How can i return null in haskell?

Question

I want to make a function that recieves a list of pairs (e.g. [(3,'a'),(7,'d'),(1,'c')]) and a key (e.g. 3) and returns the value for that key (in this example 'a').

The function I've written works fine if the key exists but if i try to return null when i did not find a key that matches - I get an error.

the function is :

lookup1 k lst = lookupH1 k lst
    where
        lookupH1 k lst 
            | ((length lst) == 0) = null   -- this is the problematic line
            | ((fst (head lst)) == k )= snd (head lst)
            | otherwise = lookupH1 k (tail (lst))

what can I do to solve this?

Answer 1

It's usually best to first stub out your types and then let the compiler guide you.

In this case, it looks like the signature you are looking at right now is:

lookup1 :: Int -> [(Int,Char)] -> Char

However, Haskell doesn't have a null that can replace the Char as the return type. Therefore, it's common practice to use Maybe instead. The new type signature would then be.

lookup1 :: Int -> [(Int,Char)] -> Maybe Char

This would mean that upon finding the value v, you would yield "Just v" and in the case where you don't, it would yield "Nothing".

I hope I'm not giving away too much, but this is a slightly simpler version of your function using pattern matching.

lookup1 :: Int -> [(Int,Char)] -> Maybe Char                                    
lookup1 k lst = lookupH1 lst                                                    
    where                                                                       
      lookupH1 [] = Nothing                                                     
      lookupH1 ((k',v):lst')                                                    
        | k == k' = Just v                                                      
        | otherwise = lookupH1 lst'