September 2017 Update

Task/List-comprehensions/Haskell/list-comprehensions-1.hs

@@ -1,2 +1,7 @@
-pyth :: (Enum t, Eq t, Num t) => t -> [(t, t, t)]
-pyth n = [(x,y,z) | x <- [1..n], y <- [x..n], z <- [y..n], x^2 + y^2 == z^2]
+pyth :: Int -> [(Int, Int, Int)]
+pyth n =
+  [ (x, y, z)
+  | x <- [1 .. n]
+  , y <- [x .. n]
+  , z <- [y .. n]
+  , x ^ 2 + y ^ 2 == z ^ 2 ]

Task/List-comprehensions/Haskell/list-comprehensions-2.hs

@@ -1,9 +1,8 @@
-import Control.Monad (guard)
-
-pyth :: (Enum t, Eq t, Num t) => t -> [(t, t, t)]
+pyth :: Int -> [(Int, Int, Int)]
 pyth n = do
-  x <- [1..n]
-  y <- [x..n]
-  z <- [y..n]
-  guard $ x^2 + y^2 == z^2
-  return (x,y,z)
+  x <- [1 .. n]
+  y <- [x .. n]
+  z <- [y .. n]
+  if x ^ 2 + y ^ 2 == z ^ 2
+  then [(x, y, z)]
+  else []

Task/List-comprehensions/Haskell/list-comprehensions-3.hs

@@ -0,0 +1,11 @@
+pyth :: Int -> [(Int, Int, Int)]
+pyth n =
+  [1 .. n] >>=
+  \x ->
+     [x .. n] >>=
+     \y ->
+        [y .. n] >>=
+        \z ->
+           case x ^ 2 + y ^ 2 == z ^ 2 of
+             True -> [(x, y, z)]
+             _ -> []

Task/List-comprehensions/Haskell/list-comprehensions-4.hs

@@ -0,0 +1,17 @@
+pyth :: Int -> [(Int, Int, Int)]
+pyth n =
+  concatMap
+    (\x ->
+        concatMap
+          (\y ->
+              concatMap
+                (\z ->
+                    if x ^ 2 + y ^ 2 == z ^ 2
+                      then [(x, y, z)]
+                      else [])
+                [y .. n])
+          [x .. n])
+    [1 .. n]
+
+main :: IO ()
+main = print $ pyth 25