Skip/Optionally in ParserPrinter

[tikitu]

I've got input that I don't want to round-trip completely: there are some optionals I'm happy to remove (e.g. an optional trailing comma in a list of items), and some variable input that's just plain irrelevant for my use case (think PrefixUpTo("interesting stuff starts here") and Rest() and similar). But I would still like to use ParserPrinters to get round-tripping for free for the parts of the input that I do care about. What are my best options?

• Skip gives me Generic struct 'ParsePrint' requires the types 'Substring' and '()' be equivalent.
• Optionally { "," } still produces a ()? because the printer wants to know whether the option was chosen or not.

I can get the commas with

OneOf {
    ","
    Always(())
}

which seems awfully roundabout to me but does work. I don't know any way at all to drop the variable stuff, so for the moment I'm just carrying it around as unnecessary Substring arguments until I get to an AnyConversion where I can ignore it in apply and give "" in unapply.

I'm sure there are better patterns than these, what am I missing?

[oskarek]

Sounds like you should be able to make use of the .printing(_:) operator on Parser. It allows you to turn any Parser into a ParserPrinter, by explicitly telling it how to print.

Can be used like this:

let input = """
don't care don't care don't care
don't care really don't care
:123 don't care again
"""

struct MyParser: ParserPrinter {
  var body: some ParserPrinter<Substring, Int> {
    Skip { PrefixThrough(":") }.printing { _, _ in } // don't print anything
    Int.parser()
    Skip { Rest() }.printing { _, _ in } // don't print anything
  }
}

// parsing
let output = try MyParser().parse(input)
output // 123

// printing
let printedValue = try MyParser().print(100)
printedValue // "100"

[tikitu]

Beautiful, thanks!