Proposal: Support grammar aliases.

[alecthomas]

In moderately complex grammars it's fairly common to see duplicate patterns emerge. For example, when matching a dot-separated identifier (eg. foo.bar) the pattern (Ident { "." Ident }) is used repeatedly. This can be handled by a Go type alias implementing the Parseable interface, but that is quite onerous.

I propose adding support for grammar aliases. Here's an example creating and using an Identifier alias:

type Assignment struct {
  Name string `@Identifier=(Ident { "." Ident })`
  Variable string `"=" @Identifier`
}

[shahms]

Alternatively (or in addition to) just making it easier to go from a grammar string to something which implements Parseable would be nice. One way of factoring out the common patterns is to put them in a separate struct and using @@, but that introduces an otherwise unnecessary layer of indirection.

Having a facility parallel to Build which took a "root" instance and grammar string and returned a semi-parser would go a long way, e.g.

var identParser = participle.MustParseable(&Identifier{}, `@(Ident { "." Ident })`)

type Identifer string
type Assignment struct {
  Name Identifier `@@`
  Variable Identifier `"=" @@`
}

func (i *Identifer) Parse(l lexer.PeekingLexer) error {
  return identParser.ParseInto(i, l)
}

[alecthomas]

That's not a bad idea at all. All it would take would be to add an extra method to the Parser.

[alecthomas]

Though it's a bit inconvenient in that, as you say, you end up with a separate struct where you might only need a string or int or whatever.

[testitm]

any updates?

[alecthomas]

I haven't thought of a good API for this yet unfortunately.

[alecthomas]

My usual pattern is to pull the field out into a struct. It's a bit uglier for the AST though.

[jmalloc]

Is there any accepted approach that I could use to do what @shahms suggested in this comment?

I'm trying to understand the most appropriate way to "jump back" into the normal grammar from within an implementation of Parseable that implements precedence climbing as per the example.