Repository : ssh://darcs.haskell.org//srv/darcs/ghc On branch : master
http://hackage.haskell.org/trac/ghc/changeset/82879d95c8e941191bfa329604716f78ec4faa83 >--------------------------------------------------------------- commit 82879d95c8e941191bfa329604716f78ec4faa83 Author: Simon Peyton Jones <simo...@microsoft.com> Date: Mon Oct 22 09:09:21 2012 +0100 Be careful when combining two CFunEqCans, in the case where one has a unification variable on the right. See Note [Carefully solve the right CFunEqCan]. This makes the error message in SimplFail16 come out the right way round. >--------------------------------------------------------------- compiler/typecheck/TcInteract.lhs | 31 +++++++++++++++++++++++++++++-- 1 files changed, 29 insertions(+), 2 deletions(-) diff --git a/compiler/typecheck/TcInteract.lhs b/compiler/typecheck/TcInteract.lhs index ac0439d..d6044d0 100644 --- a/compiler/typecheck/TcInteract.lhs +++ b/compiler/typecheck/TcInteract.lhs @@ -708,7 +708,8 @@ doInteractWithInert ii@(CFunEqCan { cc_ev = ev1, cc_fun = tc1 , cc_tyargs = args1, cc_rhs = xi1, cc_loc = d1 }) wi@(CFunEqCan { cc_ev = ev2, cc_fun = tc2 , cc_tyargs = args2, cc_rhs = xi2, cc_loc = d2 }) - | fl1 `canSolve` fl2 + | i_solves_w && (not (w_solves_i && isMetaTyVarTy xi1)) + -- See Note [Carefully solve the right CFunEqCan] = ASSERT( lhss_match ) -- extractRelevantInerts ensures this do { traceTcS "interact with inerts: FunEq/FunEq" $ vcat [ text "workItem =" <+> ppr wi @@ -753,10 +754,36 @@ doInteractWithInert ii@(CFunEqCan { cc_ev = ev1, cc_fun = tc1 fl1 = ctEvFlavour ev1 fl2 = ctEvFlavour ev2 -doInteractWithInert _ _ = return (IRKeepGoing "NOP") + i_solves_w = fl1 `canSolve` fl2 + w_solves_i = fl2 `canSolve` fl1 + +doInteractWithInert _ _ = return (IRKeepGoing "NOP") \end{code} +Note [Carefully solve the right CFunEqCan] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider the constraints + c1 :: F Int ~ a -- Arising from an application line 5 + c2 :: F Int ~ Bool -- Arising from an application line 10 +Suppose that 'a' is a unification variable, arising only from +flattening. So there is no error on line 5; it's just a flattening +variable. But there is (or might be) an error on line 10. + +Two ways to combine them, leaving either (Plan A) + c1 :: F Int ~ a -- Arising from an application line 5 + c3 :: a ~ Bool -- Arising from an application line 10 +or (Plan B) + c2 :: F Int ~ Bool -- Arising from an application line 10 + c4 :: a ~ Bool -- Arising from an application line 5 + +Plan A will unify c3, leaving c1 :: F Int ~ Bool as an error +on the *totally innocent* line 5. An example is test SimpleFail16 +where the expected/actual message comes out backwards if we use +the wrong plan. + +The second is the right thing to do. Hence the isMetaTyVarTy +test when solving pairwise CFunEqCan. Note [Shadowing of Implicit Parameters] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ _______________________________________________ Cvs-ghc mailing list Cvs-ghc@haskell.org http://www.haskell.org/mailman/listinfo/cvs-ghc
