strange "instance .. => .. => .. where ..."
I don't understand what's happening here:
GHCi, version 8.6.3: http://www.haskell.org/ghc/ :? for help
Prelude> data T a
Prelude> instance Eq a => Eq a => Eq (T a) where (==) = undefined
<interactive>:2:10: error:
• Class ‘Eq’ does not have a method ‘==’
• In the instance declaration for ‘Eq (T a)’
Is that even syntactically correct? (ghc-8.4.4 says "malformed instance") If so, then I'd assume that A => B => C
means A && B => C
, that is, (A,B) => C
in this case,
but it does not, since this works:
instance (Eq a, Eq a)=> Eq (T a) where (==) = undefined
Trac metadata
Trac field | Value |
---|---|
Version | 8.6.3 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |