import Class
import Var
import CoreUnfold ( mkDFunUnfolding )
+import CoreUtils ( mkPiTypes )
import PrelNames ( inlineIdName )
import Id
import MkId
{-# RULE "op1@C[a]" forall a, d:C a.
op1 [a] (df_i d) = op1_i a d #-}
-* We want to inline the dictionary function itself as vigorously as we
- possibly can, so that we expose that dictionary constructor to
- selectors as much as poss. We don't actually inline it; rather, we
- use a Builtin RULE for the ClassOps (see MkId.mkDictSelId) to short
- circuit such applications. But the RULE only applies if it can "see"
- the dfun's DFunUnfolding.
-
+Note [Instances and loop breakers]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* Note that df_i may be mutually recursive with both op1_i and op2_i.
It's crucial that df_i is not chosen as the loop breaker, even
though op1_i has a (user-specified) INLINE pragma.
a RULE (the magic ClassOp rule above), and RULES work inside InlineRule
unfoldings. See Note [RULEs enabled in SimplGently] in SimplUtils
+Note [ClassOp/DFun selection]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+One thing we see a lot is stuff like
+ op2 (df d1 d2)
+where 'op2' is a ClassOp and 'df' is DFun. Now, we could inline *both*
+'op2' and 'df' to get
+ case (MkD ($cop1 d1 d2) ($cop2 d1 d2) ... of
+ MkD _ op2 _ _ _ -> op2
+And that will reduce to ($cop2 d1 d2) which is what we wanted.
+
+But it's tricky to make this work in practice, because it requires us to
+inline both 'op2' and 'df'. But neither is keen to inline without having
+seen the other's result; and it's very easy to get code bloat (from the
+big intermediate) if you inline a bit too much.
+
+Instead we use a cunning trick.
+ * We arrange that 'df' and 'op2' NEVER inline.
+
+ * We arrange that 'df' is ALWAYS defined in the sylised form
+ df d1 d2 = MkD ($cop1 d1 d2) ($cop2 d1 d2) ...
+
+ * We give 'df' a magical unfolding (DFunUnfolding [$cop1, $cop2, ..])
+ that lists its methods.
+
+ * We make CoreUnfold.exprIsConApp_maybe spot a DFunUnfolding and return
+ a suitable constructor application -- inlining df "on the fly" as it
+ were.
+
+ * We give the ClassOp 'op2' a BuiltinRule that extracts the right piece
+ iff its argument satisfies exprIsConApp_maybe. This is done in
+ MkId mkDictSelId
+
+ * We make 'df' CONLIKE, so that shared uses stil match; eg
+ let d = df d1 d2
+ in ...(op2 d)...(op1 d)...
+
+Note [Single-method classes]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+If the class has just one method (or, more accurately, just one elemen
+of {superclasses + methods}), then we want a different strategy.
+
+ class C a where op :: a -> a
+ instance C a => C [a] where op = <blah>
+
+We translate the class decl into a newtype, which just gives
+a top-level axiom:
+
+ axiom Co:C a :: C a ~ (a->a)
+
+ op :: forall a. C a -> (a -> a)
+ op a d = d |> (Co:C a)
+
+ df :: forall a. C a => C [a]
+ {-# INLINE df #-}
+ df = $cop_list |> (forall a. C a -> (sym (Co:C a))
+
+ $cop_list :: forall a. C a => a -> a
+ $cop_list = <blah>
+
+So the ClassOp is just a cast; and so is the dictionary function.
+(The latter doesn't even have any lambdas.) We can inline both freely.
+No need for fancy BuiltIn rules. Indeed the BuiltinRule stuff does
+not work well for newtypes because it uses exprIsConApp_maybe.
+
+The INLINE on df is vital, else $cop_list occurs just once and is inlined,
+which is a disaster if $cop_list *itself* has an INLINE pragma.
+
+Notice, also, that we go to the trouble of generating a complicated cast,
+rather than do this:
+ df = /\a. \d. MkD ($cop_list a d)
+where the MkD "constructor" willl expand to a suitable cast:
+ df = /\a. \d. ($cop_list a d) |> (...)
+Reason: suppose $cop_list has an INLINE pragma. We want to avoid the
+nasty possibility that we eta-expand df, to get
+ df = (/\a \d \x. $cop_list a d x) |> (...)
+and now $cop_list may get inlined into the df, rather than at
+the actual call site. Of course, eta reduction may get there first,
+but it seems less fragile to generate the Right Thing in the first place.
+See Trac #3772.
+
Note [Subtle interaction of recursion and overlap]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
; checkSigTyVars inst_tyvars'
-- Create the result bindings
- ; let dict_constr = classDataCon clas
- this_dict_id = instToId this_dict
- dict_bind = mkVarBind this_dict_id dict_rhs
- dict_rhs = foldl mk_app inst_constr (sc_ids ++ meth_ids)
- inst_constr = L loc $ wrapId (mkWpTyApps inst_tys')
- (dataConWrapId dict_constr)
- -- We don't produce a binding for the dict_constr; instead we
- -- rely on the simplifier to unfold this saturated application
- -- We do this rather than generate an HsCon directly, because
- -- it means that the special cases (e.g. dictionary with only one
- -- member) are dealt with by the common MkId.mkDataConWrapId code rather
- -- than needing to be repeated here.
-
- mk_app :: LHsExpr Id -> Id -> LHsExpr Id
- mk_app fun arg_id = L loc (HsApp fun (L loc (wrapId arg_wrapper arg_id)))
- arg_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
-
- dfun_id_w_fun = dfun_id
- `setIdUnfolding` mkDFunUnfolding dict_constr (sc_ids ++ meth_ids)
- `setInlinePragma` dfunInlinePragma
-
- main_bind = noLoc $ AbsBinds
- inst_tyvars'
- dfun_lam_vars
- [(inst_tyvars', dfun_id_w_fun, this_dict_id, spec_inst_prags)]
- (unitBag dict_bind)
+ ; let this_dict_id = instToId this_dict
+ arg_ids = sc_ids ++ meth_ids
+ arg_binds = listToBag meth_binds `unionBags`
+ listToBag sc_binds
; showLIE (text "instance")
- ; return (unitBag main_bind `unionBags`
- listToBag meth_binds `unionBags`
- listToBag sc_binds) }
+ ; case newTyConCo_maybe (classTyCon clas) of
+ Nothing -- A multi-method class
+ -> return (unitBag (L loc data_bind) `unionBags` arg_binds)
+ where
+ data_dfun_id = dfun_id -- Do not inline; instead give it a magic DFunFunfolding
+ -- See Note [ClassOp/DFun selection]
+ `setIdUnfolding` mkDFunUnfolding dict_constr arg_ids
+ `setInlinePragma` dfunInlinePragma
+
+ data_bind = AbsBinds inst_tyvars' dfun_lam_vars
+ [(inst_tyvars', data_dfun_id, this_dict_id, spec_inst_prags)]
+ (unitBag dict_bind)
+
+ dict_bind = mkVarBind this_dict_id dict_rhs
+ dict_rhs = foldl mk_app inst_constr arg_ids
+ dict_constr = classDataCon clas
+ inst_constr = L loc $ wrapId (mkWpTyApps inst_tys')
+ (dataConWrapId dict_constr)
+ -- We don't produce a binding for the dict_constr; instead we
+ -- rely on the simplifier to unfold this saturated application
+ -- We do this rather than generate an HsCon directly, because
+ -- it means that the special cases (e.g. dictionary with only one
+ -- member) are dealt with by the common MkId.mkDataConWrapId code rather
+ -- than needing to be repeated here.
+
+ mk_app :: LHsExpr Id -> Id -> LHsExpr Id
+ mk_app fun arg_id = L loc (HsApp fun (L loc (wrapId arg_wrapper arg_id)))
+ arg_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
+
+ Just the_nt_co -- (Just co) for a single-method class
+ -> return (unitBag (L loc nt_bind) `unionBags` arg_binds)
+ where
+ nt_dfun_id = dfun_id -- Just let the dfun inline; see Note [Single-method classes]
+ `setInlinePragma` alwaysInlinePragma
+
+ local_nt_dfun = setIdType this_dict_id inst_ty -- A bit of a hack, but convenient
+
+ nt_bind = AbsBinds [] []
+ [([], nt_dfun_id, local_nt_dfun, spec_inst_prags)]
+ (unitBag (mkVarBind local_nt_dfun (L loc (wrapId nt_cast the_meth_id))))
+
+ the_meth_id = ASSERT( length arg_ids == 1 ) head arg_ids
+ nt_cast = WpCast $ mkPiTypes (inst_tyvars' ++ dfun_lam_vars) $
+ mkSymCoercion (mkTyConApp the_nt_co inst_tys')
+ }
------------------------------
isn't what the user expected
b) We use the magic 'inline' Id to ensure that $dmop1 really is
- inlined in $cop1, even though the latter itself has an INLINE pragma
+ inlined in $cop1, even though
+ (i) the latter itself has an INLINE pragma
+ (ii) $dmop1 is not saturated
That is important to allow the mutual recursion between $fooInt and
$cop1 to be broken
-This is all regrettably delicate.
-
%************************************************************************
%* *