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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/partial-sigs/should_compile | |
| 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/partial-sigs/should_compile')
| -rw-r--r-- | testsuite/tests/partial-sigs/should_compile/SomethingShowable.stderr | 6 |
1 files changed, 5 insertions, 1 deletions
diff --git a/testsuite/tests/partial-sigs/should_compile/SomethingShowable.stderr b/testsuite/tests/partial-sigs/should_compile/SomethingShowable.stderr index b0957a0b9b..5f761f73bb 100644 --- a/testsuite/tests/partial-sigs/should_compile/SomethingShowable.stderr +++ b/testsuite/tests/partial-sigs/should_compile/SomethingShowable.stderr @@ -4,5 +4,9 @@ TYPE CONSTRUCTORS COERCION AXIOMS Dependent modules: [] Dependent packages: [base-4.9.0.0, ghc-prim-0.5.0.0, - integer-gmp-1.0.0.0] + integer-gmp-1.0.0.1] +SomethingShowable.hs:5:1: warning: [-Wsimplifiable-class-constraints (in -Wdefault)] + The constraint ‘Show Bool’ matches an instance declaration + This makes type inference very fragile; + try simplifying it using the instance |