— see https://wiki.haskell.org/Safe_Haskell
{-# LANGUAGE Safe #-}
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————————– DO **NOT** MODIFY THIS FILE ————————–
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— Functional parsing library from chapter 13 of Programming in Haskell,
— Graham Hutton, Cambridge University Press, 2016.
module Parsing (module Parsing, module Control.Applicative) where
import Control.Applicative
import Data.Char
— Basic definitions
newtype Parser a = P (String -> [(a,String)])
parse :: Parser a -> String -> [(a,String)]
parse (P p) inp = p inp
item :: Parser Char
item = P (\inp -> case inp of
[] -> []
(x:xs) -> [(x,xs)])
— Sequencing parsers
instance Functor Parser where
— fmap :: (a -> b) -> Parser a -> Parser b
fmap g p = P (\inp -> case parse p inp of
[] -> []
[(v,out)] -> [(g v, out)])
instance Applicative Parser where
— pure :: a -> Parser a
pure v = P (\inp -> [(v,inp)])
— <*> :: Parser (a -> b) -> Parser a -> Parser b
pg <*> px = P (\inp -> case parse pg inp of
[] -> []
[(g,out)] -> parse (fmap g px) out)
instance Monad Parser where
— (>>=) :: Parser a -> (a -> Parser b) -> Parser b
p >>= f = P (\inp -> case parse p inp of
[] -> []
[(v,out)] -> parse (f v) out)
— Making choices
instance Alternative Parser where
— empty :: Parser a
empty = P (\inp -> [])
— (<|>) :: Parser a -> Parser a -> Parser a
p <|> q = P (\inp -> case parse p inp of
[] -> parse q inp
[(v,out)] -> [(v,out)])
— Derived primitives
sat :: (Char -> Bool) -> Parser Char
sat p = do x <- item
if p x then return x else empty
digit :: Parser Char
digit = sat isDigit
lower :: Parser Char
lower = sat isLower
upper :: Parser Char
upper = sat isUpper
letter :: Parser Char
letter = sat isAlpha
alphanum :: Parser Char
alphanum = sat isAlphaNum
char :: Char -> Parser Char
char x = sat (== x)
string :: String -> Parser String
string [] = return []
string (x:xs) = do char x
string xs
return (x:xs)
ident :: Parser String
ident = do x <- lower
xs <- many alphanum
return (x:xs)
nat :: Parser Int
nat = do xs <- some digit
return (read xs)
int :: Parser Int
int = do char '-'
n <- nat
return (-n)
<|> nat
— Handling spacing
space :: Parser ()
space = do many (sat isSpace)
return ()
token :: Parser a -> Parser a
token p = do space
v <- p
space
return v
identifier :: Parser String
identifier = token ident
natural :: Parser Int
natural = token nat
integer :: Parser Int
integer = token int
symbol :: String -> Parser String
symbol xs = token (string xs)