CODEWITHSUNDEEP
haskellrecursion
Marcus
Marcus

Reputation: 128

Recursion with more parameters

I am trying to write an equivalent function to Javascript's "indexOf".(getting the index of a character in a string), but I am having problems when calling the recursive function.

This is the error:

    Couldn't match expected type `Int'
            with actual type `a0 -> [a0] -> Int'
In the return type of a call of `get_index'
Probable cause: `get_index' is applied to too few arguments
In the expression: get_index (index + 1 char str)
In an equation for `get_index':
    get_index index char str
      | index < 0 || index >= (length str) = - 1
      | (str !! index) == char = index
      | otherwise = get_index (index + 1 char str)
Failed, modules loaded: none.

This is my code:

index_of char str =
  get_index 0 char str

get_index index char str
  | index < 0 || index >= (length str) = -1
  | (str !! index) == char = index
  | otherwise = get_index(index + 1 char str)

First function's purpose is solely to call the recursion with the index parameter, nothing more, the problem I have is in the second function, the recursion.

Upvotes: 1

Views: 141

Answers (1)

Zoey Hewll
Zoey Hewll

Reputation: 5385

it seems you're trying to use c-style function call syntax. for a c-style function, e.g. 

// defining the function
int plus(int a, int b)
{
    return a + b;
}

// elsewhere, calling the function
plus(a, b); // brackets surrounding only the arguments, comma separated

the equivalent Haskell code will be

-- defining the function
plus :: Int -> Int -> Int
plus a b = a + b

-- elsewhere, calling the function:
(plus a b) -- brackets surrounding the function and the arguments, no commas
-- note, plus (a b) would be equivalent to c: plus(a(b));

Note, these brackets are only ever needed for disambiguation, and in this case they can be removed, leaving plus a b.
In the following case, they would be needed:

plus(a, times(b, c));

This becomes:

(plus a (times b c))

Which is the same as:

plus a (times b c)

However it is not the same as:

plus a times b c

Upvotes: 2

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