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.