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| author | Simon Peyton Jones <simonpj@microsoft.com> | 2016-04-21 13:06:54 +0100 |
|---|---|---|
| committer | Simon Peyton Jones <simonpj@microsoft.com> | 2016-04-22 11:32:31 +0100 |
| commit | 9421b0c77122d40bf72665ea9f90dca64a0a0ae2 (patch) | |
| tree | e6a76ff1e6f6dab8590086d7ef89fc54bb842a0b /testsuite/tests/polykinds | |
| parent | edf54d72b5b8a6dd0deafa036dc16dcfc3fcb29f (diff) | |
| download | haskell-9421b0c77122d40bf72665ea9f90dca64a0a0ae2.tar.gz | |
Warn about simplifiable class constraints
Provoked by Trac #11948, this patch adds a new warning to GHC
-Wsimplifiable-class-constraints
It warns if you write a class constraint in a type signature that
can be simplified by an existing instance declaration. Almost always
this means you should simplify it right now; type inference is very
fragile without it, as #11948 shows.
I've put the warning as on-by-default, but I suppose that if there are
howls of protest we can move it out (as happened for -Wredundant-constraints.
It actually found an example of an over-complicated context in CmmNode.
Quite a few tests use these weird contexts to trigger something else,
so I had to suppress the warning in those.
The 'haskeline' library has a few occurrences of the warning (which
I think should be fixed), so I switched it off for that library in
warnings.mk.
The warning itself is done in TcValidity.check_class_pred.
HOWEVER, when type inference fails we get a type error; and the error
suppresses the (informative) warning. So as things stand, the warning
only happens when it doesn't cause a problem. Not sure what to do
about this, but this patch takes us forward, I think.
Diffstat (limited to 'testsuite/tests/polykinds')
| -rw-r--r-- | testsuite/tests/polykinds/T6020a.hs | 4 |
1 files changed, 3 insertions, 1 deletions
diff --git a/testsuite/tests/polykinds/T6020a.hs b/testsuite/tests/polykinds/T6020a.hs index abdee4d4f1..8c5416681b 100644 --- a/testsuite/tests/polykinds/T6020a.hs +++ b/testsuite/tests/polykinds/T6020a.hs @@ -1,4 +1,4 @@ -{-# OPTIONS_GHC -fno-warn-redundant-constraints #-} +{-# OPTIONS_GHC -Wno-redundant-constraints -Wno-simplifiable-class-constraints #-} {-# LANGUAGE DataKinds, FunctionalDependencies, FlexibleInstances, UndecidableInstances, PolyKinds, KindSignatures, ConstraintKinds, FlexibleContexts, GADTs #-} @@ -10,8 +10,10 @@ instance a ~ b => Id a b class Test (x :: a) (y :: a) instance (Id x y, Id y z) => Test x z +-- Weird test case: (Id x y) and (Id y z) are both simplifiable test :: Test True True => () +-- Weird test case: (Test True True) is simplifiable test = () |