Bad error message: varying arguments

[guest]

A function definition like

f :: Maybe () -> Char
f Nothing = 'a'
f Just () = 'b'

yields the error message

Varying number of arguments for function `f'

under 6.4.1 (pointing to the line with Nothing as containing the error). The following error message would be easier to understand for beginners (pointing to the line with Just):

The type signature for f specifies that the function should take one argument, but you have given it two arguments.

[simonmar]

The error message is correct: the two equations for f have differing numbers of arguments.

[guest]

The error message is indeed correct, but hard to understand for newbies (I've observed this in practice, several times). In the wake of the GHC survey Simon PJ asked for error messages that could be improved. This is such an error message.

[simonpj]

'guest' writes "In the wake of the GHC survey Simon PJ asked for error messages that could be improved. This is such an error message."

And indeed I have improved it:

Foo4.hs:4:0:
    Equations for `f' have different numbers of arguments
      Foo4.hs:4:0-14
      Foo4.hs:5:0-14

It's nothing to do with the type signature, which might be absent, or might have more args than the defintion.

Simon