private static Expression ParseTerm(TokenStream tokenStream, SymbolTable symbols)
{
if (tokenStream.Peek() == '(')
{
tokenStream.Consume('(');
var inner = ParseExp(tokenStream, symbols);
tokenStream.Consume(')');
return inner;
}
if (tokenStream.Peek() == 'λ')
{
tokenStream.Consume('λ');
char id = tokenStream.Peek();
if (!Char.IsLetter(id))
throw new ParserException(String.Format("Expected identifier at position {0}", tokenStream.Position));
tokenStream.Consume(id);
tokenStream.Consume('.');
var variable = new Symbol(id);
var body = ParseExp(tokenStream, symbols.Plus(id, variable));
return new Lambda(variable, body);
}
if (Char.IsLetter(tokenStream.Peek()))
{
char id = tokenStream.Peek();
tokenStream.Consume(id);
var variable = symbols.Lookup(id);
if (variable != null)
return new BoundVariable(variable);
else
return new FreeVariable(id);
}
throw new ParserException(String.Format("Unexpected character \"{0}\" at position {1}", tokenStream.Peek(), tokenStream.Position));
}
private static Expression ParseExp(TokenStream tokenStream, SymbolTable symbols)
{
var head = ParseTerm(tokenStream, symbols);
while (true)
{
if (tokenStream.Done() || tokenStream.Peek() == ')')
return head;
var next = ParseTerm(tokenStream, symbols);
head = new Apply(head, next);
}
}