DefaultSignatures: error message mentions internal name
{-# LANGUAGE DefaultSignatures #-}
class C a where
reflexive :: a -> Bool
default reflexive :: Eq a => a -> Bool
reflexive x = x == x
data D
instance C D where
-- /home/abel/play/haskell/bugs/DefaultSig.hs:10:10:
-- No instance for (Eq D) arising from a use of ‘Main.$gdmreflexive’
-- In the expression: Main.$gdmreflexive
-- In an equation for ‘reflexive’: reflexive = Main.$gdmreflexive
-- In the instance declaration for ‘C D’Error looks odd: The user has not written $gdmreflexive in his code.
TODO: Better error message.
Maybe this should just trigger a warning that method reflexive is undefined for instance D of C. Like when I remove the default method.
/home/abel/play/haskell/bugs/DefaultSig.hs:10:10: Warning:
No explicit implementation for
‘reflexive’
In the instance declaration for ‘C D’It seems the semantics of a default signature is that each instance *must* implement this method.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 7.8.4 |
| Type | FeatureRequest |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |