Parsing Lambda expression pre-evalutes it with given arguments

When Parsing lambda expression, it evaluates the predicate and cannot be reused (e.g. from cache) with another parameter value. See an example that demonstrates it.

var target = new Interpreter(InterpreterOptions.Default | InterpreterOptions.LambdaExpressions);
var list = new Parameter("list", new[] { 1, 2, 3 });
var value1 = new Parameter("value", 1);
var value2 = new Parameter("value", 2);
var expression = "list.Where(x => x > value)";
var lambda = target.Parse(expression, list, value1);
var result = lambda.Invoke(list, value2);
Assert.AreEqual(new[] { 3 }, result);

[metoule]

I have an idea that might work, but I’m not yet sure it will. The idea is to create a dictionary of all the captured variables, and access it when needed:

• create a body (instead of a single expression) for inner lambda expressions
• create an expression that sets the dictionary’s value for all parent parameters on enter
• parse the expression, and when an identifier is encountered, check whether it’s a captured variable

Proof of concept of such a body’s creation:

var dict = new Dictionary<string, object>();
var captured = Expression.Constant(dict);

// create an expression for x + 5
var param = Expression.Parameter(typeof(int), "x");
var five = Expression.Constant(5);
var add = Expression.Add(param, five);

// create the expression that sets the captured variable's value
// => dict["x"] = 5
var indexer = Expression.Property(captured, dict.GetType().GetProperty("Item"), new Expression[] { Expression.Constant(param.Name) });
var set = Expression.Assign(indexer, Expression.Convert(param, typeof(object)));

// create the delegate with a body
var block = Expression.Block(set, add);
var lambda = Expression.Lambda(block, param);
var del = lambda.Compile();

del.DynamicInvoke(2).Dump();
dict.Dump(); // contains x == 2

del.DynamicInvoke(3).Dump();
dict.Dump(); // contains x == 3

[metoule]

I have a working prototype, that also fixes the limitation I highlighted in #200. It still needs some work, but I might wait until your PR is merged because it impacts the Lambda class.