\begin{code}
module TcInstUtil (
- InstInfo(..),
- buildInstanceEnv,
- instInfoClass, simpleInstInfoTy, simpleInstInfoTyCon, classDataCon,
- pprInstInfo
+ InstInfo(..), pprInstInfo,
+ instInfoClass, simpleInstInfoTy, simpleInstInfoTyCon,
+
+ -- Instance environment
+ InstEnv, emptyInstEnv, buildInstanceEnv,
+ lookupInstEnv, InstLookupResult(..),
+ classInstEnv, classDataCon
) where
#include "HsVersions.h"
import Outputable
\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{The InstInfo type}
+%* *
+%************************************************************************
+
+The InstInfo type summarises the information in an instance declaration
+
instance c => k (t tvs) where b
\begin{code}
\end{code}
-%************************************************************************
-%* *
-\subsection{Creating instance related Ids}
-%* *
-%************************************************************************
-
A tiny function which doesn't belong anywhere else.
It makes a nasty mutual-recursion knot if you put it in Class.
| isLocallyDefined dfun = ptext SLIT("defined at") <+> ppr (getSrcLoc dfun)
| otherwise = ptext SLIT("imported from module") <+> quotes (ppr (nameModule (idName dfun)))
\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{Instance environments: InstEnv and ClsInstEnv}
+%* *
+%************************************************************************
+
+The actual type declarations are in HscTypes.
+
+\begin{code}
+emptyInstEnv :: InstEnv
+emptyInstEnv = emptyUFM
+
+classInstEnv :: InstEnv -> Class -> ClsInstEnv
+classInstEnv env cls = lookupWithDefaultUFM env [] cls
+\end{code}
+
+A @ClsInstEnv@ lives inside a class, and identifies all the instances
+of that class. The @Id@ inside a ClsInstEnv mapping is the dfun for
+that instance.
+
+If class C maps to a list containing the item ([a,b], [t1,t2,t3], dfun), then
+
+ forall a b, C t1 t2 t3 can be constructed by dfun
+
+or, to put it another way, we have
+
+ instance (...) => C t1 t2 t3, witnessed by dfun
+
+There is an important consistency constraint in the elements of a ClsInstEnv:
+
+ * [a,b] must be a superset of the free vars of [t1,t2,t3]
+
+ * The dfun must itself be quantified over [a,b]
+
+Thus, the @ClassInstEnv@ for @Eq@ might contain the following entry:
+ [a] ===> dfun_Eq_List :: forall a. Eq a => Eq [a]
+The "a" in the pattern must be one of the forall'd variables in
+the dfun type.
+
+
+
+Notes on overlapping instances
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+In some ClsInstEnvs, overlap is prohibited; that is, no pair of templates unify.
+
+In others, overlap is permitted, but only in such a way that one can make
+a unique choice when looking up. That is, overlap is only permitted if
+one template matches the other, or vice versa. So this is ok:
+
+ [a] [Int]
+
+but this is not
+
+ (Int,a) (b,Int)
+
+If overlap is permitted, the list is kept most specific first, so that
+the first lookup is the right choice.
+
+
+For now we just use association lists.
+
+\subsection{Avoiding a problem with overlapping}
+
+Consider this little program:
+
+\begin{pseudocode}
+ class C a where c :: a
+ class C a => D a where d :: a
+
+ instance C Int where c = 17
+ instance D Int where d = 13
+
+ instance C a => C [a] where c = [c]
+ instance ({- C [a], -} D a) => D [a] where d = c
+
+ instance C [Int] where c = [37]
+
+ main = print (d :: [Int])
+\end{pseudocode}
+
+What do you think `main' prints (assuming we have overlapping instances, and
+all that turned on)? Well, the instance for `D' at type `[a]' is defined to
+be `c' at the same type, and we've got an instance of `C' at `[Int]', so the
+answer is `[37]', right? (the generic `C [a]' instance shouldn't apply because
+the `C [Int]' instance is more specific).
+
+Ghc-4.04 gives `[37]', while ghc-4.06 gives `[17]', so 4.06 is wrong. That
+was easy ;-) Let's just consult hugs for good measure. Wait - if I use old
+hugs (pre-September99), I get `[17]', and stranger yet, if I use hugs98, it
+doesn't even compile! What's going on!?
+
+What hugs complains about is the `D [a]' instance decl.
+
+\begin{pseudocode}
+ ERROR "mj.hs" (line 10): Cannot build superclass instance
+ *** Instance : D [a]
+ *** Context supplied : D a
+ *** Required superclass : C [a]
+\end{pseudocode}
+
+You might wonder what hugs is complaining about. It's saying that you
+need to add `C [a]' to the context of the `D [a]' instance (as appears
+in comments). But there's that `C [a]' instance decl one line above
+that says that I can reduce the need for a `C [a]' instance to the
+need for a `C a' instance, and in this case, I already have the
+necessary `C a' instance (since we have `D a' explicitly in the
+context, and `C' is a superclass of `D').
+
+Unfortunately, the above reasoning indicates a premature commitment to the
+generic `C [a]' instance. I.e., it prematurely rules out the more specific
+instance `C [Int]'. This is the mistake that ghc-4.06 makes. The fix is to
+add the context that hugs suggests (uncomment the `C [a]'), effectively
+deferring the decision about which instance to use.
+
+Now, interestingly enough, 4.04 has this same bug, but it's covered up
+in this case by a little known `optimization' that was disabled in
+4.06. Ghc-4.04 silently inserts any missing superclass context into
+an instance declaration. In this case, it silently inserts the `C
+[a]', and everything happens to work out.
+
+(See `basicTypes/MkId:mkDictFunId' for the code in question. Search for
+`Mark Jones', although Mark claims no credit for the `optimization' in
+question, and would rather it stopped being called the `Mark Jones
+optimization' ;-)
+
+So, what's the fix? I think hugs has it right. Here's why. Let's try
+something else out with ghc-4.04. Let's add the following line:
+
+ d' :: D a => [a]
+ d' = c
+
+Everyone raise their hand who thinks that `d :: [Int]' should give a
+different answer from `d' :: [Int]'. Well, in ghc-4.04, it does. The
+`optimization' only applies to instance decls, not to regular
+bindings, giving inconsistent behavior.
+
+Old hugs had this same bug. Here's how we fixed it: like GHC, the
+list of instances for a given class is ordered, so that more specific
+instances come before more generic ones. For example, the instance
+list for C might contain:
+ ..., C Int, ..., C a, ...
+When we go to look for a `C Int' instance we'll get that one first.
+But what if we go looking for a `C b' (`b' is unconstrained)? We'll
+pass the `C Int' instance, and keep going. But if `b' is
+unconstrained, then we don't know yet if the more specific instance
+will eventually apply. GHC keeps going, and matches on the generic `C
+a'. The fix is to, at each step, check to see if there's a reverse
+match, and if so, abort the search. This prevents hugs from
+prematurely chosing a generic instance when a more specific one
+exists.
+
+--Jeff
+
+
+@lookupInstEnv@ looks up in a @InstEnv@, using a one-way match. Since
+the env is kept ordered, the first match must be the only one. The
+thing we are looking up can have an arbitrary "flexi" part.
+
+\begin{code}
+lookupInstEnv :: InstEnv -- The envt
+ -> Class -> [Type] -- Key
+ -> InstLookupResult
+
+data InstLookupResult
+ = FoundInst -- There is a (template,substitution) pair
+ -- that makes the template match the key,
+ -- and no template is an instance of the key
+ TyVarSubstEnv Id
+
+ | NoMatch Bool -- Boolean is true iff there is at least one
+ -- template that matches the key.
+ -- (but there are other template(s) that are
+ -- instances of the key, so we don't report
+ -- FoundInst)
+ -- The NoMatch True case happens when we look up
+ -- Foo [a]
+ -- in an InstEnv that has entries for
+ -- Foo [Int]
+ -- Foo [b]
+ -- Then which we choose would depend on the way in which 'a'
+ -- is instantiated. So we say there is no match, but identify
+ -- it as ambiguous case in the hope of giving a better error msg.
+ -- See the notes above from Jeff Lewis
+
+lookupInstEnv env key_cls key_tys
+ = find (classInstEnv env key_cls)
+ where
+ key_vars = tyVarsOfTypes key_tys
+
+ find [] = NoMatch False
+ find ((tpl_tyvars, tpl, val) : rest)
+ = case matchTys tpl_tyvars tpl key_tys of
+ Nothing ->
+ case matchTys key_vars key_tys tpl of
+ Nothing -> find rest
+ Just (_, _) -> NoMatch (any_match rest)
+ Just (subst, leftovers) -> ASSERT( null leftovers )
+ FoundInst subst val
+
+ any_match rest = or [ maybeToBool (matchTys tvs tpl key_tys)
+ | (tvs,tpl,_) <- rest
+ ]
+\end{code}
+
+@addToClsInstEnv@ extends a @ClsInstEnv@, checking for overlaps.
+
+A boolean flag controls overlap reporting.
+
+True => overlap is permitted, but only if one template matches the other;
+ not if they unify but neither is
+
+\begin{code}
+addToInstEnv :: Bool -- True <=> overlap permitted
+ -> InstEnv -- Envt
+ -> Class -> [TyVar] -> [Type] -> Id -- New item
+ -> MaybeErr InstEnv -- Success...
+ ([Type], Id) -- Failure: Offending overlap
+
+addToInstEnv overlap_ok inst_env clas ins_tvs ins_tys value
+ = case insert_into (classInstEnv inst_env clas) of
+ Failed stuff -> Failed stuff
+ Succeeded new_env -> Succeeded (addToUFM inst_env clas new_env)
+
+ where
+ ins_tv_set = mkVarSet ins_tvs
+ ins_item = (ins_tv_set, ins_tys, value)
+
+ insert_into [] = returnMaB [ins_item]
+ insert_into env@(cur_item@(tpl_tvs, tpl_tys, val) : rest)
+
+ -- FAIL if:
+ -- (a) they are the same, or
+ -- (b) they unify, and any sort of overlap is prohibited,
+ -- (c) they unify but neither is more specific than t'other
+ | identical
+ || (unifiable && not overlap_ok)
+ || (unifiable && not (ins_item_more_specific || cur_item_more_specific))
+ = failMaB (tpl_tys, val)
+
+ -- New item is an instance of current item, so drop it here
+ | ins_item_more_specific = returnMaB (ins_item : env)
+
+ -- Otherwise carry on
+ | otherwise = insert_into rest `thenMaB` \ rest' ->
+ returnMaB (cur_item : rest')
+ where
+ unifiable = maybeToBool (unifyTyListsX (ins_tv_set `unionVarSet` tpl_tvs) tpl_tys ins_tys)
+ ins_item_more_specific = maybeToBool (matchTys tpl_tvs tpl_tys ins_tys)
+ cur_item_more_specific = maybeToBool (matchTys ins_tv_set ins_tys tpl_tys)
+ identical = ins_item_more_specific && cur_item_more_specific
+\end{code}
+
+
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