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Sergey Tromimovich writes: Trying to build random packages with fresh ghc-7.8.1-rc2 I've come up with a strange bit:

https://github.com/trofi/Idris-dev/commit/9f93122ba1aa075c2fa1555fea68a6c403697e04

Is it an intended behaviour that standalone deriving (A)

    deriving instance Parsing IdrisInnerParser

is capable of doing more, than a deriving clause (B)

    newtype IdrisInnerParser a 
      = IdrisInnerParser { runInnerParser :: Parser a } 
      deriving (Parsing)

where the class is defined thus

class Alternative m => Parsing m where
    ....
    notFollowedBy :: (Monad m, Show a) => m a -> m ()
    notFollowedBy p = try ((try p >>= unexpected . show) <|> pure ())

The error message from the (B) is

[50 of 75] Compiling Idris.ParseHelpers ( src/Idris/ParseHelpers.hs, dist/build/Idris/ParseHelpers.o )

src/Idris/ParseHelpers.hs:40:97:
    Could not coerce from ‘Monad Parser’ to ‘Monad IdrisInnerParser’
      because the first type argument of ‘Monad’ has role Nominal,
      but the arguments ‘Parser’ and ‘IdrisInnerParser’ differ
      arising from the coercion of the method ‘notFollowedBy’ from type
                   ‘forall a. (Monad Parser, Show a) => Parser a -> Parser ()’ to type
                   ‘forall a.
                    (Monad IdrisInnerParser, Show a) =>
                    IdrisInnerParser a -> IdrisInnerParser ()’
    Possible fix:
      use a standalone 'deriving instance' declaration,
        so you can specify the instance context yourself
    When deriving the instance for (Parsing IdrisInnerParser)

Answer: no they should not behave differently.

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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