Commit 9a4c93a5 authored by simonpj@microsoft.com's avatar simonpj@microsoft.com

Undo the fix for Trac #3772 and do it a new way

The main idea is that I'm now treating a single-method dictionary very
much like a multi-method dictionary.  In particular, it respond to
exprIsConApp_maybe, even though newtypes aren't *really* proper
constructors.

See long comments with Note [Single-method classes] for why
this slight hack is justified.
parent 8ae8c865
......@@ -467,13 +467,9 @@ mkDictSelId no_unf name clas
-- becuase we use that to generate a top-level binding
-- for the ClassOp
info | new_tycon = base_info
-- For newtype dictionaries, just inline the class op
-- See Note [Single-method classes] in TcInstDcls
| otherwise = base_info
`setSpecInfo` mkSpecInfo [rule]
info = base_info `setSpecInfo` mkSpecInfo [rule]
`setInlinePragInfo` neverInlinePragma
-- Otherwise add a magic BuiltinRule, and never inline it
-- Add a magic BuiltinRule, and never inline it
-- so that the rule is always available to fire.
-- See Note [ClassOp/DFun selection] in TcInstDcls
......
......@@ -33,7 +33,7 @@ import DataCon
import Class
import Var
import CoreUnfold ( mkDFunUnfolding )
import CoreUtils ( mkPiTypes )
-- import CoreUtils ( mkPiTypes )
import PrelNames ( inlineIdName )
import Id
import MkId
......@@ -180,8 +180,8 @@ Instead we use a cunning trick.
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.
If the class has just one method (or, more accurately, just one element
of {superclasses + methods}), then we still use the *same* strategy
class C a where op :: a -> a
instance C a => C [a] where op = <blah>
......@@ -194,34 +194,39 @@ a top-level axiom:
op :: forall a. C a -> (a -> a)
op a d = d |> (Co:C a)
MkC :: forall a. (a->a) -> C a
MkC = /\a.\op. op |> (sym Co:C a)
df :: forall a. C a => C [a]
{-# INLINE df #-}
df = $cop_list |> (forall a. C a -> (sym (Co:C a))
{-# NOINLINE df DFun[ $cop_list ] #-}
df = /\a. \d. MkD ($cop_list a d)
$cop_list :: forall a. C a => a -> a
$cop_list = <blah>
So the ClassOp is just a cast; and so is the dictionary function.
(The latter doesn't even have any lambdas.) We can inline both freely.
No need for fancy BuiltIn rules. Indeed the BuiltinRule stuff does
not work well for newtypes because it uses exprIsConApp_maybe.
The "constructor" MkD expands to a cast, as does the class-op selector.
The RULE works just like for multi-field dictionaries:
* (df a d) returns (Just (MkD,..,[$cop_list a d]))
to exprIsConApp_Maybe
The INLINE on df is vital, else $cop_list occurs just once and is inlined,
which is a disaster if $cop_list *itself* has an INLINE pragma.
* The RULE for op picks the right result
Notice, also, that we go to the trouble of generating a complicated cast,
rather than do this:
df = /\a. \d. MkD ($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.
This is a bit of a hack, because (df a d) isn't *really* a constructor
application. But it works just fine in this case, exprIsConApp_maybe
is otherwise used only when we hit a case expression which will have
a real data constructor in it.
The biggest reason for doing it this way, apart form uniformity, is
that we want to be very careful when we have
instance C a => C [a] where
{-# INLINE op #-}
op = ...
then we'll get an INLINE pragma on $cop_list. The danger is that
we'll get something like
foo = /\a.\d. $cop_list a d
and then we'll eta expand, and then we'll inline TOO EARLY. This happened in
Trac #3772 and I spent far too long fiddling arond trying to fix it.
Look at the test for Trac #3772.
Note [Subtle interaction of recursion and overlap]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
......@@ -768,6 +773,44 @@ tc_inst_decl2 dfun_id (VanillaInst monobinds uprags standalone_deriv)
-- something in the envt with one of the inst_tyvars'
; 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')
-- Do not inline the dfun; instead give it a magic DFunFunfolding
-- See Note [ClassOp/DFun selection]
-- See also note [Single-method classes]
dfun_id_w_fun = dfun_id
`setIdUnfolding` mkDFunUnfolding dict_constr (sc_ids ++ meth_ids)
`setInlinePragma` dfunInlinePragma
main_bind = AbsBinds
inst_tyvars'
dfun_lam_vars
[(inst_tyvars', dfun_id_w_fun, this_dict_id, spec_inst_prags)]
(unitBag dict_bind)
; showLIE (text "instance")
; return (unitBag (L loc main_bind) `unionBags`
listToBag meth_binds `unionBags`
listToBag sc_binds)
}
{-
-- Create the result bindings
; let this_dict_id = instToId this_dict
arg_ids = sc_ids ++ meth_ids
......@@ -819,8 +862,7 @@ tc_inst_decl2 dfun_id (VanillaInst monobinds uprags standalone_deriv)
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')
}
-}
------------------------------
tcSuperClass :: InstLoc -> [TyVar] -> [Inst]
......
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