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 Apply(Expression head, Expression tail)
{
_head = head;
_tail = tail;
}
public override Expression Replace(Symbol variable, Expression argument)
{
return this;
}
public override Expression Replace(Symbol variable, Expression argument)
{
if (_variable == variable)
return argument;
return this;
}
public Expression Apply(Expression argument)
{
return _body.Replace(_variable, argument);
}
public Lambda(Symbol variable, Expression body)
{
_variable = variable;
_body = body;
}
public abstract Expression Replace(Symbol variable, Expression argument);