Option<Term> singleTerm(TokenStream& tokens) {
if (tokens->type == Token::LAMBDA) {
return lambdaTerm(tokens);
} else if (tokens->type == Token::OPEN_BRACKET) {
tokens.advance();
Option<Term> t = term(tokens);
if (tokens->type == Token::CLOSE_BRACKET) {
tokens.advance();
return t;
} else return errorTerm("expected closing bracket");
} else if (tokens->type == Token::IDENTIFIER) {
string id = tokens->identifierValue;
tokens.advance();
return Option<Term>(variableTerm(id));
} else if (tokens->type == Token::INTEGER) {
int value = tokens->intValue;
tokens.advance();
return Option<Term>(integerTerm(value));
} else if (tokens->type == Token::CLOSE_BRACKET) {
return errorTerm("unexpected closing bracket");
} else if (!tokens.good()) {
return errorTerm("expected a term");
} else return errorTerm("unknown token");
}
Option<Term> term(TokenStream& tokens) {
Option<Term> current = singleTerm(tokens);
if (current.exists()) {
while (tokens.good() && tokens->type != Token::CLOSE_BRACKET) {
Option<Term> next = singleTerm(tokens);
if (next.exists()) {
current = Option<Term>(applicationTerm(*current, *next));
} else return next;
}
return current;
} else return current;
}