[Haskell-beginners] pattern matching on function names or algebraic type data constructors

[Daniel Trstenjak]

Hi TP,

> What is the reason for this message?

'f' and 'g' in the 'case' expression are local names/bindings
and have nothing in common with the 'f' and 'g' functions.

It's the same like introducing names/bindings with the 'let' expression:
let f = ...
    g = ...
    in ...

The bindings are overlapping, because both match all the time and
GHC will always use the first one.


> Why does it not work? If Mult and Plus are functions, then I should not need 
> to put their arguments in pattern matching.

'Mult' and 'Plus' are data constructors which both expect one argument: a 'Float'.

You also can't match the data constructors against a function, but only
against a value of its type.

pat :: Foobar -> Bool
pat t = case t of
        Mult f -> True
        Plus f -> False


Greetings,
Daniel