Question

getPara :: (Num [Char])  =>  [Char] -> Integer -> [Char]
getPara "" _ = ""


getPara str nr 
    | (nr == 0 ) && ((head str) == ')' ) = ')' : getPara "" 0 
    | ( nr == 0 ) && ( (head str) == '(' ) = '(' : (getPara (tail str) 0)
    | (nr /= 0 ) && ( (head str) == '(') = (getPara (tail str) nr-1) 
    | (nr == 0 ) && ( (head str) /= '(' ) = (head str) : (getPara (tail str) 0 )
    | otherwise = (getPara (tail str) nr)

What I am trying to do is to get from a String the nr set of brackets and the error I get is:

Illegal Haskell 98 class constraint in type declaration
*** Expression : getPara
*** Type       : Num [Char] => [Char] -> Integer -> [Char]

What is the problem?

Answer 1

Your type signature for getPara is not allowed, but the underlying problem is that you’re missing parentheses somewhere deep in the definition. If your change your code to:

getPara :: [Char] -> Integer -> [Char]
getPara "" _ = ""

getPara str nr 
    | (nr == 0 ) && ((head str) == ')' )  = ')' : getPara "" 0 
    | (nr == 0 ) && ( (head str) == '(' ) = '(' : (getPara (tail str) 0)
    | (nr /= 0 ) && ( (head str) == '(')  = (getPara (tail str) (nr-1))  -- here!
    | (nr == 0 ) && ( (head str) /= '(' ) = (head str) : (getPara (tail str) 0 )
    | otherwise = (getPara (tail str) nr)

it compiles … but I’m not sure if it works.

There were two changes:

1. type signature of getPara

2. surround nr-1 in parentheses in

   | (nr /= 0 ) && ( (head str) == '(')  = (getPara (tail str) (nr-1))