Day2, prelude additions/fixes, better fc on do block errors

aoc2024/Day2.newt

@@ -0,0 +1,51 @@
+module Day2
+
+import Prelude
+import Node
+import Aoc
+
+decr : List Int → Bool
+decr (x :: y :: _) = y < x
+decr _ = False
+
+diff : Int → Int → Int
+diff x y = if x < y then y - x else x - y
+
+isSafe : Bool → List Int → Bool
+isSafe decr (x :: y :: rest) =
+  let d = diff x y
+      good = 0 < d && d < 4
+      safe = if x < y then not decr && good else decr && good in
+  if safe then isSafe decr (y :: rest) else False
+isSafe _ _ = True
+
+check : List Int → Bool
+check x = isSafe (decr x) x
+
+any : ∀ a. (a → Bool) → List a → Bool
+any f xs = foldl (_||_) False $ map f xs
+
+alts : List Int → List (List Int)
+alts Nil = Nil
+alts (x :: xs) = xs :: map (_::_ x) (alts xs)
+
+-- I want lean's #eval here
+
+parse : String → List (List Int)
+parse text = map nums $ split (trim text) "\n"
+
+run : String -> IO Unit
+run fn = do
+  putStrLn fn
+  text <- readFile fn
+  let stuff = parse text
+  let good = filter check stuff
+  putStrLn $ "part1 " ++ show (length good)
+  let good = filter (any check ∘ alts) stuff
+  putStrLn $ "part2 " ++ show (length good)
+
+main : IO Unit
+main = do
+  run "aoc2024/day2/eg.txt"
+  run "aoc2024/day2/input.txt"
+