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

23 Jun 2013

      Hi,

I ask myself if there is a way to make the following code work.
I would like to pattern match on function names:

-----------
f :: Float -> Int
f x = 1

g :: Float -> Int
g x = 2

pat :: (Float -> Int) -> Bool
pat t = case t of
        f -> True
        g -> False

main = do

print $ pat f
print $ pat g
-----------

I don't understand the output:

-----------
$ runghc test_pattern_match_on_function_2.hs 

test_pattern_match_on_function_2.hs:8:9: Warning:
    Pattern match(es) are overlapped
    In a case alternative: g -> ...
True
True
-----------

What is the reason for this message?

Now, I want to do it on data constructors in a GADT (which are functions if 
I am right):
-----------
{-# LANGUAGE GADTs #-}

data Foobar where
    Mult :: Float -> Foobar
    Plus :: Float -> Foobar

pat :: (Float -> Foobar) -> Bool
pat t = case t of
        Mult -> True
        Plus -> False

main = do

print $ pat Mult
print $ pat Plus
-----------

I obtain:

-----------
$ runghc test_pattern_match_on_function.hs 

test_pattern_match_on_function.hs:9:9:
    Constructor `Mult' should have 1 argument, but has been given none
    In the pattern: Mult
    In a case alternative: Mult -> True
    In the expression:
      case t of {
        Mult -> True
        Plus -> False }

test_pattern_match_on_function.hs:9:9:
    Couldn't match expected type `Float -> Foobar'
                with actual type `Foobar'
    In the pattern: Mult
    In a case alternative: Mult -> True
    In the expression:
      case t of {
        Mult -> True
        Plus -> False }
-----------

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

Thanks in advance,

TP