Use functional dependencies to give more specific error messages

An example from http://www.reddit.com/r/haskell/comments/2gryy8/stdlib_monad_error_messages_unhelpful/:

f ::(Eq a) => a -> (Int, a) -> Writer [(Int, a)] (Int, a)
f y (n,x) | y == x    = return (n+1, x)
          | otherwise = do tell (n,x)
                           return (1,y)

    Could not deduce (MonadWriter
                        (Int, a) (WriterT [(Int, a)] Data.Functor.Identity.Identity))
      arising from a use of ‘tell’
    from the context (Eq a)
      bound by the type signature for
                 f :: Eq a => a -> (Int, a) -> Writer [(Int, a)] (Int, a)
      at 180.hs:42:5-57
    In a stmt of a 'do' block: tell (n, x)
    In the expression:
      do { tell (n, x);
           return (1, y) }
    In an equation for ‘f’:
        f y (n, x)
          | y == x = return (n + 1, x)
          | otherwise
          = do { tell (n, x);
                 return (1, y) }

GHC could realize that the class MonadWriter m w has a functional dependency w -> m and notice that there is an instance MonadWriter w (WriterT w m1) whose m-part unifies with that of the needed constraint. Therefore the error cannot be a missing instance, and GHC could give a more friendly error message like

    Couldn't match expected type `[(Int, a)]'
        with actual type `(Int, a)'
    when unifying the instance `MonadWriter w (WriterT w m1)'
    with the constraint `MonadWriter (Int, a) (WriterT [(Int, a)] Identity)'
    arising from a use of 'tell'
    In a stmt of a 'do' block: tell (n, x)
    In the expression:
      ...

(Or, if necessary, Could not deduce [(Int, a)] ~ (Int, a) from the context (Eq a) ...)

Trac metadata

Trac field	Value
Version	7.8.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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