func (p *Parser) parseTree(tree *ast.ListCell) (ast.Cell, error) {
for {
switch t := <-p.tokens; t.Type {
// tree is done
case token.RParen:
return tree, nil
// EOF found before tree is done, error
case token.EOF:
return ast.NewEOF(), fmt.Errorf(
"syntax error: hanging parenthese, s-expression %v does not close", tree)
default:
val, err := p.parseAST(t)
if err != nil {
return val, err
}
tree.Add(val)
}
}
}
func (p *Parser) parseAST(t token.Item) (ast.Cell, error) {
var val ast.Cell
var err error
switch t.Type {
// use following tokens to recursivly build the tree
case token.LParen:
val, err = p.parseTree(ast.NewList())
// this "tree" is a single sym
case token.Sym:
val, err = ast.NewSym(t.Value)
// this "tree" is a single constant
case token.Const:
val, err = p.parseConst(t)
// the next token is quoted
case token.Quote:
val, err = p.parseQuote(t)
// close paren found without open paren, error
case token.RParen:
val, err = nil, errors.New("syntax error: close parenthese ')' without s-expression")
case token.EOF:
val, err = ast.NewEOF(), nil
}
return val, err
}