[Haskell-cafe] Monad transformer responsibilities

[Claus Reinke]

| bar :: (C T) => T
| *Main> :t bar
| 
| <interactive>:1:0:
|     No instance for (C T)
|       arising from a use of `bar' at <interactive>:1:0-2
|     Possible fix: add an instance declaration for (C T)
|     In the expression: bar

I'm not sure where that comes from, but it does seem to be an 
artifact of GHC's type inference, which seems unwilling to infer
a flexible context even if flexible contexts are enabled:

*Main> :show languages
active language flags:
  -XImplicitPrelude
  -XFlexibleContexts
*Main> let f _ = negate []
*Main> :t f
f :: (Num [a]) => t -> [a]
*Main> let f _ = negate [()]

<interactive>:1:10:
    No instance for (Num [()])
      arising from a use of `negate' at <interactive>:1:10-20
    Possible fix: add an instance declaration for (Num [()])
    In the expression: negate [()]
    In the definition of `f': f _ = negate [()]
*Main> let f :: Num [()] => t -> [()]; f _ = negate [()]
*Main> :t f

<interactive>:1:0:
    No instance for (Num [()])
      arising from a use of `f' at <interactive>:1:0
    Possible fix: add an instance declaration for (Num [()])
    In the expression: f

This does look like a bug to me? Compare with Hugs (Hugs mode):

Main> :t let f _ = negate [] in f
let {...} in f :: Num [a] => b -> [a]
Main> :t let f _ = negate [()] in f
let {...} in f :: Num [()] => a -> [()]

Claus