indeed,
+one could make a similar argument for abolishing the monomorphism
+restriction altogether.
+
+BOTTOM LINE: we choose (B) at present. See tcSimplifyRestricted
+
+
+
+%************************************************************************
+%* *
+\subsection{tcSimplifyInfer}
+%* *
+%************************************************************************
+
+tcSimplify is called when we *inferring* a type. Here's the overall game plan:
+
+ 1. Compute Q = grow( fvs(T), C )
+
+ 2. Partition C based on Q into Ct and Cq. Notice that ambiguous
+ predicates will end up in Ct; we deal with them at the top level
+
+ 3. Try improvement, using functional dependencies
+
+ 4. If Step 3 did any unification, repeat from step 1
+ (Unification can change the result of 'grow'.)
+
+Note: we don't reduce dictionaries in step 2. For example, if we have
+Eq (a,b), we don't simplify to (Eq a, Eq b). So Q won't be different
+after step 2. However note that we may therefore quantify over more
+type variables than we absolutely have to.
+
+For the guts, we need a loop, that alternates context reduction and
+improvement with unification. E.g. Suppose we have
+
+ class C x y | x->y where ...
+
+and tcSimplify is called with:
+ (C Int a, C Int b)
+Then improvement unifies a with b, giving
+ (C Int a, C Int a)
+
+If we need to unify anything, we rattle round the whole thing all over
+again.
+
+
+\begin{code}
+tcSimplifyInfer
+ :: SDoc
+ -> TcTyVarSet -- fv(T); type vars
+ -> [Inst] -- Wanted
+ -> TcM ([TcTyVar], -- Tyvars to quantify (zonked and quantified)
+ [Inst], -- Dict Ids that must be bound here (zonked)
+ TcDictBinds) -- Bindings
+ -- Any free (escaping) Insts are tossed into the environment
+\end{code}
+
+
+\begin{code}
+tcSimplifyInfer doc tau_tvs wanted
+ = do { tau_tvs1 <- zonkTcTyVarsAndFV (varSetElems tau_tvs)
+ ; wanted' <- mapM zonkInst wanted -- Zonk before deciding quantified tyvars
+ ; gbl_tvs <- tcGetGlobalTyVars
+ ; let preds1 = fdPredsOfInsts wanted'
+ gbl_tvs1 = oclose preds1 gbl_tvs
+ qtvs = growInstsTyVars wanted' tau_tvs1 `minusVarSet` gbl_tvs1
+ -- See Note [Choosing which variables to quantify]
+
+ -- To maximise sharing, remove from consideration any
+ -- constraints that don't mention qtvs at all
+ ; let (free, bound) = partition (isFreeWhenInferring qtvs) wanted'
+ ; extendLIEs free
+
+ -- To make types simple, reduce as much as possible
+ ; traceTc (text "infer" <+> (ppr preds1 $$ ppr (growInstsTyVars wanted' tau_tvs1) $$ ppr gbl_tvs $$
+ ppr gbl_tvs1 $$ ppr free $$ ppr bound))
+ ; (irreds1, binds1) <- tryHardCheckLoop doc bound
+
+ -- Note [Inference and implication constraints]
+ ; let want_dict d = tyVarsOfInst d `intersectsVarSet` qtvs
+ ; (irreds2, binds2) <- approximateImplications doc want_dict irreds1
+
+ -- Now work out all over again which type variables to quantify,
+ -- exactly in the same way as before, but starting from irreds2. Why?
+ -- a) By now improvment may have taken place, and we must *not*
+ -- quantify over any variable free in the environment
+ -- tc137 (function h inside g) is an example
+ --
+ -- b) Do not quantify over constraints that *now* do not
+ -- mention quantified type variables, because they are
+ -- simply ambiguous (or might be bound further out). Example:
+ -- f :: Eq b => a -> (a, b)
+ -- g x = fst (f x)
+ -- From the RHS of g we get the MethodInst f77 :: alpha -> (alpha, beta)
+ -- We decide to quantify over 'alpha' alone, but free1 does not include f77
+ -- because f77 mentions 'alpha'. Then reducing leaves only the (ambiguous)
+ -- constraint (Eq beta), which we dump back into the free set
+ -- See test tcfail181
+ --
+ -- c) irreds may contain type variables not previously mentioned,
+ -- e.g. instance D a x => Foo [a]
+ -- wanteds = Foo [a]
+ -- Then after simplifying we'll get (D a x), and x is fresh
+ -- We must quantify over x else it'll be totally unbound
+ ; tau_tvs2 <- zonkTcTyVarsAndFV (varSetElems tau_tvs1)
+ ; gbl_tvs2 <- zonkTcTyVarsAndFV (varSetElems gbl_tvs1)
+ -- Note that we start from gbl_tvs1
+ -- We use tcGetGlobalTyVars, then oclose wrt preds2, because
+ -- we've already put some of the original preds1 into frees
+ -- E.g. wanteds = C a b (where a->b)
+ -- gbl_tvs = {a}
+ -- tau_tvs = {b}
+ -- Then b is fixed by gbl_tvs, so (C a b) will be in free, and
+ -- irreds2 will be empty. But we don't want to generalise over b!
+ ; let preds2 = fdPredsOfInsts irreds2 -- irreds2 is zonked
+ qtvs = growInstsTyVars irreds2 tau_tvs2 `minusVarSet` oclose preds2 gbl_tvs2
+ ---------------------------------------------------
+ -- BUG WARNING: there's a nasty bug lurking here
+ -- fdPredsOfInsts may return preds that mention variables quantified in
+ -- one of the implication constraints in irreds2; and that is clearly wrong:
+ -- we might quantify over too many variables through accidental capture
+ ---------------------------------------------------
+ ; let (free, irreds3) = partition (isFreeWhenInferring qtvs) irreds2
+ ; extendLIEs free
+
+ -- Turn the quantified meta-type variables into real type variables
+ ; qtvs2 <- zonkQuantifiedTyVars (varSetElems qtvs)
+
+ -- We can't abstract over any remaining unsolved
+ -- implications so instead just float them outwards. Ugh.
+ ; let (q_dicts0, implics) = partition isAbstractableInst irreds3
+ ; loc <- getInstLoc (ImplicOrigin doc)
+ ; implic_bind <- bindIrreds loc qtvs2 q_dicts0 implics
+
+ -- Prepare equality instances for quantification
+ ; let (q_eqs0,q_dicts) = partition isEqInst q_dicts0
+ ; q_eqs <- mapM finalizeEqInst q_eqs0
+
+ ; return (qtvs2, q_eqs ++ q_dicts, binds1 `unionBags` binds2 `unionBags` implic_bind) }
+ -- NB: when we are done, we might have some bindings, but
+ -- the final qtvs might be empty. See Note [NO TYVARS] below.
+
+approximateImplications :: SDoc -> (Inst -> Bool) -> [Inst] -> TcM ([Inst], TcDictBinds)
+-- Note [Inference and implication constraints]
+-- Given a bunch of Dict and ImplicInsts, try to approximate the implications by
+-- - fetching any dicts inside them that are free
+-- - using those dicts as cruder constraints, to solve the implications
+-- - returning the extra ones too
+
+approximateImplications doc want_dict irreds
+ | null extra_dicts
+ = return (irreds, emptyBag)
+ | otherwise
+ = do { extra_dicts' <- mapM cloneDict extra_dicts
+ ; tryHardCheckLoop doc (extra_dicts' ++ irreds) }
+ -- By adding extra_dicts', we make them
+ -- available to solve the implication constraints
+ where
+ extra_dicts = get_dicts (filter isImplicInst irreds)
+
+ get_dicts :: [Inst] -> [Inst] -- Returns only Dicts
+ -- Find the wanted constraints in implication constraints that satisfy
+ -- want_dict, and are not bound by forall's in the constraint itself
+ get_dicts ds = concatMap get_dict ds
+
+ get_dict d@(Dict {}) | want_dict d = [d]
+ | otherwise = []
+ get_dict (ImplicInst {tci_tyvars = tvs, tci_wanted = wanteds})
+ = [ d | let tv_set = mkVarSet tvs
+ , d <- get_dicts wanteds
+ , not (tyVarsOfInst d `intersectsVarSet` tv_set)]
+ get_dict i@(EqInst {}) | want_dict i = [i]
+ | otherwise = []
+ get_dict other = pprPanic "approximateImplications" (ppr other)
+\end{code}
+
+Note [Inference and implication constraints]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Suppose we have a wanted implication constraint (perhaps arising from
+a nested pattern match) like
+ C a => D [a]
+and we are now trying to quantify over 'a' when inferring the type for
+a function. In principle it's possible that there might be an instance
+ instance (C a, E a) => D [a]
+so the context (E a) would suffice. The Right Thing is to abstract over
+the implication constraint, but we don't do that (a) because it'll be
+surprising to programmers and (b) because we don't have the machinery to deal
+with 'given' implications.
+
+So our best approximation is to make (D [a]) part of the inferred
+context, so we can use that to discharge the implication. Hence
+the strange function get_dicts in approximateImplications.
+
+The common cases are more clear-cut, when we have things like
+ forall a. C a => C b
+Here, abstracting over (C b) is not an approximation at all -- but see
+Note [Freeness and implications].
+
+See Trac #1430 and test tc228.
+
+
+\begin{code}
+-----------------------------------------------------------
+-- tcSimplifyInferCheck is used when we know the constraints we are to simplify
+-- against, but we don't know the type variables over which we are going to quantify.
+-- This happens when we have a type signature for a mutually recursive group
+tcSimplifyInferCheck
+ :: InstLoc
+ -> TcTyVarSet -- fv(T)
+ -> [Inst] -- Given
+ -> [Inst] -- Wanted
+ -> TcM ([TyVar], -- Fully zonked, and quantified
+ TcDictBinds) -- Bindings
+
+tcSimplifyInferCheck loc tau_tvs givens wanteds
+ = do { traceTc (text "tcSimplifyInferCheck <-" <+> ppr wanteds)
+ ; (irreds, binds) <- gentleCheckLoop loc givens wanteds
+
+ -- Figure out which type variables to quantify over
+ -- You might think it should just be the signature tyvars,
+ -- but in bizarre cases you can get extra ones
+ -- f :: forall a. Num a => a -> a
+ -- f x = fst (g (x, head [])) + 1
+ -- g a b = (b,a)
+ -- Here we infer g :: forall a b. a -> b -> (b,a)
+ -- We don't want g to be monomorphic in b just because
+ -- f isn't quantified over b.
+ ; let all_tvs = varSetElems (tau_tvs `unionVarSet` tyVarsOfInsts givens)
+ ; all_tvs <- zonkTcTyVarsAndFV all_tvs
+ ; gbl_tvs <- tcGetGlobalTyVars
+ ; let qtvs = varSetElems (all_tvs `minusVarSet` gbl_tvs)
+ -- We could close gbl_tvs, but its not necessary for
+ -- soundness, and it'll only affect which tyvars, not which
+ -- dictionaries, we quantify over
+
+ ; qtvs' <- zonkQuantifiedTyVars qtvs
+
+ -- Now we are back to normal (c.f. tcSimplCheck)
+ ; implic_bind <- bindIrreds loc qtvs' givens irreds
+
+ ; traceTc (text "tcSimplifyInferCheck ->" <+> ppr (implic_bind))
+ ; return (qtvs', binds `unionBags` implic_bind) }
+\end{code}
+
+Note [Squashing methods]
+~~~~~~~~~~~~~~~~~~~~~~~~~
+Be careful if you want to float methods more:
+ truncate :: forall a. RealFrac a => forall b. Integral b => a -> b
+From an application (truncate f i) we get
+ t1 = truncate at f
+ t2 = t1 at i
+If we have also have a second occurrence of truncate, we get
+ t3 = truncate at f
+ t4 = t3 at i
+When simplifying with i,f free, we might still notice that
+t1=t3; but alas, the binding for t2 (which mentions t1)
+may continue to float out!
+
+
+Note [NO TYVARS]
+~~~~~~~~~~~~~~~~~
+ class Y a b | a -> b where
+ y :: a -> X b
+
+ instance Y [[a]] a where
+ y ((x:_):_) = X x
+
+ k :: X a -> X a -> X a
+
+ g :: Num a => [X a] -> [X a]
+ g xs = h xs
+ where
+ h ys = ys ++ map (k (y [[0]])) xs
+
+The excitement comes when simplifying the bindings for h. Initially
+try to simplify {y @ [[t1]] t2, 0 @ t1}, with initial qtvs = {t2}.
+From this we get t1~t2, but also various bindings. We can't forget
+the bindings (because of [LOOP]), but in fact t1 is what g is
+polymorphic in.
+
+The net effect of [NO TYVARS]
+
+\begin{code}
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{tcSimplifyCheck}
+%* *
+%************************************************************************
+
+@tcSimplifyCheck@ is used when we know exactly the set of variables
+we are going to quantify over. For example, a class or instance declaration.
+
+\begin{code}
+-----------------------------------------------------------
+-- tcSimplifyCheck is used when checking expression type signatures,
+-- class decls, instance decls etc.
+tcSimplifyCheck :: InstLoc
+ -> [TcTyVar] -- Quantify over these
+ -> [Inst] -- Given
+ -> [Inst] -- Wanted
+ -> TcM TcDictBinds -- Bindings
+tcSimplifyCheck loc qtvs givens wanteds
+ = ASSERT( all isTcTyVar qtvs && all isSkolemTyVar qtvs )
+ do { traceTc (text "tcSimplifyCheck")
+ ; (irreds, binds) <- gentleCheckLoop loc givens wanteds
+ ; implic_bind <- bindIrreds loc qtvs givens irreds
+ ; return (binds `unionBags` implic_bind) }
+
+-----------------------------------------------------------
+-- tcSimplifyCheckPat is used for existential pattern match
+tcSimplifyCheckPat :: InstLoc
+ -> [TcTyVar] -- Quantify over these
+ -> [Inst] -- Given
+ -> [Inst] -- Wanted
+ -> TcM TcDictBinds -- Bindings
+tcSimplifyCheckPat loc qtvs givens wanteds
+ = ASSERT( all isTcTyVar qtvs && all isSkolemTyVar qtvs )
+ do { traceTc (text "tcSimplifyCheckPat")
+ ; (irreds, binds) <- gentleCheckLoop loc givens wanteds
+ ; implic_bind <- bindIrredsR loc qtvs givens irreds
+ ; return (binds `unionBags` implic_bind) }
+
+-----------------------------------------------------------
+bindIrreds :: InstLoc -> [TcTyVar]
+ -> [Inst] -> [Inst]
+ -> TcM TcDictBinds
+bindIrreds loc qtvs givens irreds
+ = bindIrredsR loc qtvs givens irreds
+
+bindIrredsR :: InstLoc -> [TcTyVar] -> [Inst] -> [Inst] -> TcM TcDictBinds
+-- Make a binding that binds 'irreds', by generating an implication
+-- constraint for them, *and* throwing the constraint into the LIE
+bindIrredsR loc qtvs givens irreds
+ | null irreds
+ = return emptyBag
+ | otherwise
+ = do { let givens' = filter isAbstractableInst givens
+ -- The givens can (redundantly) include methods
+ -- We want to retain both EqInsts and Dicts
+ -- There should be no implicadtion constraints
+ -- See Note [Pruning the givens in an implication constraint]
+
+ -- If there are no 'givens', then it's safe to
+ -- partition the 'wanteds' by their qtvs, thereby trimming irreds
+ -- See Note [Freeness and implications]
+ ; irreds' <- if null givens'
+ then do
+ { let qtv_set = mkVarSet qtvs
+ (frees, real_irreds) = partition (isFreeWrtTyVars qtv_set) irreds
+ ; extendLIEs frees
+ ; return real_irreds }
+ else return irreds
+
+ ; (implics, bind) <- makeImplicationBind loc qtvs givens' irreds'
+ -- This call does the real work
+ -- If irreds' is empty, it does something sensible
+ ; extendLIEs implics
+ ; return bind }
+
+
+makeImplicationBind :: InstLoc -> [TcTyVar]
+ -> [Inst] -> [Inst]
+ -> TcM ([Inst], TcDictBinds)
+-- Make a binding that binds 'irreds', by generating an implication
+-- constraint for them.
+--
+-- The binding looks like
+-- (ir1, .., irn) = f qtvs givens
+-- where f is (evidence for) the new implication constraint
+-- f :: forall qtvs. givens => (ir1, .., irn)
+-- qtvs includes coercion variables
+--
+-- This binding must line up the 'rhs' in reduceImplication
+makeImplicationBind loc all_tvs
+ givens -- Guaranteed all Dicts or EqInsts
+ irreds
+ | null irreds -- If there are no irreds, we are done
+ = return ([], emptyBag)
+ | otherwise -- Otherwise we must generate a binding
+ = do { uniq <- newUnique
+ ; span <- getSrcSpanM
+ ; let (eq_givens, dict_givens) = partition isEqInst givens
+
+ -- extract equality binders
+ eq_cotvs = map eqInstType eq_givens
+
+ -- make the implication constraint instance
+ name = mkInternalName uniq (mkVarOcc "ic") span
+ implic_inst = ImplicInst { tci_name = name,
+ tci_tyvars = all_tvs,
+ tci_given = eq_givens ++ dict_givens,
+ -- same order as binders
+ tci_wanted = irreds,
+ tci_loc = loc }
+
+ -- create binders for the irreducible dictionaries
+ dict_irreds = filter (not . isEqInst) irreds
+ dict_irred_ids = map instToId dict_irreds
+ lpat = mkBigLHsPatTup (map (L span . VarPat) dict_irred_ids)
+
+ -- create the binding
+ rhs = L span (mkHsWrap co (HsVar (instToId implic_inst)))
+ co = mkWpApps (map instToId dict_givens)
+ <.> mkWpTyApps eq_cotvs
+ <.> mkWpTyApps (mkTyVarTys all_tvs)
+ bind | [dict_irred_id] <- dict_irred_ids
+ = mkVarBind dict_irred_id rhs
+ | otherwise
+ = L span $
+ PatBind { pat_lhs = lpat
+ , pat_rhs = unguardedGRHSs rhs
+ , pat_rhs_ty = hsLPatType lpat
+ , bind_fvs = placeHolderNames
+ }
+
+ ; traceTc $ text "makeImplicationBind" <+> ppr implic_inst
+ ; return ([implic_inst], unitBag bind)
+ }
+
+-----------------------------------------------------------
+tryHardCheckLoop :: SDoc
+ -> [Inst] -- Wanted
+ -> TcM ([Inst], TcDictBinds)
+
+tryHardCheckLoop doc wanteds
+ = do { (irreds,binds) <- checkLoop (mkInferRedEnv doc try_me) wanteds
+ ; return (irreds,binds)
+ }
+ where
+ try_me _ = ReduceMe
+ -- Here's the try-hard bit
+
+-----------------------------------------------------------
+gentleCheckLoop :: InstLoc
+ -> [Inst] -- Given
+ -> [Inst] -- Wanted
+ -> TcM ([Inst], TcDictBinds)
+
+gentleCheckLoop inst_loc givens wanteds
+ = do { (irreds,binds) <- checkLoop env wanteds
+ ; return (irreds,binds)
+ }
+ where
+ env = mkRedEnv (pprInstLoc inst_loc) try_me givens
+
+ try_me inst | isMethodOrLit inst = ReduceMe
+ | otherwise = Stop
+ -- When checking against a given signature
+ -- we MUST be very gentle: Note [Check gently]
+
+gentleInferLoop :: SDoc -> [Inst]
+ -> TcM ([Inst], TcDictBinds)
+gentleInferLoop doc wanteds
+ = do { (irreds, binds) <- checkLoop env wanteds
+ ; return (irreds, binds) }
+ where
+ env = mkInferRedEnv doc try_me
+ try_me inst | isMethodOrLit inst = ReduceMe
+ | otherwise = Stop
+\end{code}
+
+Note [Check gently]
+~~~~~~~~~~~~~~~~~~~~
+We have to very careful about not simplifying too vigorously
+Example:
+ data T a where
+ MkT :: a -> T [a]
+
+ f :: Show b => T b -> b
+ f (MkT x) = show [x]
+
+Inside the pattern match, which binds (a:*, x:a), we know that
+ b ~ [a]
+Hence we have a dictionary for Show [a] available; and indeed we
+need it. We are going to build an implication contraint
+ forall a. (b~[a]) => Show [a]
+Later, we will solve this constraint using the knowledge (Show b)
+
+But we MUST NOT reduce (Show [a]) to (Show a), else the whole
+thing becomes insoluble. So we simplify gently (get rid of literals
+and methods only, plus common up equal things), deferring the real
+work until top level, when we solve the implication constraint
+with tryHardCheckLooop.
+
+
+\begin{code}
+-----------------------------------------------------------
+checkLoop :: RedEnv
+ -> [Inst] -- Wanted
+ -> TcM ([Inst], TcDictBinds)
+-- Precondition: givens are completely rigid
+-- Postcondition: returned Insts are zonked
+
+checkLoop env wanteds
+ = go env wanteds
+ where go env wanteds
+ = do { -- We do need to zonk the givens; cf Note [Zonking RedEnv]
+ ; env' <- zonkRedEnv env
+ ; wanteds' <- zonkInsts wanteds
+
+ ; (improved, tybinds, binds, irreds)
+ <- reduceContext env' wanteds'
+ ; execTcTyVarBinds tybinds
+
+ ; if null irreds || not improved then
+ return (irreds, binds)
+ else do
+
+ -- If improvement did some unification, we go round again.
+ -- We start again with irreds, not wanteds
+ -- Using an instance decl might have introduced a fresh type
+ -- variable which might have been unified, so we'd get an
+ -- infinite loop if we started again with wanteds!
+ -- See Note [LOOP]
+ { (irreds1, binds1) <- go env' irreds
+ ; return (irreds1, binds `unionBags` binds1) } }
+\end{code}
+
+Note [Zonking RedEnv]
+~~~~~~~~~~~~~~~~~~~~~
+It might appear as if the givens in RedEnv are always rigid, but that is not
+necessarily the case for programs involving higher-rank types that have class
+contexts constraining the higher-rank variables. An example from tc237 in the
+testsuite is
+
+ class Modular s a | s -> a
+
+ wim :: forall a w. Integral a
+ => a -> (forall s. Modular s a => M s w) -> w
+ wim i k = error "urk"
+
+ test5 :: (Modular s a, Integral a) => M s a
+ test5 = error "urk"
+
+ test4 = wim 4 test4'
+
+Notice how the variable 'a' of (Modular s a) in the rank-2 type of wim is
+quantified further outside. When type checking test4, we have to check
+whether the signature of test5 is an instance of
+
+ (forall s. Modular s a => M s w)
+
+Consequently, we will get (Modular s t_a), where t_a is a TauTv into the
+givens.
+
+Given the FD of Modular in this example, class improvement will instantiate
+t_a to 'a', where 'a' is the skolem from test5's signatures (due to the
+Modular s a predicate in that signature). If we don't zonk (Modular s t_a) in
+the givens, we will get into a loop as improveOne uses the unification engine
+Unify.tcUnifyTys, which doesn't know about mutable type variables.
+
+
+Note [LOOP]
+~~~~~~~~~~~
+ class If b t e r | b t e -> r
+ instance If T t e t
+ instance If F t e e
+ class Lte a b c | a b -> c where lte :: a -> b -> c
+ instance Lte Z b T
+ instance (Lte a b l,If l b a c) => Max a b c
+
+Wanted: Max Z (S x) y
+
+Then we'll reduce using the Max instance to:
+ (Lte Z (S x) l, If l (S x) Z y)
+and improve by binding l->T, after which we can do some reduction
+on both the Lte and If constraints. What we *can't* do is start again
+with (Max Z (S x) y)!
+
+
+
+%************************************************************************
+%* *
+ tcSimplifySuperClasses
+%* *
+%************************************************************************
+
+Note [SUPERCLASS-LOOP 1]
+~~~~~~~~~~~~~~~~~~~~~~~~
+We have to be very, very careful when generating superclasses, lest we
+accidentally build a loop. Here's an example:
+
+ class S a
+
+ class S a => C a where { opc :: a -> a }
+ class S b => D b where { opd :: b -> b }
+
+ instance C Int where
+ opc = opd
+
+ instance D Int where
+ opd = opc
+
+From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
+Simplifying, we may well get:
+ $dfCInt = :C ds1 (opd dd)
+ dd = $dfDInt
+ ds1 = $p1 dd
+Notice that we spot that we can extract ds1 from dd.
+
+Alas! Alack! We can do the same for (instance D Int):
+
+ $dfDInt = :D ds2 (opc dc)
+ dc = $dfCInt
+ ds2 = $p1 dc
+
+And now we've defined the superclass in terms of itself.
+Two more nasty cases are in
+ tcrun021
+ tcrun033
+
+Solution:
+ - Satisfy the superclass context *all by itself*
+ (tcSimplifySuperClasses)
+ - And do so completely; i.e. no left-over constraints
+ to mix with the constraints arising from method declarations
+
+
+Note [Recursive instances and superclases]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider this code, which arises in the context of "Scrap Your
+Boilerplate with Class".
+
+ class Sat a
+ class Data ctx a
+ instance Sat (ctx Char) => Data ctx Char
+ instance (Sat (ctx [a]), Data ctx a) => Data ctx [a]
+
+ class Data Maybe a => Foo a
+
+ instance Foo t => Sat (Maybe t)
+
+ instance Data Maybe a => Foo a
+ instance Foo a => Foo [a]
+ instance Foo [Char]
+
+In the instance for Foo [a], when generating evidence for the superclasses
+(ie in tcSimplifySuperClasses) we need a superclass (Data Maybe [a]).
+Using the instance for Data, we therefore need
+ (Sat (Maybe [a], Data Maybe a)
+But we are given (Foo a), and hence its superclass (Data Maybe a).
+So that leaves (Sat (Maybe [a])). Using the instance for Sat means
+we need (Foo [a]). And that is the very dictionary we are bulding
+an instance for! So we must put that in the "givens". So in this
+case we have
+ Given: Foo a, Foo [a]
+ Watend: Data Maybe [a]
+
+BUT we must *not not not* put the *superclasses* of (Foo [a]) in
+the givens, which is what 'addGiven' would normally do. Why? Because
+(Data Maybe [a]) is the superclass, so we'd "satisfy" the wanted
+by selecting a superclass from Foo [a], which simply makes a loop.
+
+On the other hand we *must* put the superclasses of (Foo a) in
+the givens, as you can see from the derivation described above.
+
+Conclusion: in the very special case of tcSimplifySuperClasses
+we have one 'given' (namely the "this" dictionary) whose superclasses
+must not be added to 'givens' by addGiven.
+
+There is a complication though. Suppose there are equalities
+ instance (Eq a, a~b) => Num (a,b)
+Then we normalise the 'givens' wrt the equalities, so the original
+given "this" dictionary is cast to one of a different type. So it's a
+bit trickier than before to identify the "special" dictionary whose
+superclasses must not be added. See test
+ indexed-types/should_run/EqInInstance
+
+We need a persistent property of the dictionary to record this
+special-ness. Current I'm using the InstLocOrigin (a bit of a hack,
+but cool), which is maintained by dictionary normalisation.
+Specifically, the InstLocOrigin is
+ NoScOrigin
+then the no-superclass thing kicks in. WATCH OUT if you fiddle
+with InstLocOrigin!
+
+\begin{code}
+tcSimplifySuperClasses
+ :: InstLoc
+ -> Inst -- The dict whose superclasses
+ -- are being figured out
+ -> [Inst] -- Given
+ -> [Inst] -- Wanted
+ -> TcM TcDictBinds
+tcSimplifySuperClasses loc this givens sc_wanteds
+ = do { traceTc (text "tcSimplifySuperClasses")
+
+ -- Note [Recursive instances and superclases]
+ ; no_sc_loc <- getInstLoc NoScOrigin
+ ; let no_sc_this = setInstLoc this no_sc_loc
+
+ ; let env = RedEnv { red_doc = pprInstLoc loc,
+ red_try_me = try_me,
+ red_givens = no_sc_this : givens,
+ red_stack = (0,[]),
+ red_improve = False } -- No unification vars
+
+
+ ; (irreds,binds1) <- checkLoop env sc_wanteds
+ ; let (tidy_env, tidy_irreds) = tidyInsts irreds
+ ; reportNoInstances tidy_env (Just (loc, givens)) [] tidy_irreds
+ ; return binds1 }
+ where
+ try_me _ = ReduceMe -- Try hard, so we completely solve the superclass
+ -- constraints right here. See Note [SUPERCLASS-LOOP 1]
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{tcSimplifyRestricted}
+%* *
+%************************************************************************
+
+tcSimplifyRestricted infers which type variables to quantify for a
+group of restricted bindings. This isn't trivial.
+
+Eg1: id = \x -> x
+ We want to quantify over a to get id :: forall a. a->a
+
+Eg2: eq = (==)
+ We do not want to quantify over a, because there's an Eq a
+ constraint, so we get eq :: a->a->Bool (notice no forall)
+
+So, assume:
+ RHS has type 'tau', whose free tyvars are tau_tvs
+ RHS has constraints 'wanteds'
+
+Plan A (simple)
+ Quantify over (tau_tvs \ ftvs(wanteds))
+ This is bad. The constraints may contain (Monad (ST s))
+ where we have instance Monad (ST s) where...
+ so there's no need to be monomorphic in s!
+
+ Also the constraint might be a method constraint,
+ whose type mentions a perfectly innocent tyvar:
+ op :: Num a => a -> b -> a
+ Here, b is unconstrained. A good example would be
+ foo = op (3::Int)
+ We want to infer the polymorphic type
+ foo :: forall b. b -> b
+
+
+Plan B (cunning, used for a long time up to and including GHC 6.2)
+ Step 1: Simplify the constraints as much as possible (to deal
+ with Plan A's problem). Then set
+ qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
+
+ Step 2: Now simplify again, treating the constraint as 'free' if
+ it does not mention qtvs, and trying to reduce it otherwise.
+ The reasons for this is to maximise sharing.
+
+ This fails for a very subtle reason. Suppose that in the Step 2
+ a constraint (Foo (Succ Zero) (Succ Zero) b) gets thrown upstairs as 'free'.
+ In the Step 1 this constraint might have been simplified, perhaps to
+ (Foo Zero Zero b), AND THEN THAT MIGHT BE IMPROVED, to bind 'b' to 'T'.
+ This won't happen in Step 2... but that in turn might prevent some other
+ constraint (Baz [a] b) being simplified (e.g. via instance Baz [a] T where {..})
+ and that in turn breaks the invariant that no constraints are quantified over.
+
+ Test typecheck/should_compile/tc177 (which failed in GHC 6.2) demonstrates
+ the problem.
+
+
+Plan C (brutal)
+ Step 1: Simplify the constraints as much as possible (to deal
+ with Plan A's problem). Then set
+ qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
+ Return the bindings from Step 1.
+
+
+A note about Plan C (arising from "bug" reported by George Russel March 2004)
+Consider this:
+
+ instance (HasBinary ty IO) => HasCodedValue ty
+
+ foo :: HasCodedValue a => String -> IO a
+
+ doDecodeIO :: HasCodedValue a => () -> () -> IO a
+ doDecodeIO codedValue view
+ = let { act = foo "foo" } in act
+
+You might think this should work becuase the call to foo gives rise to a constraint
+(HasCodedValue t), which can be satisfied by the type sig for doDecodeIO. But the
+restricted binding act = ... calls tcSimplifyRestricted, and PlanC simplifies the
+constraint using the (rather bogus) instance declaration, and now we are stuffed.
+
+I claim this is not really a bug -- but it bit Sergey as well as George. So here's
+plan D
+
+
+Plan D (a variant of plan B)
+ Step 1: Simplify the constraints as much as possible (to deal
+ with Plan A's problem), BUT DO NO IMPROVEMENT. Then set
+ qtvs = tau_tvs \ ftvs( simplify( wanteds ) )
+
+ Step 2: Now simplify again, treating the constraint as 'free' if
+ it does not mention qtvs, and trying to reduce it otherwise.
+
+ The point here is that it's generally OK to have too few qtvs; that is,
+ to make the thing more monomorphic than it could be. We don't want to
+ do that in the common cases, but in wierd cases it's ok: the programmer
+ can always add a signature.
+
+ Too few qtvs => too many wanteds, which is what happens if you do less
+ improvement.
+
+
+\begin{code}
+tcSimplifyRestricted -- Used for restricted binding groups
+ -- i.e. ones subject to the monomorphism restriction
+ :: SDoc
+ -> TopLevelFlag
+ -> [Name] -- Things bound in this group
+ -> TcTyVarSet -- Free in the type of the RHSs
+ -> [Inst] -- Free in the RHSs
+ -> TcM ([TyVar], -- Tyvars to quantify (zonked and quantified)
+ TcDictBinds) -- Bindings
+ -- tcSimpifyRestricted returns no constraints to
+ -- quantify over; by definition there are none.
+ -- They are all thrown back in the LIE
+
+tcSimplifyRestricted doc top_lvl bndrs tau_tvs wanteds
+ -- Zonk everything in sight
+ = do { traceTc (text "tcSimplifyRestricted")
+ ; wanteds_z <- zonkInsts wanteds
+
+ -- 'ReduceMe': Reduce as far as we can. Don't stop at
+ -- dicts; the idea is to get rid of as many type
+ -- variables as possible, and we don't want to stop
+ -- at (say) Monad (ST s), because that reduces
+ -- immediately, with no constraint on s.
+ --
+ -- BUT do no improvement! See Plan D above
+ -- HOWEVER, some unification may take place, if we instantiate
+ -- a method Inst with an equality constraint
+ ; let env = mkNoImproveRedEnv doc (\_ -> ReduceMe)
+ ; (_imp, _tybinds, _binds, constrained_dicts)
+ <- reduceContext env wanteds_z
+
+ -- Next, figure out the tyvars we will quantify over
+ ; tau_tvs' <- zonkTcTyVarsAndFV (varSetElems tau_tvs)
+ ; gbl_tvs' <- tcGetGlobalTyVars
+ ; constrained_dicts' <- zonkInsts constrained_dicts
+
+ ; let qtvs1 = tau_tvs' `minusVarSet` oclose (fdPredsOfInsts constrained_dicts) gbl_tvs'
+ -- As in tcSimplifyInfer
+
+ -- Do not quantify over constrained type variables:
+ -- this is the monomorphism restriction
+ constrained_tvs' = tyVarsOfInsts constrained_dicts'
+ qtvs = qtvs1 `minusVarSet` constrained_tvs'
+ pp_bndrs = pprWithCommas (quotes . ppr) bndrs
+
+ -- Warn in the mono
+ ; warn_mono <- doptM Opt_WarnMonomorphism
+ ; warnTc (warn_mono && (constrained_tvs' `intersectsVarSet` qtvs1))
+ (vcat[ ptext (sLit "the Monomorphism Restriction applies to the binding")
+ <> plural bndrs <+> ptext (sLit "for") <+> pp_bndrs,
+ ptext (sLit "Consider giving a type signature for") <+> pp_bndrs])
+
+ ; traceTc (text "tcSimplifyRestricted" <+> vcat [
+ pprInsts wanteds, pprInsts constrained_dicts',
+ ppr _binds,
+ ppr constrained_tvs', ppr tau_tvs', ppr qtvs ])
+
+ -- The first step may have squashed more methods than
+ -- necessary, so try again, this time more gently, knowing the exact
+ -- set of type variables to quantify over.
+ --
+ -- We quantify only over constraints that are captured by qtvs;
+ -- these will just be a subset of non-dicts. This in contrast
+ -- to normal inference (using isFreeWhenInferring) in which we quantify over
+ -- all *non-inheritable* constraints too. This implements choice
+ -- (B) under "implicit parameter and monomorphism" above.
+ --
+ -- Remember that we may need to do *some* simplification, to
+ -- (for example) squash {Monad (ST s)} into {}. It's not enough
+ -- just to float all constraints
+ --
+ -- At top level, we *do* squash methods because we want to
+ -- expose implicit parameters to the test that follows
+ ; let is_nested_group = isNotTopLevel top_lvl
+ try_me inst | isFreeWrtTyVars qtvs inst,
+ (is_nested_group || isDict inst) = Stop
+ | otherwise = ReduceMe
+ env = mkNoImproveRedEnv doc try_me
+ ; (_imp, tybinds, binds, irreds) <- reduceContext env wanteds_z
+ ; execTcTyVarBinds tybinds
+
+ -- See "Notes on implicit parameters, Question 4: top level"
+ ; ASSERT( all (isFreeWrtTyVars qtvs) irreds ) -- None should be captured
+ if is_nested_group then
+ extendLIEs irreds
+ else do { let (bad_ips, non_ips) = partition isIPDict irreds
+ ; addTopIPErrs bndrs bad_ips
+ ; extendLIEs non_ips }
+
+ ; qtvs' <- zonkQuantifiedTyVars (varSetElems qtvs)
+ ; return (qtvs', binds) }
+\end{code}
+
+
+%************************************************************************
+%* *
+ tcSimplifyRuleLhs
+%* *
+%************************************************************************
+
+On the LHS of transformation rules we only simplify methods and constants,
+getting dictionaries. We want to keep all of them unsimplified, to serve
+as the available stuff for the RHS of the rule.
+
+Example. Consider the following left-hand side of a rule
+
+ f (x == y) (y > z) = ...
+
+If we typecheck this expression we get constraints
+
+ d1 :: Ord a, d2 :: Eq a
+
+We do NOT want to "simplify" to the LHS
+
+ forall x::a, y::a, z::a, d1::Ord a.
+ f ((==) (eqFromOrd d1) x y) ((>) d1 y z) = ...
+
+Instead we want
+
+ forall x::a, y::a, z::a, d1::Ord a, d2::Eq a.
+ f ((==) d2 x y) ((>) d1 y z) = ...
+
+Here is another example:
+
+ fromIntegral :: (Integral a, Num b) => a -> b
+ {-# RULES "foo" fromIntegral = id :: Int -> Int #-}
+
+In the rule, a=b=Int, and Num Int is a superclass of Integral Int. But
+we *dont* want to get
+
+ forall dIntegralInt.
+ fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
+
+because the scsel will mess up RULE matching. Instead we want
+
+ forall dIntegralInt, dNumInt.
+ fromIntegral Int Int dIntegralInt dNumInt = id Int
+
+Even if we have
+
+ g (x == y) (y == z) = ..
+
+where the two dictionaries are *identical*, we do NOT WANT
+
+ forall x::a, y::a, z::a, d1::Eq a
+ f ((==) d1 x y) ((>) d1 y z) = ...
+
+because that will only match if the dict args are (visibly) equal.
+Instead we want to quantify over the dictionaries separately.
+
+In short, tcSimplifyRuleLhs must *only* squash LitInst and MethInts, leaving
+all dicts unchanged, with absolutely no sharing. It's simpler to do this
+from scratch, rather than further parameterise simpleReduceLoop etc.
+Simpler, maybe, but alas not simple (see Trac #2494)
+
+* Type errors may give rise to an (unsatisfiable) equality constraint
+
+* Applications of a higher-rank function on the LHS may give
+ rise to an implication constraint, esp if there are unsatisfiable
+ equality constraints inside.
+
+\begin{code}
+tcSimplifyRuleLhs :: [Inst] -> TcM ([Inst], TcDictBinds)
+tcSimplifyRuleLhs wanteds
+ = do { wanteds' <- zonkInsts wanteds
+
+ -- Simplify equalities
+ -- It's important to do this: Trac #3346 for example
+ ; (_, wanteds'', tybinds, binds1) <- tcReduceEqs [] wanteds'
+ ; execTcTyVarBinds tybinds
+
+ -- Simplify other constraints
+ ; (irreds, binds2) <- go [] emptyBag wanteds''
+
+ -- Report anything that is left
+ ; let (dicts, bad_irreds) = partition isDict irreds
+ ; traceTc (text "tcSimplifyrulelhs" <+> pprInsts bad_irreds)
+ ; addNoInstanceErrs (nub bad_irreds)
+ -- The nub removes duplicates, which has
+ -- not happened otherwise (see notes above)
+
+ ; return (dicts, binds1 `unionBags` binds2) }
+ where
+ go :: [Inst] -> TcDictBinds -> [Inst] -> TcM ([Inst], TcDictBinds)
+ go irreds binds []
+ = return (irreds, binds)
+ go irreds binds (w:ws)
+ | isDict w
+ = go (w:irreds) binds ws
+ | isImplicInst w -- Have a go at reducing the implication
+ = do { (binds1, irreds1) <- reduceImplication red_env w
+ ; let (bad_irreds, ok_irreds) = partition isImplicInst irreds1
+ ; go (bad_irreds ++ irreds)
+ (binds `unionBags` binds1)
+ (ok_irreds ++ ws)}
+ | otherwise
+ = do { w' <- zonkInst w -- So that (3::Int) does not generate a call
+ -- to fromInteger; this looks fragile to me
+ ; lookup_result <- lookupSimpleInst w'
+ ; case lookup_result of
+ NoInstance -> go (w:irreds) binds ws
+ GenInst ws' rhs -> go irreds binds' (ws' ++ ws)
+ where
+ binds' = addInstToDictBind binds w rhs
+ }
+
+ -- Sigh: we need to reduce inside implications
+ red_env = mkInferRedEnv doc try_me
+ doc = ptext (sLit "Implication constraint in RULE lhs")
+ try_me inst | isMethodOrLit inst = ReduceMe
+ | otherwise = Stop -- Be gentle
+\end{code}
+
+tcSimplifyBracket is used when simplifying the constraints arising from
+a Template Haskell bracket [| ... |]. We want to check that there aren't
+any constraints that can't be satisfied (e.g. Show Foo, where Foo has no
+Show instance), but we aren't otherwise interested in the results.
+Nor do we care about ambiguous dictionaries etc. We will type check
+this bracket again at its usage site.
+
+\begin{code}
+tcSimplifyBracket :: [Inst] -> TcM ()
+tcSimplifyBracket wanteds
+ = do { _ <- tryHardCheckLoop doc wanteds
+ ; return () }
+ where
+ doc = text "tcSimplifyBracket"
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{Filtering at a dynamic binding}
+%* *
+%************************************************************************
+
+When we have
+ let ?x = R in B
+
+we must discharge all the ?x constraints from B. We also do an improvement
+step; if we have ?x::t1 and ?x::t2 we must unify t1, t2.
+
+Actually, the constraints from B might improve the types in ?x. For example
+
+ f :: (?x::Int) => Char -> Char
+ let ?x = 3 in f 'c'
+
+then the constraint (?x::Int) arising from the call to f will
+force the binding for ?x to be of type Int.
+
+\begin{code}
+tcSimplifyIPs :: [Inst] -- The implicit parameters bound here
+ -> [Inst] -- Wanted
+ -> TcM TcDictBinds
+ -- We need a loop so that we do improvement, and then
+ -- (next time round) generate a binding to connect the two
+ -- let ?x = e in ?x
+ -- Here the two ?x's have different types, and improvement
+ -- makes them the same.
+
+tcSimplifyIPs given_ips wanteds
+ = do { wanteds' <- zonkInsts wanteds
+ ; given_ips' <- zonkInsts given_ips
+ -- Unusually for checking, we *must* zonk the given_ips
+
+ ; let env = mkRedEnv doc try_me given_ips'
+ ; (improved, tybinds, binds, irreds) <- reduceContext env wanteds'
+ ; execTcTyVarBinds tybinds
+
+ ; if null irreds || not improved then
+ ASSERT( all is_free irreds )
+ do { extendLIEs irreds
+ ; return binds }
+ else do
+ -- If improvement did some unification, we go round again.
+ -- We start again with irreds, not wanteds
+ -- Using an instance decl might have introduced a fresh type
+ -- variable which might have been unified, so we'd get an
+ -- infinite loop if we started again with wanteds!
+ -- See Note [LOOP]
+ { binds1 <- tcSimplifyIPs given_ips' irreds
+ ; return $ binds `unionBags` binds1
+ } }
+ where
+ doc = text "tcSimplifyIPs" <+> ppr given_ips
+ ip_set = mkNameSet (ipNamesOfInsts given_ips)
+ is_free inst = isFreeWrtIPs ip_set inst
+
+ -- Simplify any methods that mention the implicit parameter
+ try_me inst | is_free inst = Stop
+ | otherwise = ReduceMe
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection[binds-for-local-funs]{@bindInstsOfLocalFuns@}
+%* *
+%************************************************************************
+
+When doing a binding group, we may have @Insts@ of local functions.
+For example, we might have...
+\begin{verbatim}
+let f x = x + 1 -- orig local function (overloaded)
+ f.1 = f Int -- two instances of f
+ f.2 = f Float
+ in
+ (f.1 5, f.2 6.7)
+\end{verbatim}
+The point is: we must drop the bindings for @f.1@ and @f.2@ here,
+where @f@ is in scope; those @Insts@ must certainly not be passed
+upwards towards the top-level. If the @Insts@ were binding-ified up
+there, they would have unresolvable references to @f@.
+
+We pass in an @init_lie@ of @Insts@ and a list of locally-bound @Ids@.
+For each method @Inst@ in the @init_lie@ that mentions one of the
+@Ids@, we create a binding. We return the remaining @Insts@ (in an
+@LIE@), as well as the @HsBinds@ generated.
+
+\begin{code}
+bindInstsOfLocalFuns :: [Inst] -> [TcId] -> TcM TcDictBinds
+-- Simlifies only MethodInsts, and generate only bindings of form
+-- fm = f tys dicts
+-- We're careful not to even generate bindings of the form
+-- d1 = d2
+-- You'd think that'd be fine, but it interacts with what is
+-- arguably a bug in Match.tidyEqnInfo (see notes there)
+
+bindInstsOfLocalFuns wanteds local_ids
+ | null overloaded_ids = do
+ -- Common case
+ extendLIEs wanteds
+ return emptyLHsBinds
+
+ | otherwise
+ = do { (irreds, binds) <- gentleInferLoop doc for_me
+ ; extendLIEs not_for_me
+ ; extendLIEs irreds
+ ; return binds }
+ where
+ doc = text "bindInsts" <+> ppr local_ids
+ overloaded_ids = filter is_overloaded local_ids
+ is_overloaded id = isOverloadedTy (idType id)
+ (for_me, not_for_me) = partition (isMethodFor overloaded_set) wanteds
+
+ overloaded_set = mkVarSet overloaded_ids -- There can occasionally be a lot of them
+ -- so it's worth building a set, so that
+ -- lookup (in isMethodFor) is faster
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{Data types for the reduction mechanism}
+%* *
+%************************************************************************
+
+The main control over context reduction is here
+
+\begin{code}
+data RedEnv
+ = RedEnv { red_doc :: SDoc -- The context
+ , red_try_me :: Inst -> WhatToDo
+ , red_improve :: Bool -- True <=> do improvement
+ , red_givens :: [Inst] -- All guaranteed rigid
+ -- Always dicts & equalities
+ -- but see Note [Rigidity]
+
+ , red_stack :: (Int, [Inst]) -- Recursion stack (for err msg)
+ -- See Note [RedStack]
+ }
+
+-- Note [Rigidity]
+-- The red_givens are rigid so far as cmpInst is concerned.
+-- There is one case where they are not totally rigid, namely in tcSimplifyIPs
+-- let ?x = e in ...
+-- Here, the given is (?x::a), where 'a' is not necy a rigid type
+-- But that doesn't affect the comparison, which is based only on mame.
+
+-- Note [RedStack]
+-- The red_stack pair (n,insts) pair is just used for error reporting.
+-- 'n' is always the depth of the stack.
+-- The 'insts' is the stack of Insts being reduced: to produce X
+-- I had to produce Y, to produce Y I had to produce Z, and so on.
+
+
+mkRedEnv :: SDoc -> (Inst -> WhatToDo) -> [Inst] -> RedEnv
+mkRedEnv doc try_me givens
+ = RedEnv { red_doc = doc, red_try_me = try_me,
+ red_givens = givens,
+ red_stack = (0,[]),
+ red_improve = True }
+
+mkInferRedEnv :: SDoc -> (Inst -> WhatToDo) -> RedEnv
+-- No givens at all
+mkInferRedEnv doc try_me
+ = RedEnv { red_doc = doc, red_try_me = try_me,
+ red_givens = [],
+ red_stack = (0,[]),
+ red_improve = True }
+
+mkNoImproveRedEnv :: SDoc -> (Inst -> WhatToDo) -> RedEnv
+-- Do not do improvement; no givens
+mkNoImproveRedEnv doc try_me
+ = RedEnv { red_doc = doc, red_try_me = try_me,
+ red_givens = [],
+ red_stack = (0,[]),
+ red_improve = True }
+
+data WhatToDo
+ = ReduceMe -- Try to reduce this
+ -- If there's no instance, add the inst to the
+ -- irreductible ones, but don't produce an error
+ -- message of any kind.
+ -- It might be quite legitimate such as (Eq a)!
+
+ | Stop -- Return as irreducible unless it can
+ -- be reduced to a constant in one step
+ -- Do not add superclasses; see
+
+data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
+ -- of a predicate when adding it to the avails
+ -- The reason for this flag is entirely the super-class loop problem
+ -- Note [SUPER-CLASS LOOP 1]
+
+zonkRedEnv :: RedEnv -> TcM RedEnv
+zonkRedEnv env
+ = do { givens' <- mapM zonkInst (red_givens env)
+ ; return $ env {red_givens = givens'}
+ }
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection[reduce]{@reduce@}
+%* *
+%************************************************************************
+
+Note [Ancestor Equalities]
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+During context reduction, we add to the wanted equalities also those
+equalities that (transitively) occur in superclass contexts of wanted
+class constraints. Consider the following code
+
+ class a ~ Int => C a
+ instance C Int
+
+If (C a) is wanted, we want to add (a ~ Int), which will be discharged by
+substituting Int for a. Hence, we ultimately want (C Int), which we
+discharge with the explicit instance.
+
+\begin{code}
+reduceContext :: RedEnv
+ -> [Inst] -- Wanted
+ -> TcM (ImprovementDone,
+ TcTyVarBinds, -- Type variable bindings
+ TcDictBinds, -- Dictionary bindings
+ [Inst]) -- Irreducible
+
+reduceContext env wanteds0
+ = do { traceTc (text "reduceContext" <+> (vcat [
+ text "----------------------",
+ red_doc env,
+ text "given" <+> ppr (red_givens env),
+ text "wanted" <+> ppr wanteds0,
+ text "----------------------"
+ ]))
+
+ -- We want to add as wanted equalities those that (transitively)
+ -- occur in superclass contexts of wanted class constraints.
+ -- See Note [Ancestor Equalities]
+ ; ancestor_eqs <- ancestorEqualities wanteds0
+ ; traceTc $ text "reduceContext: ancestor eqs" <+> ppr ancestor_eqs
+
+ -- Normalise and solve all equality constraints as far as possible
+ -- and normalise all dictionary constraints wrt to the reduced
+ -- equalities. The returned wanted constraints include the
+ -- irreducible wanted equalities.
+ ; let wanteds = wanteds0 ++ ancestor_eqs
+ givens = red_givens env
+ ; (givens',
+ wanteds',
+ tybinds,
+ normalise_binds) <- tcReduceEqs givens wanteds
+ ; traceTc $ text "reduceContext: tcReduceEqs result" <+> vcat
+ [ppr givens', ppr wanteds', ppr tybinds,
+ ppr normalise_binds]
+
+ -- Build the Avail mapping from "given_dicts"
+ ; (init_state, _) <- getConstraints $ do
+ { init_state <- foldlM addGiven emptyAvails givens'
+ ; return init_state
+ }
+
+ -- Solve the *wanted* *dictionary* constraints (not implications)
+ -- This may expose some further equational constraints in the course
+ -- of improvement due to functional dependencies if any of the
+ -- involved unifications gets deferred.
+ ; let (wanted_implics, wanted_dicts) = partition isImplicInst wanteds'
+ ; (avails, extra_eqs) <- getConstraints (reduceList env wanted_dicts init_state)
+ -- The getConstraints is reqd because reduceList does improvement
+ -- (via extendAvails) which may in turn do unification
+ ; (dict_binds,
+ bound_dicts,
+ dict_irreds) <- extractResults avails wanted_dicts
+ ; traceTc $ text "reduceContext: extractResults" <+> vcat
+ [ppr avails, ppr wanted_dicts, ppr dict_binds]
+
+ -- Solve the wanted *implications*. In doing so, we can provide
+ -- as "given" all the dicts that were originally given,
+ -- *or* for which we now have bindings,
+ -- *or* which are now irreds
+ -- NB: Equality irreds need to be converted, as the recursive
+ -- invocation of the solver will still treat them as wanteds
+ -- otherwise.
+ ; let implic_env = env { red_givens
+ = givens ++ bound_dicts ++
+ map wantedToLocalEqInst dict_irreds }
+ ; (implic_binds_s, implic_irreds_s)
+ <- mapAndUnzipM (reduceImplication implic_env) wanted_implics
+ ; let implic_binds = unionManyBags implic_binds_s
+ implic_irreds = concat implic_irreds_s
+
+ -- Collect all irreducible instances, and determine whether we should
+ -- go round again. We do so in either of two cases:
+ -- (1) If dictionary reduction or equality solving led to
+ -- improvement (i.e., bindings for type variables).
+ -- (2) If we reduced dictionaries (i.e., got dictionary bindings),
+ -- they may have exposed further opportunities to normalise
+ -- family applications. See Note [Dictionary Improvement]
+ --
+ -- NB: We do *not* go around for new extra_eqs. Morally, we should,
+ -- but we can't without risking non-termination (see #2688). By
+ -- not going around, we miss some legal programs mixing FDs and
+ -- TFs, but we never claimed to support such programs in the
+ -- current implementation anyway.
+
+ ; let all_irreds = dict_irreds ++ implic_irreds ++ extra_eqs
+ avails_improved = availsImproved avails
+ eq_improved = anyBag (not . isCoVarBind) tybinds
+ improvedFlexible = avails_improved || eq_improved
+ reduced_dicts = not (isEmptyBag dict_binds)
+ improved = improvedFlexible || reduced_dicts
+ --
+ improvedHint = (if avails_improved then " [AVAILS]" else "") ++
+ (if eq_improved then " [EQ]" else "")
+
+ ; traceTc (text "reduceContext end" <+> (vcat [
+ text "----------------------",
+ red_doc env,
+ text "given" <+> ppr givens,
+ text "wanted" <+> ppr wanteds0,
+ text "----",
+ text "tybinds" <+> ppr tybinds,
+ text "avails" <+> pprAvails avails,
+ text "improved =" <+> ppr improved <+> text improvedHint,
+ text "(all) irreds = " <+> ppr all_irreds,
+ text "dict-binds = " <+> ppr dict_binds,
+ text "implic-binds = " <+> ppr implic_binds,
+ text "----------------------"
+ ]))
+
+ ; return (improved,
+ tybinds,
+ normalise_binds `unionBags` dict_binds
+ `unionBags` implic_binds,
+ all_irreds)
+ }
+ where
+ isCoVarBind (TcTyVarBind tv _) = isCoVar tv
+
+tcImproveOne :: Avails -> Inst -> TcM ImprovementDone
+tcImproveOne avails inst
+ | not (isDict inst) = return False
+ | otherwise
+ = do { inst_envs <- tcGetInstEnvs
+ ; let eqns = improveOne (classInstances inst_envs)
+ (dictPred inst, pprInstArising inst)
+ [ (dictPred p, pprInstArising p)
+ | p <- availsInsts avails, isDict p ]
+ -- Avails has all the superclasses etc (good)
+ -- It also has all the intermediates of the deduction (good)
+ -- It does not have duplicates (good)
+ -- NB that (?x::t1) and (?x::t2) will be held separately in
+ -- avails so that improve will see them separate
+ ; traceTc (text "improveOne" <+> ppr inst)
+ ; unifyEqns eqns }
+
+unifyEqns :: [(Equation, (PredType, SDoc), (PredType, SDoc))]
+ -> TcM ImprovementDone
+unifyEqns [] = return False
+unifyEqns eqns
+ = do { traceTc (ptext (sLit "Improve:") <+> vcat (map pprEquationDoc eqns))
+ ; improved <- mapM unify eqns
+ ; return $ or improved
+ }
+ where
+ unify ((qtvs, pairs), what1, what2)
+ = addErrCtxtM (mkEqnMsg what1 what2) $
+ do { let freeTyVars = unionVarSets (map tvs_pr pairs)
+ `minusVarSet` qtvs
+ ; (_, _, tenv) <- tcInstTyVars (varSetElems qtvs)
+ ; mapM_ (unif_pr tenv) pairs
+ ; anyM isFilledMetaTyVar $ varSetElems freeTyVars
+ }
+
+ unif_pr tenv (ty1, ty2) = unifyType (substTy tenv ty1) (substTy tenv ty2)
+
+ tvs_pr (ty1, ty2) = tyVarsOfType ty1 `unionVarSet` tyVarsOfType ty2
+
+pprEquationDoc :: (Equation, (PredType, SDoc), (PredType, SDoc)) -> SDoc
+pprEquationDoc (eqn, (p1, _), (p2, _))
+ = vcat [pprEquation eqn, nest 2 (ppr p1), nest 2 (ppr p2)]
+
+mkEqnMsg :: (TcPredType, SDoc) -> (TcPredType, SDoc) -> TidyEnv
+ -> TcM (TidyEnv, SDoc)
+mkEqnMsg (pred1,from1) (pred2,from2) tidy_env
+ = do { pred1' <- zonkTcPredType pred1
+ ; pred2' <- zonkTcPredType pred2
+ ; let { pred1'' = tidyPred tidy_env pred1'
+ ; pred2'' = tidyPred tidy_env pred2' }
+ ; let msg = vcat [ptext (sLit "When using functional dependencies to combine"),
+ nest 2 (sep [ppr pred1'' <> comma, nest 2 from1]),
+ nest 2 (sep [ppr pred2'' <> comma, nest 2 from2])]
+ ; return (tidy_env, msg) }
+\end{code}
+
+Note [Dictionary Improvement]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+In reduceContext, we first reduce equalities and then class constraints.
+However, the letter may expose further opportunities for the former. Hence,
+we need to go around again if dictionary reduction produced any dictionary
+bindings. The following example demonstrated the point:
+
+ data EX _x _y (p :: * -> *)
+ data ANY
+
+ class Base p
+
+ class Base (Def p) => Prop p where
+ type Def p
+
+ instance Base ()
+ instance Prop () where
+ type Def () = ()
+
+ instance (Base (Def (p ANY))) => Base (EX _x _y p)
+ instance (Prop (p ANY)) => Prop (EX _x _y p) where
+ type Def (EX _x _y p) = EX _x _y p
+
+ data FOO x
+ instance Prop (FOO x) where
+ type Def (FOO x) = ()
+
+ data BAR
+ instance Prop BAR where
+ type Def BAR = EX () () FOO
+
+During checking the last instance declaration, we need to check the superclass
+cosntraint Base (Def BAR), which family normalisation reduced to
+Base (EX () () FOO). Chasing the instance for Base (EX _x _y p), gives us
+Base (Def (FOO ANY)), which again requires family normalisation of Def to
+Base () before we can finish.
+
+
+The main context-reduction function is @reduce@. Here's its game plan.
+
+\begin{code}
+reduceList :: RedEnv -> [Inst] -> Avails -> TcM Avails
+reduceList env@(RedEnv {red_stack = (n,stk)}) wanteds state
+ = do { traceTc (text "reduceList " <+> (ppr wanteds $$ ppr state))
+ ; dopts <- getDOpts
+ ; when (debugIsOn && (n > 8)) $ do
+ debugDumpTcRn (hang (ptext (sLit "Interesting! Context reduction stack depth") <+> int n)
+ 2 (ifPprDebug (nest 2 (pprStack stk))))
+ ; if n >= ctxtStkDepth dopts then
+ failWithTc (reduceDepthErr n stk)
+ else
+ go wanteds state }
+ where
+ go [] state = return state
+ go (w:ws) state = do { state' <- reduce (env {red_stack = (n+1, w:stk)}) w state
+ ; go ws state' }
+
+ -- Base case: we're done!
+reduce :: RedEnv -> Inst -> Avails -> TcM Avails
+reduce env wanted avails
+
+ -- We don't reduce equalities here (and they must not end up as irreds
+ -- in the Avails!)
+ | isEqInst wanted
+ = return avails
+
+ -- It's the same as an existing inst, or a superclass thereof
+ | Just _ <- findAvail avails wanted
+ = do { traceTc (text "reduce: found " <+> ppr wanted)
+ ; return avails
+ }
+
+ | otherwise
+ = do { traceTc (text "reduce" <+> ppr wanted $$ ppr avails)
+ ; case red_try_me env wanted of {
+ Stop -> try_simple (addIrred NoSCs);
+ -- See Note [No superclasses for Stop]
+
+ ReduceMe -> do -- It should be reduced
+ { (avails, lookup_result) <- reduceInst env avails wanted
+ ; case lookup_result of
+ NoInstance -> addIrred AddSCs avails wanted
+ -- Add it and its superclasses
+
+ GenInst [] rhs -> addWanted AddSCs avails wanted rhs []
+
+ GenInst wanteds' rhs
+ -> do { avails1 <- addIrred NoSCs avails wanted
+ ; avails2 <- reduceList env wanteds' avails1
+ ; addWanted AddSCs avails2 wanted rhs wanteds' } }
+ -- Temporarily do addIrred *before* the reduceList,
+ -- which has the effect of adding the thing we are trying
+ -- to prove to the database before trying to prove the things it
+ -- needs. See note [RECURSIVE DICTIONARIES]
+ -- NB: we must not do an addWanted before, because that adds the
+ -- superclasses too, and that can lead to a spurious loop; see
+ -- the examples in [SUPERCLASS-LOOP]
+ -- So we do an addIrred before, and then overwrite it afterwards with addWanted
+ } }
+ where
+ -- First, see if the inst can be reduced to a constant in one step
+ -- Works well for literals (1::Int) and constant dictionaries (d::Num Int)
+ -- Don't bother for implication constraints, which take real work
+ try_simple do_this_otherwise
+ = do { res <- lookupSimpleInst wanted
+ ; case res of
+ GenInst [] rhs -> addWanted AddSCs avails wanted rhs []
+ _ -> do_this_otherwise avails wanted }
+\end{code}
+
+
+Note [RECURSIVE DICTIONARIES]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+ data D r = ZeroD | SuccD (r (D r));
+
+ instance (Eq (r (D r))) => Eq (D r) where
+ ZeroD == ZeroD = True
+ (SuccD a) == (SuccD b) = a == b
+ _ == _ = False;
+
+ equalDC :: D [] -> D [] -> Bool;
+ equalDC = (==);
+
+We need to prove (Eq (D [])). Here's how we go:
+
+ d1 : Eq (D [])
+
+by instance decl, holds if
+ d2 : Eq [D []]
+ where d1 = dfEqD d2
+
+by instance decl of Eq, holds if
+ d3 : D []
+ where d2 = dfEqList d3
+ d1 = dfEqD d2
+
+But now we can "tie the knot" to give
+
+ d3 = d1
+ d2 = dfEqList d3
+ d1 = dfEqD d2
+
+and it'll even run! The trick is to put the thing we are trying to prove
+(in this case Eq (D []) into the database before trying to prove its
+contributing clauses.
+
+Note [SUPERCLASS-LOOP 2]
+~~~~~~~~~~~~~~~~~~~~~~~~
+We need to be careful when adding "the constaint we are trying to prove".
+Suppose we are *given* d1:Ord a, and want to deduce (d2:C [a]) where
+
+ class Ord a => C a where
+ instance Ord [a] => C [a] where ...
+
+Then we'll use the instance decl to deduce C [a] from Ord [a], and then add the
+superclasses of C [a] to avails. But we must not overwrite the binding
+for Ord [a] (which is obtained from Ord a) with a superclass selection or we'll just
+build a loop!
+
+Here's another variant, immortalised in tcrun020
+ class Monad m => C1 m
+ class C1 m => C2 m x
+ instance C2 Maybe Bool
+For the instance decl we need to build (C1 Maybe), and it's no good if
+we run around and add (C2 Maybe Bool) and its superclasses to the avails
+before we search for C1 Maybe.
+
+Here's another example
+ class Eq b => Foo a b
+ instance Eq a => Foo [a] a
+If we are reducing
+ (Foo [t] t)
+
+we'll first deduce that it holds (via the instance decl). We must not
+then overwrite the Eq t constraint with a superclass selection!
+
+At first I had a gross hack, whereby I simply did not add superclass constraints
+in addWanted, though I did for addGiven and addIrred. This was sub-optimal,
+becuase it lost legitimate superclass sharing, and it still didn't do the job:
+I found a very obscure program (now tcrun021) in which improvement meant the
+simplifier got two bites a the cherry... so something seemed to be an Stop
+first time, but reducible next time.
+
+Now we implement the Right Solution, which is to check for loops directly
+when adding superclasses. It's a bit like the occurs check in unification.
+
+
+
+%************************************************************************
+%* *
+ Reducing a single constraint
+%* *
+%************************************************************************
+
+\begin{code}
+---------------------------------------------
+reduceInst :: RedEnv -> Avails -> Inst -> TcM (Avails, LookupInstResult)
+reduceInst _ avails other_inst
+ = do { result <- lookupSimpleInst other_inst
+ ; return (avails, result) }
+\end{code}
+
+Note [Equational Constraints in Implication Constraints]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+An implication constraint is of the form
+ Given => Wanted
+where Given and Wanted may contain both equational and dictionary
+constraints. The delay and reduction of these two kinds of constraints
+is distinct:
+
+-) In the generated code, wanted Dictionary constraints are wrapped up in an
+ implication constraint that is created at the code site where the wanted
+ dictionaries can be reduced via a let-binding. This let-bound implication
+ constraint is deconstructed at the use-site of the wanted dictionaries.
+
+-) While the reduction of equational constraints is also delayed, the delay
+ is not manifest in the generated code. The required evidence is generated
+ in the code directly at the use-site. There is no let-binding and deconstruction
+ necessary. The main disadvantage is that we cannot exploit sharing as the
+ same evidence may be generated at multiple use-sites. However, this disadvantage
+ is limited because it only concerns coercions which are erased.
+
+The different treatment is motivated by the different in representation. Dictionary
+constraints require manifest runtime dictionaries, while equations require coercions
+which are types.
+
+\begin{code}
+---------------------------------------------
+reduceImplication :: RedEnv
+ -> Inst
+ -> TcM (TcDictBinds, [Inst])
+\end{code}
+
+Suppose we are simplifying the constraint
+ forall bs. extras => wanted
+in the context of an overall simplification problem with givens 'givens'.
+
+Note that
+ * The 'givens' need not mention any of the quantified type variables
+ e.g. forall {}. Eq a => Eq [a]
+ forall {}. C Int => D (Tree Int)
+
+ This happens when you have something like
+ data T a where
+ T1 :: Eq a => a -> T a
+
+ f :: T a -> Int
+ f x = ...(case x of { T1 v -> v==v })...
+
+\begin{code}
+ -- ToDo: should we instantiate tvs? I think it's not necessary
+ --
+ -- Note on coercion variables:
+ --
+ -- The extra given coercion variables are bound at two different
+ -- sites:
+ --
+ -- -) in the creation context of the implication constraint
+ -- the solved equational constraints use these binders
+ --
+ -- -) at the solving site of the implication constraint
+ -- the solved dictionaries use these binders;
+ -- these binders are generated by reduceImplication
+ --
+ -- Note [Binders for equalities]
+ -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ -- To reuse the binders of local/given equalities in the binders of
+ -- implication constraints, it is crucial that these given equalities
+ -- always have the form
+ -- cotv :: t1 ~ t2
+ -- where cotv is a simple coercion type variable (and not a more
+ -- complex coercion term). We require that the extra_givens always
+ -- have this form and exploit the special form when generating binders.
+reduceImplication env
+ orig_implic@(ImplicInst { tci_name = name, tci_loc = inst_loc,
+ tci_tyvars = tvs,
+ tci_given = extra_givens, tci_wanted = wanteds
+ })
+ = do { -- Solve the sub-problem
+ ; let try_me _ = ReduceMe -- Note [Freeness and implications]
+ env' = env { red_givens = extra_givens ++ red_givens env
+ , red_doc = sep [ptext (sLit "reduceImplication for")
+ <+> ppr name,
+ nest 2 (parens $ ptext (sLit "within")
+ <+> red_doc env)]
+ , red_try_me = try_me }
+
+ ; traceTc (text "reduceImplication" <+> vcat
+ [ ppr (red_givens env), ppr extra_givens,
+ ppr wanteds])
+ ; (irreds, binds) <- checkLoop env' wanteds
+
+ ; traceTc (text "reduceImplication result" <+> vcat
+ [ppr irreds, ppr binds])
+
+ ; -- extract superclass binds
+ -- (sc_binds,_) <- extractResults avails []
+-- ; traceTc (text "reduceImplication sc_binds" <+> vcat
+-- [ppr sc_binds, ppr avails])
+--
+
+ -- SLPJ Sept 07: what if improvement happened inside the checkLoop?
+ -- Then we must iterate the outer loop too!
+
+ ; didntSolveWantedEqs <- allM wantedEqInstIsUnsolved wanteds
+ -- we solve wanted eqs by side effect!
+
+ -- Progress is no longer measered by the number of bindings
+ -- If there are any irreds, but no bindings and no solved
+ -- equalities, we back off and do nothing
+ ; let backOff = isEmptyLHsBinds binds && -- no new bindings
+ (not $ null irreds) && -- but still some irreds
+ didntSolveWantedEqs -- no instantiated cotv
+
+ ; if backOff then -- No progress
+ return (emptyBag, [orig_implic])
+ else do
+ { (simpler_implic_insts, bind)
+ <- makeImplicationBind inst_loc tvs extra_givens irreds
+ -- This binding is useless if the recursive simplification
+ -- made no progress; but currently we don't try to optimise that
+ -- case. After all, we only try hard to reduce at top level, or
+ -- when inferring types.
+
+ ; let -- extract Id binders for dicts and CoTyVar binders for eqs;
+ -- see Note [Binders for equalities]
+ (extra_eq_givens, extra_dict_givens) = partition isEqInst
+ extra_givens
+ eq_cotvs = map instToVar extra_eq_givens
+ dict_ids = map instToId extra_dict_givens
+
+ co = mkWpTyLams tvs
+ <.> mkWpTyLams eq_cotvs
+ <.> mkWpLams dict_ids
+ <.> WpLet (binds `unionBags` bind)
+ rhs = mkLHsWrap co payload
+ loc = instLocSpan inst_loc
+ -- wanted equalities are solved by updating their
+ -- cotv; we don't generate bindings for them
+ dict_bndrs = map (L loc . HsVar . instToId)
+ . filter (not . isEqInst)
+ $ wanteds
+ payload = mkBigLHsTup dict_bndrs
+
+ ; traceTc (vcat [text "reduceImplication" <+> ppr name,
+ ppr simpler_implic_insts,
+ text "->" <+> ppr rhs])
+ ; return (unitBag (L loc (VarBind { var_id= instToId orig_implic
+ , var_rhs = rhs
+ , var_inline = notNull dict_ids }
+ -- See Note [Always inline implication constraints]
+ )),
+ simpler_implic_insts)
+ }
+ }
+reduceImplication _ i = pprPanic "reduceImplication" (ppr i)
+\end{code}
+
+Note [Always inline implication constraints]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Suppose an implication constraint floats out of an INLINE function.
+Then although the implication has a single call site, it won't be
+inlined. And that is bad because it means that even if there is really
+*no* overloading (type signatures specify the exact types) there will
+still be dictionary passing in the resulting code. To avert this,
+we mark the implication constraints themselves as INLINE, at least when
+there is no loss of sharing as a result.
+
+Note [Freeness and implications]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+It's hard to say when an implication constraint can be floated out. Consider
+ forall {} Eq a => Foo [a]
+The (Foo [a]) doesn't mention any of the quantified variables, but it
+still might be partially satisfied by the (Eq a).
+
+There is a useful special case when it *is* easy to partition the
+constraints, namely when there are no 'givens'. Consider
+ forall {a}. () => Bar b
+There are no 'givens', and so there is no reason to capture (Bar b).
+We can let it float out. But if there is even one constraint we
+must be much more careful:
+ forall {a}. C a b => Bar (m b)
+because (C a b) might have a superclass (D b), from which we might
+deduce (Bar [b]) when m later gets instantiated to []. Ha!
+
+Here is an even more exotic example
+ class C a => D a b
+Now consider the constraint
+ forall b. D Int b => C Int
+We can satisfy the (C Int) from the superclass of D, so we don't want
+to float the (C Int) out, even though it mentions no type variable in
+the constraints!
+
+One more example: the constraint
+ class C a => D a b
+ instance (C a, E c) => E (a,c)
+
+ constraint: forall b. D Int b => E (Int,c)
+
+You might think that the (D Int b) can't possibly contribute
+to solving (E (Int,c)), since the latter mentions 'c'. But
+in fact it can, because solving the (E (Int,c)) constraint needs
+dictionaries
+ C Int, E c
+and the (C Int) can be satisfied from the superclass of (D Int b).
+So we must still not float (E (Int,c)) out.
+
+To think about: special cases for unary type classes?
+
+Note [Pruning the givens in an implication constraint]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Suppose we are about to form the implication constraint
+ forall tvs. Eq a => Ord b
+The (Eq a) cannot contribute to the (Ord b), because it has no access to
+the type variable 'b'. So we could filter out the (Eq a) from the givens.
+But BE CAREFUL of the examples above in [Freeness and implications].
+
+Doing so would be a bit tidier, but all the implication constraints get
+simplified away by the optimiser, so it's no great win. So I don't take
+advantage of that at the moment.
+
+If you do, BE CAREFUL of wobbly type variables.
+
+
+%************************************************************************
+%* *
+ Avails and AvailHow: the pool of evidence
+%* *
+%************************************************************************
+
+
+\begin{code}
+data Avails = Avails !ImprovementDone !AvailEnv
+
+type ImprovementDone = Bool -- True <=> some unification has happened
+ -- so some Irreds might now be reducible
+ -- keys that are now
+
+type AvailEnv = FiniteMap Inst AvailHow
+data AvailHow
+ = IsIrred -- Used for irreducible dictionaries,
+ -- which are going to be lambda bound
+
+ | Given Inst -- Used for dictionaries for which we have a binding
+ -- e.g. those "given" in a signature
+
+ | Rhs -- Used when there is a RHS
+ (LHsExpr TcId) -- The RHS
+ [Inst] -- Insts free in the RHS; we need these too
+
+instance Outputable Avails where
+ ppr = pprAvails
+
+pprAvails :: Avails -> SDoc
+pprAvails (Avails imp avails)
+ = vcat [ ptext (sLit "Avails") <> (if imp then ptext (sLit "[improved]") else empty)
+ , nest 2 $ braces $
+ vcat [ sep [ppr inst, nest 2 (equals <+> ppr avail)]
+ | (inst,avail) <- fmToList avails ]]
+
+instance Outputable AvailHow where
+ ppr = pprAvail
+
+-------------------------
+pprAvail :: AvailHow -> SDoc
+pprAvail IsIrred = text "Irred"
+pprAvail (Given x) = text "Given" <+> ppr x
+pprAvail (Rhs rhs bs) = sep [text "Rhs" <+> ppr bs,
+ nest 2 (ppr rhs)]
+
+-------------------------
+extendAvailEnv :: AvailEnv -> Inst -> AvailHow -> AvailEnv
+extendAvailEnv env inst avail = addToFM env inst avail
+
+findAvailEnv :: AvailEnv -> Inst -> Maybe AvailHow
+findAvailEnv env wanted = lookupFM env wanted
+ -- NB 1: the Ord instance of Inst compares by the class/type info
+ -- *not* by unique. So
+ -- d1::C Int == d2::C Int
+
+emptyAvails :: Avails
+emptyAvails = Avails False emptyFM
+
+findAvail :: Avails -> Inst -> Maybe AvailHow
+findAvail (Avails _ avails) wanted = findAvailEnv avails wanted
+
+elemAvails :: Inst -> Avails -> Bool
+elemAvails wanted (Avails _ avails) = wanted `elemFM` avails
+
+extendAvails :: Avails -> Inst -> AvailHow -> TcM Avails
+-- Does improvement
+extendAvails avails@(Avails imp env) inst avail
+ = do { imp1 <- tcImproveOne avails inst -- Do any improvement
+ ; return (Avails (imp || imp1) (extendAvailEnv env inst avail)) }
+
+availsInsts :: Avails -> [Inst]
+availsInsts (Avails _ avails) = keysFM avails
+
+availsImproved :: Avails -> ImprovementDone
+availsImproved (Avails imp _) = imp
+\end{code}
+
+Extracting the bindings from a bunch of Avails.
+The bindings do *not* come back sorted in dependency order.
+We assume that they'll be wrapped in a big Rec, so that the
+dependency analyser can sort them out later
+
+\begin{code}
+type DoneEnv = FiniteMap Inst [Id]
+-- Tracks which things we have evidence for
+
+extractResults :: Avails
+ -> [Inst] -- Wanted
+ -> TcM (TcDictBinds, -- Bindings
+ [Inst], -- The insts bound by the bindings
+ [Inst]) -- Irreducible ones
+ -- Note [Reducing implication constraints]
+
+extractResults (Avails _ avails) wanteds
+ = go emptyBag [] [] emptyFM wanteds
+ where
+ go :: TcDictBinds -- Bindings for dicts
+ -> [Inst] -- Bound by the bindings
+ -> [Inst] -- Irreds
+ -> DoneEnv -- Has an entry for each inst in the above three sets
+ -> [Inst] -- Wanted
+ -> TcM (TcDictBinds, [Inst], [Inst])
+ go binds bound_dicts irreds _ []
+ = return (binds, bound_dicts, irreds)
+
+ go binds bound_dicts irreds done (w:ws)
+ | isEqInst w
+ = go binds bound_dicts (w:irreds) done' ws
+
+ | Just done_ids@(done_id : rest_done_ids) <- lookupFM done w
+ = if w_id `elem` done_ids then
+ go binds bound_dicts irreds done ws
+ else
+ go (add_bind (nlHsVar done_id)) bound_dicts irreds
+ (addToFM done w (done_id : w_id : rest_done_ids)) ws
+
+ | otherwise -- Not yet done
+ = case findAvailEnv avails w of
+ Nothing -> pprTrace "Urk: extractResults" (ppr w) $
+ go binds bound_dicts irreds done ws
+
+ Just IsIrred -> go binds bound_dicts (w:irreds) done' ws
+
+ Just (Rhs rhs ws') -> go (add_bind rhs) (w:bound_dicts) irreds done' (ws' ++ ws)
+
+ Just (Given g) -> go binds' bound_dicts irreds (addToFM done w [g_id]) ws
+ where
+ g_id = instToId g
+ binds' | w_id == g_id = binds
+ | otherwise = add_bind (nlHsVar g_id)
+ where
+ w_id = instToId w
+ done' = addToFM done w [w_id]
+ add_bind rhs = addInstToDictBind binds w rhs
+\end{code}
+
+
+Note [No superclasses for Stop]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When we decide not to reduce an Inst -- the 'WhatToDo' --- we still
+add it to avails, so that any other equal Insts will be commoned up
+right here. However, we do *not* add superclasses. If we have
+ df::Floating a
+ dn::Num a
+but a is not bound here, then we *don't* want to derive dn from df
+here lest we lose sharing.
+
+\begin{code}
+addWanted :: WantSCs -> Avails -> Inst -> LHsExpr TcId -> [Inst] -> TcM Avails
+addWanted want_scs avails wanted rhs_expr wanteds
+ = addAvailAndSCs want_scs avails wanted avail
+ where
+ avail = Rhs rhs_expr wanteds
+
+addGiven :: Avails -> Inst -> TcM Avails
+addGiven avails given
+ = addAvailAndSCs want_scs avails given (Given given)
+ where
+ want_scs = case instLocOrigin (instLoc given) of
+ NoScOrigin -> NoSCs
+ _other -> AddSCs
+ -- Conditionally add superclasses for 'given'
+ -- See Note [Recursive instances and superclases]
+
+ -- No ASSERT( not (given `elemAvails` avails) ) because in an
+ -- instance decl for Ord t we can add both Ord t and Eq t as
+ -- 'givens', so the assert isn't true
+\end{code}
+
+\begin{code}
+addIrred :: WantSCs -> Avails -> Inst -> TcM Avails
+addIrred want_scs avails irred = ASSERT2( not (irred `elemAvails` avails), ppr irred $$ ppr avails )
+ addAvailAndSCs want_scs avails irred IsIrred
+
+addAvailAndSCs :: WantSCs -> Avails -> Inst -> AvailHow -> TcM Avails
+addAvailAndSCs want_scs avails inst avail
+ | not (isClassDict inst) = extendAvails avails inst avail
+ | NoSCs <- want_scs = extendAvails avails inst avail
+ | otherwise = do { traceTc (text "addAvailAndSCs" <+> vcat [ppr inst, ppr deps])
+ ; avails' <- extendAvails avails inst avail
+ ; addSCs is_loop avails' inst }
+ where
+ is_loop pred = any (`tcEqType` mkPredTy pred) dep_tys
+ -- Note: this compares by *type*, not by Unique
+ deps = findAllDeps (unitVarSet (instToVar inst)) avail
+ dep_tys = map idType (varSetElems deps)
+
+ findAllDeps :: IdSet -> AvailHow -> IdSet
+ -- Find all the Insts that this one depends on
+ -- See Note [SUPERCLASS-LOOP 2]
+ -- Watch out, though. Since the avails may contain loops
+ -- (see Note [RECURSIVE DICTIONARIES]), so we need to track the ones we've seen so far
+ findAllDeps so_far (Rhs _ kids) = foldl find_all so_far kids
+ findAllDeps so_far _ = so_far
+
+ find_all :: IdSet -> Inst -> IdSet
+ find_all so_far kid
+ | isEqInst kid = so_far
+ | kid_id `elemVarSet` so_far = so_far
+ | Just avail <- findAvail avails kid = findAllDeps so_far' avail
+ | otherwise = so_far'
+ where
+ so_far' = extendVarSet so_far kid_id -- Add the new kid to so_far
+ kid_id = instToId kid
+
+addSCs :: (TcPredType -> Bool) -> Avails -> Inst -> TcM Avails
+ -- Add all the superclasses of the Inst to Avails
+ -- The first param says "don't do this because the original thing
+ -- depends on this one, so you'd build a loop"
+ -- Invariant: the Inst is already in Avails.
+
+addSCs is_loop avails dict
+ = ASSERT( isDict dict )
+ do { sc_dicts <- newCtGivens (instLoc dict) sc_theta'
+ ; foldlM add_sc avails (zipEqual "add_scs" sc_dicts sc_sels) }
+ where
+ (clas, tys) = getDictClassTys dict
+ (tyvars, sc_theta, sc_sels, _) = classBigSig clas
+ sc_theta' = filter (not . isEqPred) $
+ substTheta (zipTopTvSubst tyvars tys) sc_theta
+
+ add_sc avails (sc_dict, sc_sel)
+ | is_loop (dictPred sc_dict) = return avails -- See Note [SUPERCLASS-LOOP 2]
+ | is_given sc_dict = return avails
+ | otherwise = do { avails' <- extendAvails avails sc_dict (Rhs sc_sel_rhs [dict])
+ ; addSCs is_loop avails' sc_dict }
+ where
+ sc_sel_rhs = L (instSpan dict) (HsWrap co_fn (HsVar sc_sel))
+ co_fn = WpApp (instToVar dict) <.> mkWpTyApps tys
+
+ is_given :: Inst -> Bool
+ is_given sc_dict = case findAvail avails sc_dict of
+ Just (Given _) -> True -- Given is cheaper than superclass selection
+ _ -> False
+
+-- From the a set of insts obtain all equalities that (transitively) occur in
+-- superclass contexts of class constraints (aka the ancestor equalities).
+--
+ancestorEqualities :: [Inst] -> TcM [Inst]
+ancestorEqualities
+ = mapM mkWantedEqInst -- turn only equality predicates..
+ . filter isEqPred -- ..into wanted equality insts
+ . bagToList
+ . addAEsToBag emptyBag -- collect the superclass constraints..
+ . map dictPred -- ..of all predicates in a bag
+ . filter isClassDict
+ where
+ addAEsToBag :: Bag PredType -> [PredType] -> Bag PredType
+ addAEsToBag bag [] = bag
+ addAEsToBag bag (pred:preds)
+ | pred `elemBag` bag = addAEsToBag bag preds
+ | isEqPred pred = addAEsToBag bagWithPred preds
+ | isClassPred pred = addAEsToBag bagWithPred predsWithSCs
+ | otherwise = addAEsToBag bag preds
+ where
+ bagWithPred = bag `snocBag` pred
+ predsWithSCs = preds ++ substTheta (zipTopTvSubst tyvars tys) sc_theta
+ --
+ (tyvars, sc_theta, _, _) = classBigSig clas
+ (clas, tys) = getClassPredTys pred
+\end{code}
+
+
+%************************************************************************
+%* *
+\section{tcSimplifyTop: defaulting}
+%* *
+%************************************************************************
+
+
+@tcSimplifyTop@ is called once per module to simplify all the constant
+and ambiguous Insts.
+
+We need to be careful of one case. Suppose we have
+
+ instance Num a => Num (Foo a b) where ...
+
+and @tcSimplifyTop@ is given a constraint (Num (Foo x y)). Then it'll simplify
+to (Num x), and default x to Int. But what about y??
+
+It's OK: the final zonking stage should zap y to (), which is fine.
+
+
+\begin{code}
+tcSimplifyTop, tcSimplifyInteractive :: [Inst] -> TcM TcDictBinds
+tcSimplifyTop wanteds
+ = tc_simplify_top doc False wanteds
+ where
+ doc = text "tcSimplifyTop"
+
+tcSimplifyInteractive wanteds
+ = tc_simplify_top doc True wanteds
+ where
+ doc = text "tcSimplifyInteractive"
+
+-- The TcLclEnv should be valid here, solely to improve
+-- error message generation for the monomorphism restriction
+tc_simplify_top :: SDoc -> Bool -> [Inst] -> TcM (Bag (LHsBind TcId))
+tc_simplify_top doc interactive wanteds
+ = do { dflags <- getDOpts
+ ; wanteds <- zonkInsts wanteds
+ ; mapM_ zonkTopTyVar (varSetElems (tyVarsOfInsts wanteds))
+
+ ; traceTc (text "tc_simplify_top 0: " <+> ppr wanteds)
+ ; (irreds1, binds1) <- tryHardCheckLoop doc1 wanteds
+-- ; (irreds1, binds1) <- gentleInferLoop doc1 wanteds
+ ; traceTc (text "tc_simplify_top 1: " <+> ppr irreds1)
+ ; (irreds2, binds2) <- approximateImplications doc2 (\_ -> True) irreds1
+ ; traceTc (text "tc_simplify_top 2: " <+> ppr irreds2)
+
+ -- Use the defaulting rules to do extra unification
+ -- NB: irreds2 are already zonked
+ ; (irreds3, binds3) <- disambiguate doc3 interactive dflags irreds2
+
+ -- Deal with implicit parameters
+ ; let (bad_ips, non_ips) = partition isIPDict irreds3
+ (ambigs, others) = partition isTyVarDict non_ips
+
+ ; topIPErrs bad_ips -- Can arise from f :: Int -> Int
+ -- f x = x + ?y
+ ; addNoInstanceErrs others
+ ; addTopAmbigErrs ambigs
+
+ ; return (binds1 `unionBags` binds2 `unionBags` binds3) }
+ where
+ doc1 = doc <+> ptext (sLit "(first round)")
+ doc2 = doc <+> ptext (sLit "(approximate)")
+ doc3 = doc <+> ptext (sLit "(disambiguate)")
+\end{code}
+
+If a dictionary constrains a type variable which is
+ * not mentioned in the environment
+ * and not mentioned in the type of the expression
+then it is ambiguous. No further information will arise to instantiate
+the type variable; nor will it be generalised and turned into an extra
+parameter to a function.
+
+It is an error for this to occur, except that Haskell provided for
+certain rules to be applied in the special case of numeric types.
+Specifically, if
+ * at least one of its classes is a numeric class, and
+ * all of its classes are numeric or standard
+then the type variable can be defaulted to the first type in the
+default-type list which is an instance of all the offending classes.
+
+So here is the function which does the work. It takes the ambiguous
+dictionaries and either resolves them (producing bindings) or
+complains. It works by splitting the dictionary list by type
+variable, and using @disambigOne@ to do the real business.
+
+@disambigOne@ assumes that its arguments dictionaries constrain all
+the same type variable.
+
+ADR Comment 20/6/94: I've changed the @CReturnable@ case to default to
+@()@ instead of @Int@. I reckon this is the Right Thing to do since
+the most common use of defaulting is code like:
+\begin{verbatim}
+ _ccall_ foo `seqPrimIO` bar
+\end{verbatim}
+Since we're not using the result of @foo@, the result if (presumably)
+@void@.
+
+\begin{code}
+disambiguate :: SDoc -> Bool -> DynFlags -> [Inst] -> TcM ([Inst], TcDictBinds)
+ -- Just does unification to fix the default types
+ -- The Insts are assumed to be pre-zonked
+disambiguate doc interactive dflags insts
+ | null insts
+ = return (insts, emptyBag)
+
+ | null defaultable_groups
+ = do { traceTc (text "disambigutate, no defaultable groups" <+> vcat [ppr unaries, ppr insts, ppr bad_tvs, ppr defaultable_groups])
+ ; return (insts, emptyBag) }
+
+ | otherwise
+ = do { -- Figure out what default types to use
+ default_tys <- getDefaultTys extended_defaulting ovl_strings
+
+ ; traceTc (text "disambiguate1" <+> vcat [ppr insts, ppr unaries, ppr bad_tvs, ppr defaultable_groups])
+ ; mapM_ (disambigGroup default_tys) defaultable_groups
+
+ -- disambigGroup does unification, hence try again
+ ; tryHardCheckLoop doc insts }
+
+ where
+ extended_defaulting = interactive || dopt Opt_ExtendedDefaultRules dflags
+ -- See also Trac #1974
+ ovl_strings = dopt Opt_OverloadedStrings dflags
+
+ unaries :: [(Inst, Class, TcTyVar)] -- (C tv) constraints
+ bad_tvs :: TcTyVarSet -- Tyvars mentioned by *other* constraints
+ (unaries, bad_tvs_s) = partitionWith find_unary insts
+ bad_tvs = unionVarSets bad_tvs_s
+
+ -- Finds unary type-class constraints
+ find_unary d@(Dict {tci_pred = ClassP cls [ty]})
+ | Just tv <- tcGetTyVar_maybe ty = Left (d,cls,tv)
+ find_unary inst = Right (tyVarsOfInst inst)
+
+ -- Group by type variable
+ defaultable_groups :: [[(Inst,Class,TcTyVar)]]
+ defaultable_groups = filter defaultable_group (equivClasses cmp_tv unaries)
+ cmp_tv (_,_,tv1) (_,_,tv2) = tv1 `compare` tv2
+
+ defaultable_group :: [(Inst,Class,TcTyVar)] -> Bool
+ defaultable_group ds@((_,_,tv):_)
+ = isTyConableTyVar tv -- Note [Avoiding spurious errors]
+ && not (tv `elemVarSet` bad_tvs)
+ && defaultable_classes [c | (_,c,_) <- ds]
+ defaultable_group [] = panic "defaultable_group"
+
+ defaultable_classes clss
+ | extended_defaulting = any isInteractiveClass clss
+ | otherwise = all is_std_class clss && (any is_num_class clss)
+
+ -- In interactive mode, or with -XExtendedDefaultRules,
+ -- we default Show a to Show () to avoid graututious errors on "show []"
+ isInteractiveClass cls
+ = is_num_class cls || (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
+
+ is_num_class cls = isNumericClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
+ -- is_num_class adds IsString to the standard numeric classes,
+ -- when -foverloaded-strings is enabled
+
+ is_std_class cls = isStandardClass cls || (ovl_strings && (cls `hasKey` isStringClassKey))
+ -- Similarly is_std_class
+
+-----------------------
+disambigGroup :: [Type] -- The default types
+ -> [(Inst,Class,TcTyVar)] -- All standard classes of form (C a)
+ -> TcM () -- Just does unification, to fix the default types
+
+disambigGroup default_tys dicts
+ = do { mb_chosen_ty <- try_default default_tys
+ ; case mb_chosen_ty of
+ Nothing -> return ()
+ Just chosen_ty -> do { _ <- unifyType chosen_ty (mkTyVarTy tyvar)
+ ; warnDefault dicts chosen_ty } }
+ where
+ (_,_,tyvar) = ASSERT(not (null dicts)) head dicts -- Should be non-empty
+ classes = [c | (_,c,_) <- dicts]
+
+ try_default [] = return Nothing
+ try_default (default_ty : default_tys)
+ = tryTcLIE_ (try_default default_tys) $
+ do { tcSimplifyDefault [mkClassPred clas [default_ty] | clas <- classes]
+ -- This may fail; then the tryTcLIE_ kicks in
+ -- Failure here is caused by there being no type in the
+ -- default list which can satisfy all the ambiguous classes.
+ -- For example, if Real a is reqd, but the only type in the
+ -- default list is Int.
+
+ ; return (Just default_ty) -- TOMDO: do something with the coercion
+ }
+
+
+-----------------------
+getDefaultTys :: Bool -> Bool -> TcM [Type]
+getDefaultTys extended_deflts ovl_strings
+ = do { mb_defaults <- getDeclaredDefaultTys
+ ; case mb_defaults of {
+ Just tys -> return tys ; -- User-supplied defaults
+ Nothing -> do
+
+ -- No use-supplied default
+ -- Use [Integer, Double], plus modifications
+ { integer_ty <- tcMetaTy integerTyConName
+ ; checkWiredInTyCon doubleTyCon
+ ; string_ty <- tcMetaTy stringTyConName
+ ; return (opt_deflt extended_deflts unitTy
+ -- Note [Default unitTy]
+ ++
+ [integer_ty,doubleTy]
+ ++
+ opt_deflt ovl_strings string_ty) } } }
+ where
+ opt_deflt True ty = [ty]
+ opt_deflt False _ = []
+\end{code}
+
+Note [Default unitTy]
+~~~~~~~~~~~~~~~~~~~~~
+In interative mode (or with -XExtendedDefaultRules) we add () as the first type we
+try when defaulting. This has very little real impact, except in the following case.
+Consider:
+ Text.Printf.printf "hello"
+This has type (forall a. IO a); it prints "hello", and returns 'undefined'. We don't
+want the GHCi repl loop to try to print that 'undefined'. The neatest thing is to
+default the 'a' to (), rather than to Integer (which is what would otherwise happen;
+and then GHCi doesn't attempt to print the (). So in interactive mode, we add
+() to the list of defaulting types. See Trac #1200.
+
+Note [Avoiding spurious errors]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When doing the unification for defaulting, we check for skolem
+type variables, and simply don't default them. For example:
+ f = (*) -- Monomorphic
+ g :: Num a => a -> a
+ g x = f x x
+Here, we get a complaint when checking the type signature for g,
+that g isn't polymorphic enough; but then we get another one when
+dealing with the (Num a) context arising from f's definition;
+we try to unify a with Int (to default it), but find that it's
+already been unified with the rigid variable from g's type sig
+
+
+%************************************************************************
+%* *
+\subsection[simple]{@Simple@ versions}
+%* *
+%************************************************************************
+
+Much simpler versions when there are no bindings to make!
+
+@tcSimplifyThetas@ simplifies class-type constraints formed by
+@deriving@ declarations and when specialising instances. We are
+only interested in the simplified bunch of class/type constraints.
+
+It simplifies to constraints of the form (C a b c) where
+a,b,c are type variables. This is required for the context of
+instance declarations.
+
+\begin{code}
+tcSimplifyDeriv :: InstOrigin
+ -> [TyVar]
+ -> ThetaType -- Wanted
+ -> TcM ThetaType -- Needed
+-- Given instance (wanted) => C inst_ty
+-- Simplify 'wanted' as much as possible
+
+tcSimplifyDeriv orig tyvars theta
+ = do { (tvs, _, tenv) <- tcInstTyVars tyvars
+ -- The main loop may do unification, and that may crash if
+ -- it doesn't see a TcTyVar, so we have to instantiate. Sigh
+ -- ToDo: what if two of them do get unified?
+ ; wanteds <- newCtGivensO orig (substTheta tenv theta)
+ ; (irreds, _) <- tryHardCheckLoop doc wanteds
+
+ ; let (tv_dicts, others) = partition ok irreds
+ (tidy_env, tidy_insts) = tidyInsts others
+ ; reportNoInstances tidy_env Nothing [alt_fix] tidy_insts
+ -- See Note [Exotic derived instance contexts] in TcMType
+
+ ; let rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars)
+ simpl_theta = substTheta rev_env (map dictPred tv_dicts)
+ -- This reverse-mapping is a pain, but the result
+ -- should mention the original TyVars not TcTyVars
+
+ ; return simpl_theta }
+ where
+ doc = ptext (sLit "deriving classes for a data type")
+
+ ok dict | isDict dict = validDerivPred (dictPred dict)
+ | otherwise = False
+ alt_fix = vcat [ptext (sLit "use a standalone 'deriving instance' declaration instead,"),
+ ptext (sLit "so you can specify the instance context yourself")]
+\end{code}
+
+
+@tcSimplifyDefault@ just checks class-type constraints, essentially;
+used with \tr{default} declarations. We are only interested in
+whether it worked or not.
+
+\begin{code}
+tcSimplifyDefault :: ThetaType -- Wanted; has no type variables in it
+ -> TcM ()
+
+tcSimplifyDefault theta = do
+ wanteds <- newCtGivensO DefaultOrigin theta
+ (irreds, _) <- tryHardCheckLoop doc wanteds
+ addNoInstanceErrs irreds
+ if null irreds then
+ return ()
+ else
+ traceTc (ptext (sLit "tcSimplifyDefault failing")) >> failM
+ where
+ doc = ptext (sLit "default declaration")
+\end{code}
+
+
+
+%************************************************************************
+%* *
+\section{Errors and contexts}
+%* *
+%************************************************************************
+
+ToDo: for these error messages, should we note the location as coming
+from the insts, or just whatever seems to be around in the monad just
+now?
+
+\begin{code}
+groupErrs :: ([Inst] -> TcM ()) -- Deal with one group
+ -> [Inst] -- The offending Insts
+ -> TcM ()
+-- Group together insts with the same origin
+-- We want to report them together in error messages
+
+groupErrs _ []
+ = return ()
+groupErrs report_err (inst:insts)
+ = do { do_one (inst:friends)
+ ; groupErrs report_err others }
+ where
+ -- (It may seem a bit crude to compare the error messages,
+ -- but it makes sure that we combine just what the user sees,
+ -- and it avoids need equality on InstLocs.)
+ (friends, others) = partition is_friend insts
+ loc_msg = showSDoc (pprInstLoc (instLoc inst))
+ is_friend friend = showSDoc (pprInstLoc (instLoc friend)) == loc_msg
+ do_one insts = setInstCtxt (instLoc (head insts)) (report_err insts)
+ -- Add location and context information derived from the Insts
+
+-- Add the "arising from..." part to a message about bunch of dicts
+addInstLoc :: [Inst] -> Message -> Message
+addInstLoc insts msg = msg $$ nest 2 (pprInstArising (head insts))
+
+addTopIPErrs :: [Name] -> [Inst] -> TcM ()
+addTopIPErrs _ []
+ = return ()
+addTopIPErrs bndrs ips
+ = do { dflags <- getDOpts
+ ; addErrTcM (tidy_env, mk_msg dflags tidy_ips) }
+ where
+ (tidy_env, tidy_ips) = tidyInsts ips
+ mk_msg dflags ips
+ = vcat [sep [ptext (sLit "Implicit parameters escape from"),
+ nest 2 (ptext (sLit "the monomorphic top-level binding")
+ <> plural bndrs <+> ptext (sLit "of")
+ <+> pprBinders bndrs <> colon)],
+ nest 2 (vcat (map ppr_ip ips)),
+ monomorphism_fix dflags]
+ ppr_ip ip = pprPred (dictPred ip) <+> pprInstArising ip
+
+topIPErrs :: [Inst] -> TcM ()
+topIPErrs dicts
+ = groupErrs report tidy_dicts
+ where
+ (tidy_env, tidy_dicts) = tidyInsts dicts
+ report dicts = addErrTcM (tidy_env, mk_msg dicts)
+ mk_msg dicts = addInstLoc dicts (ptext (sLit "Unbound implicit parameter") <>
+ plural tidy_dicts <+> pprDictsTheta tidy_dicts)
+
+addNoInstanceErrs :: [Inst] -- Wanted (can include implications)
+ -> TcM ()
+addNoInstanceErrs insts
+ = do { let (tidy_env, tidy_insts) = tidyInsts insts
+ ; reportNoInstances tidy_env Nothing [] tidy_insts }
+
+reportNoInstances
+ :: TidyEnv
+ -> Maybe (InstLoc, [Inst]) -- Context
+ -- Nothing => top level
+ -- Just (d,g) => d describes the construct
+ -- with givens g
+ -> [SDoc] -- Alternative fix for no-such-instance
+ -> [Inst] -- What is wanted (can include implications)
+ -> TcM ()
+
+reportNoInstances tidy_env mb_what alt_fix insts
+ = groupErrs (report_no_instances tidy_env mb_what alt_fix) insts
+
+report_no_instances :: TidyEnv -> Maybe (InstLoc, [Inst]) -> [SDoc] -> [Inst] -> TcM ()
+report_no_instances tidy_env mb_what alt_fixes insts
+ = do { inst_envs <- tcGetInstEnvs
+ ; let (implics, insts1) = partition isImplicInst insts
+ (insts2, overlaps) = partitionWith (check_overlap inst_envs) insts1
+ (eqInsts, insts3) = partition isEqInst insts2
+ ; traceTc (text "reportNoInstances" <+> vcat
+ [ppr insts, ppr implics, ppr insts1, ppr insts2])
+ ; mapM_ complain_implic implics
+ ; mapM_ (\doc -> addErrTcM (tidy_env, doc)) overlaps
+ ; groupErrs complain_no_inst insts3
+ ; mapM_ (addErrTcM . mk_eq_err) eqInsts
+ }
+ where
+ complain_no_inst insts = addErrTcM (tidy_env, mk_no_inst_err insts)
+
+ complain_implic inst -- Recurse!
+ = reportNoInstances tidy_env
+ (Just (tci_loc inst, tci_given inst))
+ alt_fixes (tci_wanted inst)
+
+ check_overlap :: (InstEnv,InstEnv) -> Inst -> Either Inst SDoc
+ -- Right msg => overlap message
+ -- Left inst => no instance
+ check_overlap inst_envs wanted
+ | not (isClassDict wanted) = Left wanted
+ | otherwise
+ = case lookupInstEnv inst_envs clas tys of
+ ([], _) -> Left wanted -- No match
+ -- The case of exactly one match and no unifiers means a
+ -- successful lookup. That can't happen here, because dicts
+ -- only end up here if they didn't match in Inst.lookupInst
+ ([_],[])
+ | debugIsOn -> pprPanic "reportNoInstance" (ppr wanted)
+ res -> Right (mk_overlap_msg wanted res)
+ where
+ (clas,tys) = getDictClassTys wanted
+
+ mk_overlap_msg dict (matches, unifiers)
+ = ASSERT( not (null matches) )
+ vcat [ addInstLoc [dict] ((ptext (sLit "Overlapping instances for")
+ <+> pprPred (dictPred dict))),
+ sep [ptext (sLit "Matching instances") <> colon,
+ nest 2 (vcat [pprInstances ispecs, pprInstances unifiers])],
+ if not (isSingleton matches)
+ then -- Two or more matches
+ empty
+ else -- One match, plus some unifiers
+ ASSERT( not (null unifiers) )
+ parens (vcat [ptext (sLit "The choice depends on the instantiation of") <+>
+ quotes (pprWithCommas ppr (varSetElems (tyVarsOfInst dict))),
+ ptext (sLit "To pick the first instance above, use -XIncoherentInstances"),
+ ptext (sLit "when compiling the other instance declarations")])]
+ where
+ ispecs = [ispec | (ispec, _) <- matches]
+
+ mk_eq_err :: Inst -> (TidyEnv, SDoc)
+ mk_eq_err inst = misMatchMsg tidy_env (eqInstTys inst)
+
+ mk_no_inst_err insts
+ | null insts = empty
+
+ | Just (loc, givens) <- mb_what, -- Nested (type signatures, instance decls)
+ not (isEmptyVarSet (tyVarsOfInsts insts))
+ = vcat [ addInstLoc insts $
+ sep [ ptext (sLit "Could not deduce") <+> pprDictsTheta insts
+ , nest 2 $ ptext (sLit "from the context") <+> pprDictsTheta givens]
+ , show_fixes (fix1 loc : fixes2 ++ alt_fixes) ]
+
+ | otherwise -- Top level
+ = vcat [ addInstLoc insts $
+ ptext (sLit "No instance") <> plural insts
+ <+> ptext (sLit "for") <+> pprDictsTheta insts
+ , show_fixes (fixes2 ++ alt_fixes) ]
+
+ where
+ fix1 loc = sep [ ptext (sLit "add") <+> pprDictsTheta insts
+ <+> ptext (sLit "to the context of"),
+ nest 2 (ppr (instLocOrigin loc)) ]
+ -- I'm not sure it helps to add the location
+ -- nest 2 (ptext (sLit "at") <+> ppr (instLocSpan loc)) ]
+
+ fixes2 | null instance_dicts = []
+ | otherwise = [sep [ptext (sLit "add an instance declaration for"),
+ pprDictsTheta instance_dicts]]
+ instance_dicts = [d | d <- insts, isClassDict d, not (isTyVarDict d)]
+ -- Insts for which it is worth suggesting an adding an instance declaration
+ -- Exclude implicit parameters, and tyvar dicts
+
+ show_fixes :: [SDoc] -> SDoc
+ show_fixes [] = empty
+ show_fixes (f:fs) = sep [ptext (sLit "Possible fix:"),
+ nest 2 (vcat (f : map (ptext (sLit "or") <+>) fs))]
+
+addTopAmbigErrs :: [Inst] -> TcRn ()
+addTopAmbigErrs dicts
+-- Divide into groups that share a common set of ambiguous tyvars
+ = ifErrsM (return ()) $ -- Only report ambiguity if no other errors happened
+ -- See Note [Avoiding spurious errors]
+ mapM_ report (equivClasses cmp [(d, tvs_of d) | d <- tidy_dicts])
+ where
+ (tidy_env, tidy_dicts) = tidyInsts dicts
+
+ tvs_of :: Inst -> [TcTyVar]
+ tvs_of d = varSetElems (tyVarsOfInst d)
+ cmp (_,tvs1) (_,tvs2) = tvs1 `compare` tvs2
+
+ report :: [(Inst,[TcTyVar])] -> TcM ()
+ report pairs@((inst,tvs) : _) = do -- The pairs share a common set of ambiguous tyvars
+ (tidy_env, mono_msg) <- mkMonomorphismMsg tidy_env tvs
+ setSrcSpan (instSpan inst) $
+ -- the location of the first one will do for the err message
+ addErrTcM (tidy_env, msg $$ mono_msg)
+ where
+ dicts = map fst pairs
+ msg = sep [text "Ambiguous type variable" <> plural tvs <+>
+ pprQuotedList tvs <+> in_msg,
+ nest 2 (pprDictsInFull dicts)]
+ in_msg = text "in the constraint" <> plural dicts <> colon
+ report [] = panic "addTopAmbigErrs"
+
+
+mkMonomorphismMsg :: TidyEnv -> [TcTyVar] -> TcM (TidyEnv, Message)
+-- There's an error with these Insts; if they have free type variables
+-- it's probably caused by the monomorphism restriction.
+-- Try to identify the offending variable
+-- ASSUMPTION: the Insts are fully zonked
+mkMonomorphismMsg tidy_env inst_tvs
+ = do { dflags <- getDOpts
+ ; (tidy_env, docs) <- findGlobals (mkVarSet inst_tvs) tidy_env
+ ; return (tidy_env, mk_msg dflags docs) }
+ where
+ mk_msg _ _ | any isRuntimeUnk inst_tvs
+ = vcat [ptext (sLit "Cannot resolve unknown runtime types:") <+>
+ (pprWithCommas ppr inst_tvs),
+ ptext (sLit "Use :print or :force to determine these types")]
+ mk_msg _ [] = ptext (sLit "Probable fix: add a type signature that fixes these type variable(s)")
+ -- This happens in things like
+ -- f x = show (read "foo")
+ -- where monomorphism doesn't play any role
+ mk_msg dflags docs
+ = vcat [ptext (sLit "Possible cause: the monomorphism restriction applied to the following:"),
+ nest 2 (vcat docs),
+ monomorphism_fix dflags]
+
+monomorphism_fix :: DynFlags -> SDoc
+monomorphism_fix dflags
+ = ptext (sLit "Probable fix:") <+> vcat
+ [ptext (sLit "give these definition(s) an explicit type signature"),
+ if dopt Opt_MonomorphismRestriction dflags
+ then ptext (sLit "or use -XNoMonomorphismRestriction")
+ else empty] -- Only suggest adding "-XNoMonomorphismRestriction"
+ -- if it is not already set!
+
+warnDefault :: [(Inst, Class, Var)] -> Type -> TcM ()
+warnDefault ups default_ty = do
+ warn_flag <- doptM Opt_WarnTypeDefaults
+ setInstCtxt (instLoc (head (dicts))) (warnTc warn_flag warn_msg)
+ where
+ dicts = [d | (d,_,_) <- ups]
+
+ -- Tidy them first
+ (_, tidy_dicts) = tidyInsts dicts
+ warn_msg = vcat [ptext (sLit "Defaulting the following constraint(s) to type") <+>
+ quotes (ppr default_ty),
+ pprDictsInFull tidy_dicts]
+
+reduceDepthErr :: Int -> [Inst] -> SDoc
+reduceDepthErr n stack
+ = vcat [ptext (sLit "Context reduction stack overflow; size =") <+> int n,
+ ptext (sLit "Use -fcontext-stack=N to increase stack size to N"),
+ nest 4 (pprStack stack)]
+
+pprStack :: [Inst] -> SDoc
+pprStack stack = vcat (map pprInstInFull stack)
+\end{code}
projects / ghc-hetmet.git / blobdiff