I found this function in a pure script tutorial and I'm trying to convert to Haskell because sometimes it would be nice to do complicated list comprehension in this way, I think. I'm stuck on the guard line (commented out) that only lets right angle triangles through to the solution set.
triples :: Int -> [[Int]]
triples n = do
z <- [1 .. n]
y <- [1 .. z]
x <- [1 .. y]
-- guard x * x + y * y == z * z ↼↽ not sure how to do this
return [x, y, z]
in purescript:
-- Find Pythagorean triples using an array comprehension.
triples :: Int -> Array (Array Int)
triples n = do
z <- 1 .. n
y <- 1 .. z
x <- 1 .. y
guard $ x * x + y * y == z * z
pure [x, y, z]
The solution is nearly the same code:
import Control.Monad (guard)
triples :: Int -> [[Int]]
triples n = do
z <- [1 .. n]
y <- [1 .. z]
x <- [1 .. y]
guard $ x * x + y * y == z * z
return [x, y, z]
Just remember to import guard. Note that, in this case you can pretend that guard is defined as:
guard :: Bool -> [()]
guard True = [()]
guard False = []
The library definition is more general, but it does not matter here. In principle, you could even use this definition for guard and avoid importing from the library.