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)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tfeature request Trac import typechecker labels

Which version of GHC are you using? With HEAD or 7.8 I get

T9612.hs:6:33:
    Couldn't match expected type ‘[(Int, a)]’
                with actual type ‘(Int, a)’
    Relevant bindings include
      x :: a (bound at T9612.hs:5:8)
      y :: a (bound at T9612.hs:5:3)
      f :: a -> (Int, a) -> Writer [(Int, a)] (Int, a)
        (bound at T9612.hs:5:1)
    In the first argument of ‘tell’, namely ‘(n, x)’
    In a stmt of a 'do' block: tell (n, x)

which is certainly better than the one you display.

I had to add import Control.Monad.Trans.Writer.Strict at the top.

Simon

Oh, I meant for tell to be the one from mtl Control.Monad.Writer, sorry.

class (Monoid w, Monad m) => MonadWriter w m | m -> w where
    writer :: (a,w) -> m a
    tell   :: w -> m ()
    listen :: m a -> m (a, w)
    pass   :: m (a, w -> w) -> m a

mentioned in commit 20632d37

mentioned in commit 1a88f9a4

closed

Good idea, thank you. The error message is now

T9612.hs:16:9:
    Couldn't match type ‘[(Int, a)]’ with ‘(Int, a)’
    arising from a functional dependency between:
      constraint ‘MonadWriter (Int, a) (WriterT [(Int, a)] Identity)’
        arising from a use of ‘tell’
      instance ‘MonadWriter w (WriterT w m)’ at T9612.hs:20:10-59

Simon

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed
Test case	→ typecheck/should_fail/T9612

added Pnormal label