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Text.ParserCombinators.Parsec.Prim | Portability | portable | Stability | provisional | Maintainer | derek.a.elkins@gmail.com |
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Description |
Parsec compatibility module
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Synopsis |
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Documentation |
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The parser p ? msg behaves as parser p, but whenever the
parser p fails without consuming any input, it replaces expect
error messages with the expect error message msg.
This is normally used at the end of a set alternatives where we want
to return an error message in terms of a higher level construct
rather than returning all possible characters. For example, if the
expr parser from the try example would fail, the error
message is: '...: expecting expression'. Without the (<?>)
combinator, the message would be like '...: expecting "let" or
letter', which is less friendly.
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This combinator implements choice. The parser p <|> q first
applies p. If it succeeds, the value of p is returned. If p
fails without consuming any input, parser q is tried. This
combinator is defined equal to the mplus member of the MonadPlus
class and the (Control.Applicative.<|>) member of Control.Applicative.Alternative.
The parser is called predictive since q is only tried when
parser p didn't consume any input (i.e.. the look ahead is 1).
This non-backtracking behaviour allows for both an efficient
implementation of the parser combinators and the generation of good
error messages.
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parse p filePath input runs a parser p over Identity without user
state. The filePath is only used in error messages and may be the
empty string. Returns either a ParseError (Left)
or a value of type a (Right).
main = case (parse numbers "" "11, 2, 43") of
Left err -> print err
Right xs -> print (sum xs)
numbers = commaSep integer
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parseFromFile p filePath runs a string parser p on the
input read from filePath using readFile. Returns either a ParseError
(Left) or a value of type a (Right).
main = do{ result <- parseFromFile numbers "digits.txt"
; case result of
Left err -> print err
Right xs -> print (sum xs)
}
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The expression parseTest p input applies a parser p against
input input and prints the result to stdout. Used for testing
parsers.
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:: Stream s Identity t | | => t -> String | Computes the position of a token.
| -> t -> SourcePos | Matching function for the token to parse.
| -> t -> Maybe a | | -> Parsec s u a | | The parser token showTok posFromTok testTok accepts a token t
with result x when the function testTok t returns Just x. The
source position of the t should be returned by posFromTok t and
the token can be shown using showTok t.
This combinator is expressed in terms of tokenPrim.
It is used to accept user defined token streams. For example,
suppose that we have a stream of basic tokens tupled with source
positions. We can than define a parser that accepts single tokens as:
mytoken x
= token showTok posFromTok testTok
where
showTok (pos,t) = show t
posFromTok (pos,t) = pos
testTok (pos,t) = if x == t then Just t else Nothing
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:: Stream s m t | | => t -> String | Next position calculating function.
| -> SourcePos -> t -> s -> SourcePos | Matching function for the token to parse.
| -> t -> Maybe a | | -> ParsecT s u m a | | The parser token showTok nextPos testTok accepts a token t
with result x when the function testTok t returns Just x. The
token can be shown using showTok t. The position of the next
token should be returned when nextPos is called with the current
source position pos, the current token t and the rest of the
tokens toks, nextPos pos t toks.
This is the most primitive combinator for accepting tokens. For
example, the Text.Parsec.Char.char parser could be implemented as:
char c
= tokenPrim showChar nextPos testChar
where
showChar x = "'" ++ x ++ "'"
testChar x = if x == c then Just x else Nothing
nextPos pos x xs = updatePosChar pos x
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The parser unexpected msg always fails with an unexpected error
message msg without consuming any input.
The parsers fail, (<?>) and unexpected are the three parsers
used to generate error messages. Of these, only (<?>) is commonly
used. For an example of the use of unexpected, see the definition
of Text.Parsec.Combinator.notFollowedBy.
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many p applies the parser p zero or more times. Returns a
list of the returned values of p.
identifier = do{ c <- letter
; cs <- many (alphaNum <|> char '_')
; return (c:cs)
}
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skipMany p applies the parser p zero or more times, skipping
its result.
spaces = skipMany space
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Returns the current user state.
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An alias for putState for backwards compatibility.
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An alias for modifyState for backwards compatibility.
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Returns the current source position. See also SourcePos.
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setPosition pos sets the current source position to pos.
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Returns the current input
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setInput input continues parsing with input. The getInput and
setInput functions can for example be used to deal with #include
files.
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Constructors | State | | stateInput :: s | | statePos :: !SourcePos | | stateUser :: !u | |
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Returns the full parser state as a State record.
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setParserState st set the full parser state to st.
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Produced by Haddock version 2.6.0 |