Creating a parser for expression trees about ecsharp

[qwertie]

Well, me, I haven't ever had a direct use for expression trees. I don't even remember how they deal with typing information (is it... optionally typed?). I probably used them in Linq-to-SQL, but that's it, since I find parsing of "any executable code block" or "any code block" to be a much more useful concept. So I am curious, if you had a expression-only parser, what would you use it for?

It would be really appreciated if you could figure out how to parse LINQ expressions into LNodes first by modifying the existing Ecs/Parser/EcsParserGrammar.les in this repo... but if you prefer to go the other route and directly produce expression trees from all expessions (including LINQ), I'd be curious to see the results. Back when I looked into parsing LINQ expressions, I had difficulty finding complete information about the various kinds of LINQ expressions and how they are translated into C#-without-lambda-expressions. Technically, that information can be found in the C# language reference, but it is not easy reading.

If you want to rip out the EC# expression parser and turn it into a plain-C# expression parser, you'd want to remove the EC#-specific things. The following comes to mind:

• EC# allows braced blocks in expressions, like 2 + {bool c = x*y; if (c > 0) x else y}
• EC# allows tuples like (x, y)
• EC# allows variable declarations in expressions, like (var Foo2 = Foo*Foo) * Foo2

Making it more complicated to remove the latter two features is the fact that there is no special rule for lambda expressions; lambda is treated like any other infix operator! Thus 5 => 3+2 is valid, and (x, y) => z has a tuple on the LHS. So... it would be easier to write a converter from LNode to expression trees, but then your code will have more dependencies than you probably wanted.

Just a reminder: if you add Ecs.exe and its dependencies like Loyc.Syntax.dll as references to your project, you can parse a specified expression by calling

LNode expr = Ecs.Parser.EcsLanguageService.Value.ParseSingle(
    "your_expression(here)", null, ParsingService.Exprs);

To avoid creating intermediate LNodes, I think you could make new derived classes of System.Linq.Expressions.Expression to represent tuples and variable declarations, and then postprocess the output of the parser from (A a, B b) => expr (where the left-hand side is a tuple) into an Expression<A, B>. (Btw I'm a little confused: when I look at the documentation of Expression it doesn't seem to have type parameters, and I don't see any links to the version of Expression that does have type parameters.)

[qwertie]

P.S. Thanks for your interest in LLLPG/Loyc, and let me know if I forgot to respond to anything!

[qwertie]

@glopesdev it appears based on this question that you can already parse a string into an expression tree at run-time.

However, it would be straightforward to do it in Enhanced C# using matchCode - have a look at this example and imagine replacing the "Eval" function with a "CreateExpressionTree" function.