CODEWITHSUNDEEP
haskell
Justin Tyler
Justin Tyler

Reputation: 225

Haskell Even Function?

This is probably a fairly obvious question, but I just can't figure it out.

I am trying to write a function that squares the even numbers in a list. When I try to run it, I am getting an error about my use of the even function. How can I fix this?

module SquareEvens where

squareEvens :: [Integer] -> [Integer]

squareEvens n = [ns * ns | ns <- n, even n]

Upvotes: 4

Views: 5383

Answers (2)

Landei
Landei

Reputation: 54574

As you can see, it's easy to get variable names wrong. So why not do it without?

squareEvens = map (^2) . filter even

I think this is clearer than the comprehension. You can read it from right to left: Keep only the even numbers, then square them.

Upvotes: 7

dave4420
dave4420

Reputation: 47042

The code works fine if you change even n to even ns:

squareEvens n = [ns * ns | ns <- n, even ns]

But note that the convention is to use the plural to name the list and the singular to name an element from that list. So swap n and ns to follow idiomatic Haskell usage:

squareEvens ns = [n * n | n <- ns, even n]

Upvotes: 12

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