How to retrieve value from optional parser in Parsec?
Sorry if it's a novice question - I want to parse something defined by
Exp ::= Mandatory_Part Optional_Part0 Optional_Part1
I thought I could do this:
proc::Parser String
proc = do {
;str<-parserMandatoryPart
;str0<-optional(parserOptionalPart0) --(1)
;str1<-optional(parserOptionalPart1) --(2)
;return str++str0开发者_JAVA百科++str1
}
I want to get str0/str1 if optional parts are present, otherwise, str0/str1 would be "". But (1) and (2) won't work since optional() doesn't allow extracting result from its parameters, in this case, parserOptionalPart0/parserOptionalPart1.
Now What would be the proper way to do it?
Many thanks!
Billy R
The function you're looking for is optionMaybe
. It returns Nothing
if the parser failed, and returns the content in Just
if it consumed input.
From the docs:
option x p
tries to apply parserp
. Ifp
fails without consuming input, it returns the valuex
, otherwise the value returned byp
.
So you could do:
proc :: Parser String
proc = do
str <- parserMandatoryPart
str0 <- option "" parserOptionalPart0
str1 <- option "" parserOptionalPart1
return (str++str0++str1)
Watch out for the "without consuming input" part. You may need to wrap either or both optional parsers with try
.
I've also adjusted your code style to be more standard, and fixed an error on the last line. return
isn't a keyword; it's an ordinary function. So return a ++ b
is (return a) ++ b
, i.e. almost never what you want.
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