debrief.hs

data Maybe a = Nothing | Just a
data Expr = SumExpr Expr Expr
          | ProdExpr Expr Expr
          | Power Expr Expr
          | Val Float


String -> Maybe Expr

-- "1+2*3" → Just (SumExpr (Val 1.0) (ProdExpr (Val 2.0) (Val 3.0))
-- "1+(2*3" → Nothing

--

data Cmd = GET | POST | PUT | DELETE

parseCmd :: Parser Cmd
         :: String -> Maybe (Cmd, String)

parseCmd "GET123" → Just (GET, "123")
parseCmd "Robert123" → Nothing

type String = [Char]

[]
(x:xs)

"GET" = 'G':('E':'T:[])

parseCmd ('G':('E':('T':rest))) = Just (GET, rest) 
parseCmd ('P':('U':('T':rest))) = Just (PUT, rest) 
parseCmd ('P':('O':('S':('T':rest)))) = Just (POST, rest) 
parseCmd ('D':('E':('L':('E':('T':('E':rest)))))) = Just (DELETE, rest) 
parseCmd _ = Nothing

--

type Parser a = String → Maybe (a, String)

parseString :: a -> String -> Parser a
            :: a -> String -> (String -> Maybe (a, String))


parseString a "" input = Just (a, input)
-- ^ Même chose que: parseString a "" = \input -> Just (a, input)
parseString a (x:xs) "" = Nothing
parseString a (x:xs) (y:ys) | x == y = parseString a xs ys
parseString a _ _ = Nothing

parseGet :: Parser Cmd
parseGet = parseString GET "GET"
parsePost = …
parsePut  = …

anyOf :: [Parser a] -> Parser a
-- anyOf :: [String -> Maybe (a, String)] -> (String -> Maybe (a, String))
anyOf [] _ = Nothing
anyOf (p:ps) input =
  case p input of
    Nothing -> anyOf ps input
    ret -> ret 

parseCmd :: Parser Cmd
parseCmd = anyOf [parseGet, parsePost, parsePut, parseDelete]