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Text.ParserCombinators.PolyStateLazy |
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Synopsis |
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The Parser datatype.
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Parsers do not return explicit failure. An exception is raised
instead. This allows partial results to be returned before a
full parse is complete.
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The Parser datatype is a fairly generic parsing monad with error
reporting and a running state. It can be used for arbitrary token
types, not just String input.
| Constructors | P (s -> [t] -> (Either String a, s, [t])) | |
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Apply a parser to an initial state and input token sequence.
The parser cannot return an error value explicitly, so errors
raise an exception. Thus, results can be partial (lazily constructed,
but containing undefined).
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Simple failure can be corrected, but when a simple fail is not strong
enough, use failBad for emphasis. It guarantees parsing will terminate
with an exception.
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Commit is a way of raising the severity of any errors found within
its argument. Used in the middle of a parser definition, it means that
any operations prior to commitment fail softly, but after commitment,
they fail hard.
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Combinators:
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Primitives
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One token
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One token satifying a predicate
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Apply a parsed function to a parsed value
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x discard y parses both x and y, but discards the result of y
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Error-handling
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p adjustErr f applies the transformation f to any error message
generated in p, having no effect if p succeeds.
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adjustErrBad is just like adjustErr except it also raises the
severity of the error.
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indent :: Int -> String -> String | Source |
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Helper for formatting error messages: indents all lines by a fixed amount.
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Choices
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p onFail q means parse p unless p fails in which case parse q instead.
Can be chained together to give multiple attempts to parse something.
(Note that q could itself be a failing parser, e.g. to change the error
message from that defined in p to something different.)
However, a *severe* failure in p cannot be ignored.
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Parse the first alternative in the list that succeeds.
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Parse the first alternative that succeeds, but if none succeed,
report only the severe errors, and if none of those, then report
all the soft errors.
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optional indicates whether the parser succeeded through the Maybe type.
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Sequences
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'exactly n p' parses a precise number of items, n, using the parser
p, in sequence.
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'many p' parses a list of elements with individual parser p.
Cannot fail, since an empty list is a valid return value.
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Parse a non-empty list of items.
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Parse a list of items separated by discarded junk.
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Parse a non-empty list of items separated by discarded junk.
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Parse a list of items, discarding the start, end, and separator
items.
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Parse a bracketed item, discarding the brackets.
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'manyFinally e t' parses a possibly-empty sequence of e's,
terminated by a t. Any parse failures could be due either to
a badly-formed terminator or a badly-formed element, so raise
both possible errors.
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State-handling
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Update the internal state.
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Query the internal state.
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Deliver the entire internal state.
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Re-parsing
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Push some tokens back onto the front of the input stream and reparse.
This is useful e.g. for recursively expanding macros. When the
user-parser recognises a macro use, it can lookup the macro
expansion from the parse state, lex it, and then stuff the
lexed expansion back down into the parser.
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Produced by Haddock version 0.8 |