public Expression CreateList(IEnumerable<Expression> list)
{
BuiltinList head = null;
BuiltinList tail = null;
foreach (var item in list)
{
if (head == null)
{
head = new BuiltinList { First = item, Second = null };
tail = head;
}
else
{
tail.Second = new BuiltinList { First = item, Second = null };
tail = tail.Second;
}
}
return CreateList(head);
}
public Expression CreateList(BuiltinList head)
{
return new FreeSymbolExpression { Name = "_List", Tag = head, Display = _ListDisplay };
}
public static bool RecognizeList(Expression expression, out BuiltinList value)
{
BuiltinList head = null;
BuiltinList tail = null;
Expression current = expression;
// \x,y,z.y is Null
// \f.f <FIRST> <SECOND> is Pair
value = null;
while (true)
{
if (current is AbstractionExpression)
{
var abs1 = (AbstractionExpression)current;
if (abs1.Right is ApplicationExpression)
{
var app1 = (ApplicationExpression)abs1.Right;
if (app1.Left is ApplicationExpression)
{
var app2 = (ApplicationExpression)app1.Left;
if (app2.Left is BoundSymbolExpression)
{
var bound = (BoundSymbolExpression)app2.Left;
if (bound.Symbol == abs1.Left)
{
// Pair
if (head == null)
{
head = new BuiltinList { First = app2.Right, Second = null };
tail = head;
}
else
{
tail.Second = new BuiltinList { First = app2.Right, Second = null };
tail = tail.Second;
}
current = app1.Right;
continue;
}
}
}
}
else if (abs1.Right is AbstractionExpression)
{
var abs2 = (AbstractionExpression)abs1.Right;
if (abs2.Right is AbstractionExpression)
{
var abs3 = (AbstractionExpression)abs2.Right;
if (abs3.Right is BoundSymbolExpression)
{
var bound = (BoundSymbolExpression)abs3.Right;
if (bound.Symbol == abs2.Left)
{
// Null
value = head;
return true;
}
}
}
}
}
return false;
}
}