CODEWITHSUNDEEP
haskellfilterfunctional-programming
Thanasi Poulos
Thanasi Poulos

Reputation: 278

Implementing a filter whose function takes an argument

I am writing some Haskell which aims to take a list of numbers and filter out any number that isn't triangular.

main :: IO()
main = do
    interact tri

tri :: String -> String
tri =   unwords . 
        map show .
        filter (isSquare (\n -> (8*n+1))) . 
        map (read :: String -> Int) . 
        words

isSquare :: (Integral a) => a -> Bool
isSquare n = (round . sqrt $ fromIntegral n) ^ 2 == n

This is what I have so far. Where I am having trouble is the filter.

I don't fully understand how I might go about actually accessing the value in that anonymous function (or, for that matter, if this is even possible in this manner). I need to test each number using this function in combination with isSquare to determine whether the number is triangular. I understand the exercise is purely academic, I'm just trying to learn about function composition and IO mostly.

Upvotes: 0

Views: 101

Answers (1)

G Philip
G Philip

Reputation: 565

  1. Does the expression (isSquare (\n -> (8*n+1))) type-check? Why not?

  2. filter has the type (a -> Bool) -> [a] -> [a]. So its first argument needs to be a function of type (a -> Bool). You are almost there with this; what function do you want here, and how can you construct it with what you have? Hint: It's a single-character change.

Upvotes: 3

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