Parsec
1 like•724 views
This document provides a quick tour of the Parsec parsing library in Haskell. It introduces the basic types and functions in Parsec, demonstrates simple parsers for characters and strings, and shows how parsers can be combined using do notation and other functions. It also gives examples of parsing phone book entries, handling parser failure, and defining expression parsers and language grammars with Parsec.
![Handling Multiple First Names
let nameSep = try $ do
space
notFollowedBy (char '(')
let fullName = do
last <- name
char ','
space
rest <- sepBy1 name nameSep
return $ rest ++ [last]](https://image.slidesharecdn.com/parsec-130814194008-phpapp02/85/Parsec-14-320.jpg)
![Example - Expression Parsers
let
number = fmap read $ many1 digit
bracket = between (char '(') (char ')') numExpr
atom = bracket <|> number
numExpr = buildExpressionParser
[ [ Infix (do { char '/'; return (/) }) AssocRight
, Infix (do { char '*'; return (*) }) AssocRight ]
, [ Infix (do { char '+'; return (+) }) AssocRight
, Infix (do { char '-'; return (-) }) AssocRight ]
, [ Prefix $ do { char '-'; return negate } ]
] atom](https://image.slidesharecdn.com/parsec-130814194008-phpapp02/85/Parsec-22-320.jpg)
Parsec
- 1. A Very Quick Tour of Parsec For the LA Haskell User Group Setup: :set +m :set -XNoMonomorphismRestriction import Text.Parsec import Control.Monad
- 2. Types :i Parsec ParsecT :i State
- 3. Escaping the Monad :t runParser :t runParserT :t parse :t parseTest
- 4. Demonstration parseTest (char 'a') "a" parseTest (char 'a') "ab" parseTest (char 'a') "d"
- 5. Simple Parsers :t oneOf :t letter :t digit :t space
- 6. Do Notation fmap f p Parse using p and apply f to the result p >>= q Parse using p first, and then using q, possibly using the result of p. do { x ← … ; y ← … ; … } Combine several parse results
- 7. Modifying Parsers :t many :t count :t chainl :t chainr
- 8. Parsing Phone Book Entries let tel = do g1 <- count 3 digit; char '-' g2 <- count 3 digit char '-' g3 <- count 4 digit return (g1, g2, g3)
- 9. Parsing Phone Book Entries let name = many1 letter let fullName = do last <- name char ',' space first <- name return (first, last)
- 10. Parsing Phone Book Entries let phoneBook = do (first, last) <- fullName space char '(' (g1, g2, g3) <- tel char ')' return (first, last, g1, g2, g3)
- 11. Parsing Phone Book Entries parseTest phoneBook "Freeman, Phillip (555-555-5555)"
- 12. Combining Parsers :t sepBy :t (<|>)
- 13. Handling Failure :t try :t <?>
- 14. Handling Multiple First Names let nameSep = try $ do space notFollowedBy (char '(') let fullName = do last <- name char ',' space rest <- sepBy1 name nameSep return $ rest ++ [last]
- 15. Handling Multiple First Names let phoneBook = do names <- fullName space char '(' (g1, g2, g3) <- tel char ')' return (names, g1, g2, g3)
- 16. Parsing Phone Book Entries parseTest phoneBook "Freeman, Phillip Antony (555-555- 5555)"
- 17. User State Parsec(T) is a state monad, so we can carry around user state as well as the parser's internal state. :t getState :t setState :t modifyState
- 18. Example - { an bn | n ≥ 0 } let a = liftM2 (curry fst) (char 'a') (modifyState succ) an bn = do as <- many a n <- getState bs <- count n $ char 'b' eof return $ as ++ bs
- 19. Example - { an bn | n ≥ 0 } runParser an bn 0 "" "aaabb" runParser an bn 0 "" "aabb" runParser an bn 0 "" "aabbb"
- 20. Example – Print Debugging let char' c = lift (print c) >> char c chars = many $ (char' 'a') <|> (char' 'b') runParserT chars () "" "aabb"
- 21. Expression Parsers import Text.Parsec.Expr :t buildExpressionParser :i Operator OperatorTable Assoc
- 22. Example - Expression Parsers let number = fmap read $ many1 digit bracket = between (char '(') (char ')') numExpr atom = bracket <|> number numExpr = buildExpressionParser [ [ Infix (do { char '/'; return (/) }) AssocRight , Infix (do { char '*'; return (*) }) AssocRight ] , [ Infix (do { char '+'; return (+) }) AssocRight , Infix (do { char '-'; return (-) }) AssocRight ] , [ Prefix $ do { char '-'; return negate } ] ] atom
- 23. Example - Expression Parsers parseText numExpr “(1+2*4)/3”
- 24. Language Definitions import Text.Parsec.Token import Text.Parsec.Language :i GenLanguageDef GenTokenParser
- 25. Language Definitions let langDef = haskellStyle -- { ... } let parser = makeTokenParser langDef let stringLiteral' = stringLiteral parser integer' = integer parser float' = float parser lexeme' = lexeme parser
- 26. Language Definitions parseTest stringLiteral' ""Test"" parseTest (lexeme' stringLiteral') ""Test" {-# Comment #-}"