可与GHC 7.10一起强制执行

Question

The following code works in GHC 7.8.4:

import Data.Coerce
coerceNewtype :: (Coercible (o r) (n m' r)) => [o r] -> [n m' r]
coerceNewtype = coerce

but in 7.10 I get the error:

Couldn't match representation of type ‘n m' r’ with that of ‘o r’
arising from trying to show that the representations of
  ‘[o r]’ and
  ‘[n m' r]’ are the same
Relevant role signatures: type role [] representational

I'm not quite sure where the magic happens in Data.Coerce , but given that coerce has the signature (Coercible ab) => a -> b , I think the function above should still compile in 7.10. Any ideas why the behavior is different, or how I can get use coercible with polymorphic types in 7.10?

Answer 1

There's nothing wrong with your code.

The error is due to a bug in the type-checker. The issue tracker says this will be fixed in GHC 7.11.

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