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gdeest/debrief.hs

Created October 23, 2019 08:42
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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]
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