You cannot do like that in the middle of pipeline, but instead you can use `
andThen
<http://package.elm-lang.org/packages/elm-tools/parser/2.0.1/Parser#andThen>`
to
make a new parser based on the previous value you parsed.
For example, you can pass `states` value to parse `acceptStates` like this.
(Simplified a lot, not tested)
dfaParser : Parser.Parser DFA
dfaParser =
statesParser
|> andThen (\states ->
succeed (DFA states)
|= alphabetParser
|= acceptStateParser states
|= ...
)
You can also make a recursive parser and pass the intermediate state to the
later parser.
deltaListParser : Context -> Parser.Parser (List Delta)
deltaListParser context =
oneOf
[ deltaParser
|> andThen (\delta ->
if checkDuplication delta context then
deltaListParser (updateContext delta context)
|> map (\rest -> delta :: rest)
else
fail "found duplicated values"
, succeed []
]
deltaParser : Parser.Parser Delta
That said, I don't think validation is necessary during the parsing
process. You can check it after everything is parsed. That is much simpler.
2017年8月2日水曜日 12時17分57秒 UTC+9 Dave Doty:
>
> The elm-tools/parser documentation recommends using parsing pipelines such
> as
>
> type alias Point = { x : Float, y : Float}
> point : Parser Pointpoint =
> succeed Point
> |. symbol "("
> |. spaces
> |= float
> |. spaces
> |. symbol ","
> |. spaces
> |= float
> |. spaces
> |. symbol ")"
> spaces : Parser ()spaces =
> ignore zeroOrMore (\c -> c == ' ')
>
>
> I am parsing text in this way, but it is much longer than just two floats.
> The high-level parser parses text with five major parts in order
> (describing portions of a finite automaton) and it looks like this (and
> uses five "subroutine" parsers that I've not shown):
>
> type alias State = String
> type alias DFATransition = State -> Char -> Result String State
> type alias DFA =
> { states : List State
> , inputAlphabet : List Char
> , startState : State
> , acceptStates : List State
> , delta : DFATransition
> }
>
> dfaParser : Parser.Parser DFA
> dfaParser =
> Parser.succeed DFA
> |. spaces
> |. Parser.keyword "states:"
> |. spaces
> |= statesParser
> |. spaces
> |. Parser.keyword "input_alphabet:"
> |. spaces
> |= alphabetParser
> |. spaces
> |. Parser.keyword "start_state:"
> |. spaces
> |= startStateParser
> |. spaces
> |. Parser.keyword "accept_states:"
> |. spaces
> |= statesParser
> |. spaces
> |. Parser.keyword "delta:"
> |. spaces
> |= deltaParser
> |. spaces
>
> to parse text such as, for instance,
>
> """
> states: {q,q0,q00,q000}
> input_alphabet: {0,1}
> start_state: q
> accept_states: {q,q0,q00}
> delta:
> q,1 -> q
> q0,1 -> q
> q00,1 -> q
> q000,1 -> q
> q,0 -> q0
> q0,0 -> q00
> q00,0 -> q000
> q000,0 -> q000
> """
>
> Here's what I want to do: insert code in the middle of the pipeline that
> can reference the data that has been parsed so far.
>
> For example, after this portion of the pipeline has succeeded:
>
> dfaParser =
> Parser.succeed DFA
> |. spaces
> |. Parser.keyword "states:"
> |. spaces
> |= statesParser
> |. spaces
> |. Parser.keyword "input_alphabet:"
> |. spaces
> |= alphabetParser
> ...
>
> then the data for states and alphabet have been successfully parsed into
> two Lists. I would like to access those lists by name, later down the
> pipeline.
>
> One reason is that I would like to pass those lists as input to subsequent
> parsers (startStateParser, acceptStatesParser, and deltaParser), to help
> them do error-checking.
>
> For example, the next thing parsed is a String parsed by startStateParser,
> and I want to ensure that the parsed String is an element of the List
> parsed by statesParser. But at the time I put the line |= startStateParser
> in the pipeline, the parsed result of statesParser doesn't have a name
> that I can refer to.
>
> Another reason is that I want to do error-checking in the middle of a
> pipeline. For example, my implementation of deltaParser reads the lines
> such as "q,0 -> q0" and "q0,1 -> q" one at a time, and I would like to
> access data parsed by previous lines when looking for errors on the current
> line. (For example, it is an error to have duplicates on the left side of
> -> such as the line "q,1 -> q" followed later by "q,1 -> q0", but to
> indicate this error and reference the correct line number, I need access to
> the lines parsed so far as I am processing the line with the error.)
>
> I get the feeling that perhaps I'm structuring this incorrectly, so I
> welcome advice on a better way to structure the parser.
>
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