Question

Studying Haskell, i’m trying to code a function takeIf that given a condition and a list returns all the list’s element that satisfies the condition.

Examples of desired output:

takeIf (>0) [] --> []
takeIf (>0) [-1, 0, 1, 2] --> [1,2]

I tried this definition:

takeIf cond [] = []
takeIf cond (x:xs) = if (cond x) 
                    then x:(takeIf cond xs)
                    else []:(takeIf cond xs)

but it doesn’t work.

My first question is: I have

:t takeIf --> ([a] -> Bool) -> [[a]] -> [[a]]

why? Why not:

:t takeIf --> (a -> Bool) -> [a] -> [a]

How can I get this code to work?

This is the error I get:

If helpful i’m using ghci

Answer 1

[]:(takeIf cond xs)

Here you’re trying to prepend [] as the new first element to the result of takeIf cond xs. Since [] is a list, GHC infers from that that the result of takeIf cond xs must be a list of lists. Since the result of takeIf has the same type as its argument, that means xs must also be a list of lists. And since x is an element of xs, x must consequently be a list.

It seems like you intended []: to mean “prepend nothing to the list”, but [] isn’t nothing, it’s the empty list. [] : [], doesn’t give you [], it gives you [[]]. Likewise [] : [1,2,3] would give you [[], 1, 2, 3] – except that that’s not well-typed, so what it really gives you is a type error.

If you want to prepend nothing to a list, just don’t prepend anything to the list, i.e. just use someList instead of [] : someList.