Haskell

Parsec

type LeafTree = Free []

leaf = Pure
node = Free

genWord = many1 (noneOf " (),\n")

parseExpr :: Parser a -> Parser (LeafTree a)
parseExpr p = (spaces >> (p >>= (return . leaf))) <|>
  do {
    char '(';
    trees <- sepEndBy (parseExpr p) spaces;
    char ')';
    return $ node trees;
  }

parseLISP' :: Parser (LeafTree String)
parseLISP' = parseExpr genWord

parseLISP :: String -> Maybe (LeafTree String)
parseLISP = fromRight . parse parseLISP' ""

Many ways to write a for loop

1. foldl, shuffle the arguments around to your preference. foldl::foldl :: Foldable t => (a -> b -> a) -> a -> t b -> a.

   for' :: [a] -> b -> (a -> b -> b) -> b
   for' li x0 f = foldl (flip f) x0 li
   
   --simple for loop
   for' [1,2,3] 0 (\y i -> i + y)
   
   --nested for loop
   for' [1..5] 0 (\x i ->
   	for' [1..x] i (\y j -> j+y))
   -- same as (in other languages): s=0; for x in [1..5] {for y in [1..x]{s = s+y}}; return y

2. Use Haskell’s built-in for! forM on State:

   execState (forM_ [1..5] (\x ->
   	forM_ [1..x] (\y ->
   		modify (+y)
   	)
   )) 0

• ST
• IO vs. ST
• STRef
• LoopWhile

See C:\Users\Owner\Dropbox\CS\hs\learning.

Useful things

• Memoization (dynamic programming)