[Haskell-beginners] Pattern matching with one, two or more arguments

legajid legajid at free.fr
Sun Nov 29 12:41:22 EST 2009

I'm writing a little word game that consists of the following : between 
a consonant and a vowel, insert "AV" so "fabien" becomes "fAVabAVien".

When i type : f "fabine", the result is ok ("fAVabAVinAVe")
When i type : f "fabien", the result is fAVabAVie *** exception :prelude 
empty list

Here's my program


est_voyelle x = x `elem` voyelles
est_consonne x = x `elem` consonnes

--f []=[] ; unuseful - conflict with f a = a

f (a:b:c) =
    case trait a b of
        True -> a : (" AV " ++ f (b:c))
        False -> a : f (b:c)
f (a:b) =
    case trait a (head b) of
        True -> a : (" EP " ++ f ( b) )
        False -> a : f ( b)
f a =  a

-- detect a vowel following a consonant
trait a b = est_consonne a && est_voyelle b

f (a:b) is never invoked (i wrote EP instead of AV to trace the 
problem). f(a:b:c) seems to always take precedence over the other patterns.
I expect f(a:b:c) to treat words greater than or equal to 3 letters, 
f(a:b) the last two letters of the word (no third part) and f a the last 

Another trick : how can i calculate consonnes with a method that makes 
the difference between  the two lists [a..z] and  voyelles, rather  than 
writing each consonant.
As we can append two lists (++), can we compute the difference between 
lists, i.e. delete from the first list the elements that are present in 
the second one ?

Last, with Ghci, is it possible to debug my program, especially for 
recursive functions, because it's hard to follow the successive calls 
and the parameters that are passed each turn.



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