incorrect type error message

[Simon Peyton-Jones]

The reason there's an error message at all is because of the
monomorphism restriction.  So that's not surprising.

So the compiler has to solve the constraints { Num (f a), Functor f }
in the absence of any other context whatsoever.  

So the constraint (Functor f) is certainly ambiguous, in the sense that
there are many different instantiations of 'f' that could resolve it.

Can you suggest a better error message?  I'm always looking for ways to
report errors more informatively.

Simon

| -----Original Message-----
| From: Martin Sulzmann [mailto:sulzmann at comp.nus.edu.sg]
| Sent: 25 November 2002 10:04
| To: glasgow-haskell-bugs at haskell.org
| Subject: incorrect type error message
| 
| Consider
| 
| module Bug where
| 
| e = fmap id 1
| 
| ghci --version
| The Glorious Glasgow Haskell Compilation System, version 5.02.2
| 
| says
| 
| :l Bug.hs
| Compiling Bug              ( Bug.hs, interpreted )
| 
| Bug.hs:5:
|     No instance for (Num (f a))
|     arising from the literal `1' at Bug.hs:5
|     In the second argument of `fmap', namely `1'
|     In the definition of `e': fmap id 1
| 
| Bug.hs:5:
|     Ambiguous type variable(s) `f' in the constraint `Functor f'
|     arising from use of `fmap' at Bug.hs:5
|     In the definition of `e': fmap id 1
| 
| although
| 
| :t (fmap id 1)
| forall f a. (Num (f a), Functor f) => f a
| 
| Clearly, 'f' is not ambiguous.
| 
| Martin
| 
| 
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