How to retrieve value from optional parser in Parsec?

Question

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++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

Answer 1

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.