Syntax for functions that take in or return functions in Haskell

Question

I'm learning Haskell via Learn You a Haskell and I'm pretty sure he went over this at some point but I can't figure out where. Say I want to have a function return a pair, list of pairs, or general tuple. How is the type signature for this written? For example, for some given even number I want to return the list of unique pairs of primes that add up to it. I have a function isPrime that tells whether or not an Int is a prime.

pairs :: Int -> [(Int a, Int b)]
pairs n
  | n < 4 = []
  | odd n = []
  | otherwise = [(a, b) | a < b, isPrime a, isPrime b, a+b == 1]

When I try to compile this, I get the error saying "Int has too few arguments." So I tried the following syntax instead:

pairs :: Int -> [((Int a), (Int b))]

But I got the same error. So in general how does typesetting functions involving pairs work?

What two primes will add up to 1?

chepner
– chepner

2017-03-14 00:25:11 +00:00
Commented Mar 14, 2017 at 0:25 — chepner
– chepner, Commented Mar 14, 2017 at 0:25
See learnyouahaskell.com/…

Yawar
– Yawar

2017-03-14 02:09:52 +00:00
Commented Mar 14, 2017 at 2:09 — Yawar
– Yawar, Commented Mar 14, 2017 at 2:09
@chepner 3 and -2, naturally.

Daniel Wagner
– Daniel Wagner

2017-03-14 03:03:51 +00:00
Commented Mar 14, 2017 at 3:03 — Daniel Wagner
– Daniel Wagner, Commented Mar 14, 2017 at 3:03

isekaijin · Accepted Answer · 2017-03-14 00:01:42Z

4

Your problem isn't one of formatting. Int a and Int b are ill-kinded. Int can't be applied because it isn't a (type-level) function. You just want Int -> [(Int, Int)].

answered Mar 14, 2017 at 0:01

isekaijin

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