Newtype deriving error messages coming out too late (#1608) · Issues · Glasgow Haskell Compiler / GHC

Newtype deriving error messages coming out too late

Consider this

newtype Ego a = Ego a deriving (Ord)

f :: Ord a => Ego a -> Ego a -> Bool
f e1 e2 = e1 < e2

GHC 6.7 reports an error like this:

    Could not deduce (Eq (Ego a)) from the context (Ord a)
      arising from a use of `<' at Foo10.hs:6:10-16
    In the expression: e1 < e2
    In the definition of `f': f e1 e2 = e1 < e2

This error should have come when you gave the instance decl. The reason is that GHC's newtype deriving is generating an instance like

instance Eq (Ego a) => Ord (Ego a)

It would make more sense to report the missing instance for Eq right there, rather than abstracting over it.

Thanks to Dougal Stanton for pointing this out: http://www.haskell.org/pipermail/haskell-cafe/2007-August/030224.html

Trac metadata

Trac field	Value
Version	6.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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Operating system	Unknown
Architecture	Unknown