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));
}
public SymbolTable Plus(char id, Symbol variable)
{
var symbols = _symbols.ContainsKey(id)
? _symbols.Remove(id)
: _symbols;
symbols = symbols.Add(id, variable);
return new SymbolTable(symbols);
}
public override Expression Replace(Symbol variable, Expression argument)
{
var newHead = _head.Replace(variable, argument);
var newTail = _tail.Replace(variable, argument);
if (newHead == _head && newTail == _tail)
return this;
return new Apply(newHead, newTail);
}
public override Expression Replace(Symbol variable, Expression argument)
{
var newBody = _body.Replace(variable, argument);
if (newBody == _body)
return this;
ImmutableList<char> taken = newBody.FreeVariableNames(new Symbol[] { _variable }.ToImmutableList());
if (taken.Contains(_variable.Default))
{
int ordinal = (int)(char.ToLower(_variable.Default) - 'a') + 1 % 26;
while (taken.Contains((char)('a' + ordinal)))
ordinal++;
var newVariable = new Symbol((char)('a' + ordinal));
newBody = newBody.Replace(_variable, new BoundVariable(newVariable));
return new Lambda(newVariable, newBody);
}
return new Lambda(_variable, newBody);
}
public override bool DependsUpon(Symbol variable)
{
return _head.DependsUpon(variable) || _tail.DependsUpon(variable);
}
public override Expression Replace(Symbol variable, Expression argument)
{
return this;
}
public override bool DependsUpon(Symbol variable)
{
return false;
}
public override Expression Replace(Symbol variable, Expression argument)
{
if (_variable == variable)
return argument;
return this;
}
public override bool DependsUpon(Symbol variable)
{
return _variable == variable;
}
public BoundVariable(Symbol variable)
{
_variable = variable;
}
public override bool DependsUpon(Symbol variable)
{
return _body.DependsUpon(variable);
}
public Lambda(Symbol variable, Expression body)
{
_variable = variable;
_body = body;
}
public abstract bool DependsUpon(Symbol variable);
public abstract Expression Replace(Symbol variable, Expression argument);
