proposal: allowing non-string types to Capture tokens

[torbenschinke]

I can only use the Capture(values []string) error interface on types with the underlying string type. It seems like an unneeded restriction.

In setField nodes.go:592 the type is asserted to a string to assemble the slice parameter for the Capture method. I wonder if we could use the lexer tokens to build the same just without the type assertion.

//			for _, v := range fieldValue {
//				ifv = append(ifv, v.Interface().(string))
//			}

			for _, token := range tokens {
				ifv = append(ifv, token.Value)
			}

[alecthomas]

That is incorrect. You can implement Capture on any type. See the hcl example where it captures into a
bool.

[torbenschinke]

Well, on structs it does not work:

type Boxes struct {
	Pos          lexer.Position
	Boxes Box `@@`
}

type Box struct{
	Pos          lexer.Position
	Val string `@Ident`
}

func (b *Box) Capture(values []string) error {
	b.Val = values[0]
	return nil
}

func TestBoxedCapture(t *testing.T) {
	lex := stateful.MustSimple([]stateful.Rule{
		{"Ident", `[a-zA-Z](\w|\.|/|:|-)*`, nil},
		{"whitespace", `\s+`, nil},
	})

	parser := participle.MustBuild(&Boxes{},
		participle.Lexer(lex),
		participle.UseLookahead(2),
	)

	boxed:=&Boxes{}
	if err:=parser.ParseString("test", "abc::cdef.abc", boxed);err!=nil{
		t.Fatal(err)
	}
}

This causes a type assertion panic:

panic: interface conversion: interface {} is parser.Box, not string [recovered]
	panic: interface conversion: interface {} is parser.Box, not string

goroutine 6 [running]:
testing.tRunner.func1.2(0x1166b80, 0xc00006c810)
	/usr/local/go/src/testing/testing.go:1144 +0x332
testing.tRunner.func1(0xc000001380)
	/usr/local/go/src/testing/testing.go:1147 +0x4b6
panic(0x1166b80, 0xc00006c810)
	/usr/local/go/src/runtime/panic.go:965 +0x1b9
github.com/alecthomas/participle/v2.setField(0xc000022880, 0x1, 0x1, 0x116f320, 0xc0000585a0, 0x199, 0x114c743, 0x5, 0x0, 0x0, ...)
	/Users/tschinke/go/pkg/mod/github.com/alecthomas/participle/v2@v2.0.0-alpha4/nodes.go:582 +0x23e5
github.com/alecthomas/participle/v2.(*parseContext).Apply(0xc0000102d0, 0xc000022880, 0x1)
	/Users/tschinke/go/pkg/mod/github.com/alecthomas/participle/v2@v2.0.0-alpha4/context.go:53 +0x150
github.com/alecthomas/participle/v2.(*strct).Parse(0xc000114d20, 0xc0000102d0, 0x116f320, 0xc0000584e0, 0x199, 0x199, 0xc00007fcc8, 0x113635e, 0x1168200, 0xc00006c4b0)
	/Users/tschinke/go/pkg/mod/github.com/alecthomas/participle/v2@v2.0.0-alpha4/nodes.go:121 +0x5cc
github.com/alecthomas/participle/v2.(*Parser).parseInto(0xc000114c40, 0xc0000102d0, 0x1156840, 0xc0000584e0, 0x16, 0x11c23c8, 0x128e940)
	/Users/tschinke/go/pkg/mod/github.com/alecthomas/participle/v2@v2.0.0-alpha4/parser.go:239 +0xd0
github.com/alecthomas/participle/v2.(*Parser).parseOne(0xc000114c40, 0xc0000102d0, 0x1156840, 0xc0000584e0, 0x16, 0xc000022800, 0x1)
	/Users/tschinke/go/pkg/mod/github.com/alecthomas/participle/v2@v2.0.0-alpha4/parser.go:222 +0x77
github.com/alecthomas/participle/v2.(*Parser).ParseFromLexer(0xc000114c40, 0xc000114e70, 0x1156840, 0xc0000584e0, 0x0, 0x0, 0x0, 0x0, 0x0)
	/Users/tschinke/go/pkg/mod/github.com/alecthomas/participle/v2@v2.0.0-alpha4/parser.go:152 +0x4c5
github.com/alecthomas/participle/v2.(*Parser).parse(0xc000114c40, 0x11c1a28, 0xc000058540, 0x1156840, 0xc0000584e0, 0x0, 0x0, 0x0, 0x0, 0xc0000584e0)
	/Users/tschinke/go/pkg/mod/github.com/alecthomas/participle/v2@v2.0.0-alpha4/parser.go:160 +0xe5
github.com/alecthomas/participle/v2.(*Parser).ParseString(0xc000114c40, 0x118ff92, 0x4, 0x11918f3, 0xd, 0x1156840, 0xc0000584e0, 0x0, 0x0, 0x0, ...)
	/Users/tschinke/go/pkg/mod/github.com/alecthomas/participle/v2@v2.0.0-alpha4/parser.go:189 +0xf5
github.com/golangee/architecture/adl/parser.TestBoxedCapture(0xc000001380)
	/Users/tschinke/git/github.com/golangee/architecture.git/adl/parser/parser_test.go:73 +0x1fa
testing.tRunner(0xc000001380, 0x119ad10)
	/usr/local/go/src/testing/testing.go:1194 +0xef
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:1239 +0x2b3

[torbenschinke]

I found a captureableWithPosition struct in parser_test.go but it is not used anywhere for testing, perhaps for a reason?