> you're basically saying you can do lexical analysis and semantic analysis in the same function.

yes

> that makes the code that much hairier

true, but it does save you the rigamarole of the lexer.

    func parseDeclaration(s string, i0 Pos) (n Node, i Pos, err error) {
        i = i0
        i = skipSpace(s, i)
        if word(s, i, "var") {
            v := &ast.VarDecl{VarPos: i}
            i += 3
            i = skipSpace(s, i)
            if v.Ident, i, err = parseIdent(s, i); err != nil {
                return
            }
            i = skipSpace(s, i)
            if is(s, i, "=") {
                i += 1
                i = skipSpace(s, i)
                if v.Value, i, err = parseExpr(s, i); err != nil {
                    return
                }
            }
            if !is(s, i, ";") {
                return nil, i, fmt.Errorf(`unexpected input or EOF (expected ";")`)
            }
            ⋮