発展: 順列表現

まんま

module Main where

import Text.ParserCombinators.Parsec
--import Text.ParserCombinators.Parsec.Char
import Text.ParserCombinators.Parsec.Perm

perm0 = permute (f (<$$>) char 'a'
                   (<||>) char 'b'
                   (<||>) char 'c')
      where
        f a b c  = [a,b,c]

で、

Prelude> :l 20080928_parsec00.hs
Compiling Main             ( 20080928_parsec00.hs, interpreted )

20080928_parsec00.hs:8:
    Couldn't match `[t]' against `t1 -> t2'
        Expected type: [t]
        Inferred type: t1 -> t2
    Probable cause: `f' is applied to too many arguments in the call
        (f (<$$>) char 'a' (<||>) char 'b' (<||>) char 'c')
    In the first argument of `permute', namely
        `(f (<$$>) char 'a' (<||>) char 'b' (<||>) char 'c')'
Failed, modules loaded: none.

？

まんま

module Main where

import Text.ParserCombinators.Parsec
--import Text.ParserCombinators.Parsec.Char
import Text.ParserCombinators.Parsec.Perm

perm1 :: Parser (String,Char,Char)
perm1 = permute (tuple <$?> ("",many1 (char 'a'))
                       <||> char 'b' 
                       <|?> ('_',char 'c'))
      where
        tuple a b c  = (a,b,c)

run :: Show a => Parser a -> String -> IO ()
run p input
        = case (parse p "" input) of
            Left err -> do{ putStr "parse error at "
                          ; print err
                          }
            Right x  -> print x

で、

*Main> run perm1 "caaaaab"
("aaaaa",'b','c')
*Main> run perm1 "cb"
("",'b','c')
*Main> run perm1 "b"
("",'b','_')
*Main> run perm1 ""
parse error at (line 1, column 1):
unexpected end of input
expecting "c", "b" or "a"
*Main> run perm1 "c"
parse error at (line 1, column 2):
unexpected end of input
expecting "b" or "a"
*Main> run perm1 "ca"
parse error at (line 1, column 3):
unexpected end of input
expecting "a" or "b"