Warnings about impossible MPTCs would be nice
In the following class declaration:
{-# LANGUAGE MultiParamTypeClasses #-}
class C a b where
f :: a -> a
it is possible to declare this class, and to make instances of it, but it will be impossible to ever call f
, even with unsafeCoerce
and other evil attempted workarounds. It seems fairly easy for GHC to spot this situation, where a method doesn't mention all of the class arguments, and it might save some confusion for newbies. I remember trying pretty hard when I was first learning haskell to tell GHC which instance I wanted there, but obviously failed quite miserably.
Trac metadata
Trac field | Value |
---|---|
Version | 7.0.4 |
Type | FeatureRequest |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
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