Iterator Expression Trees
This repository contains a prototype of expression tree support for iterators. It should be merged with the Microsoft.CSharp.Expressions effort and forms the basis for experiments with async iterators and bi-directional iterators.
Usage
An example of constructing, compiling, instantiating, and iterating over a Range iterator is shown below:
var start = Expression.Parameter(typeof(int), "start");
var count = Expression.Parameter(typeof(int), "count");
var i = Expression.Parameter(typeof(int), "i");
var max = Expression.Parameter(typeof(long), "max");
var @break = Expression.Label("__Break");
var expr =
CSharpExpression.Iterator<Func<int, int, IIterable<int>>>(
Expression.Block(
new[] { i, max },
Expression.Assign(max, Expression.Subtract(Expression.Add(Expression.Convert(start, typeof(long)), Expression.Convert(count, typeof(long))), Expression.Constant(1L, typeof(long)))),
Expression.IfThen(
Expression.OrElse(
Expression.LessThan(count, Expression.Constant(0)),
Expression.GreaterThan(max, Expression.Constant((long)int.MaxValue))
),
Expression.Throw(Expression.New(typeof(ArgumentOutOfRangeException).GetConstructor(new[] { typeof(string) }), Expression.Constant("count")))
),
Expression.Assign(i, Expression.Constant(0)),
Expression.Loop(
Expression.Block(
Expression.IfThen(
Expression.GreaterThanOrEqual(i, count),
Expression.Break(@break)
),
CSharpExpression.YieldReturn(Expression.Add(start, i)),
Expression.PostIncrementAssign(i)
),
@break
)
),
start,
count
);
IIterable<int> iterator = expr.Compile()(5, 10);
foreach (var x in iterator)
{
Console.WriteLine(x);
}Goals
The goals of this project are:
- Extend the C# expression tree extensions to support iterators. Hopefully we can get iterator lambdas at some point.
- Play with implementation techniques for async iterators and bi-directional iterators.
- Experiment with operator fusion, a technique where iterators get inlined, allowing for runtime LINQ query optimization.