Commit dd45134b authored by simonpj's avatar simonpj
Browse files

[project @ 2006-01-05 13:10:55 by simonpj]

MERGE TO STABLE

This commit fixes a nasty problem discovered by Volker Stolz.
The problem is described in Note [Multiple instantiation] in
TcExpr, which is reproduced below.

(Core Lint identifies the problem, incidentally.)

tc200 is a test case.


Note [Multiple instantiation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We are careful never to make a MethodInst that has, as its meth_id, another MethodInst.
For example, consider
	f :: forall a. Eq a => forall b. Ord b => a -> b
At a call to f, at say [Int, Bool], it's tempting to translate the call to

	f_m1
  where
	f_m1 :: forall b. Ord b => Int -> b
	f_m1 = f Int dEqInt

	f_m2 :: Int -> Bool
	f_m2 = f_m1 Bool dOrdBool

But notice that f_m2 has f_m1 as its meth_id.  Now the danger is that if we do
a tcSimplCheck with a Given f_mx :: f Int dEqInt, we may make a binding
	f_m1 = f_mx
But it's entirely possible that f_m2 will continue to float out, because it
mentions no type variables.  Result, f_m1 isn't in scope.

Here's a concrete example that does this (test tc200):

    class C a where
      f :: Eq b => b -> a -> Int
      baz :: Eq a => Int -> a -> Int

    instance C Int where
      baz = f

Current solution: only do the "method sharing" thing for the first type/dict
application, not for the iterated ones.  A horribly subtle point.
parent 82e428eb
......@@ -875,15 +875,18 @@ tcId orig id_name -- Look up the Id and instantiate its type
#endif /* GHCI */
instantiate :: TcId -> TcM (HsExpr TcId, [TcTyVar], TcRhoType)
instantiate fun_id = loop (HsVar fun_id) [] (idType fun_id)
loop (HsVar fun_id) tvs fun_ty
| want_method_inst fun_ty
instantiate fun_id
| not (want_method_inst fun_ty)
= loop (HsVar fun_id) [] fun_ty
| otherwise -- Make a MethodInst
= tcInstType fun_ty `thenM` \ (tyvars, theta, tau) ->
newMethodWithGivenTy orig fun_id
(mkTyVarTys tyvars) theta tau `thenM` \ meth_id ->
loop (HsVar meth_id) (tvs ++ tyvars) tau
loop (HsVar meth_id) tyvars tau
where
fun_ty = idType fun_id
-- See Note [Multiple instantiation]
loop fun tvs fun_ty
| isSigmaTy fun_ty
= tcInstCall orig fun_ty `thenM` \ (inst_fn, new_tvs, tau) ->
......@@ -907,6 +910,40 @@ tcId orig id_name -- Look up the Id and instantiate its type
(_,theta,_) -> not (any isLinearPred theta)
\end{code}
Note [Multiple instantiation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We are careful never to make a MethodInst that has, as its meth_id, another MethodInst.
For example, consider
f :: forall a. Eq a => forall b. Ord b => a -> b
At a call to f, at say [Int, Bool], it's tempting to translate the call to
f_m1
where
f_m1 :: forall b. Ord b => Int -> b
f_m1 = f Int dEqInt
f_m2 :: Int -> Bool
f_m2 = f_m1 Bool dOrdBool
But notice that f_m2 has f_m1 as its meth_id. Now the danger is that if we do
a tcSimplCheck with a Given f_mx :: f Int dEqInt, we may make a binding
f_m1 = f_mx
But it's entirely possible that f_m2 will continue to float out, because it
mentions no type variables. Result, f_m1 isn't in scope.
Here's a concrete example that does this (test tc200):
class C a where
f :: Eq b => b -> a -> Int
baz :: Eq a => Int -> a -> Int
instance C Int where
baz = f
Current solution: only do the "method sharing" thing for the first type/dict
application, not for the iterated ones. A horribly subtle point.
%************************************************************************
%* *
\subsection{Record bindings}
......
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