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the Lexical Macro Processor for C#


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LeMP is a new open-source LISP-style macro processor for C#, comparable to sweet.js for JavaScript. Are you a good developer, but reluctant to “buy into” commercial tools such as PostSharp to enhance your productivity? If so, LeMP — a preprocessor built on a parser called Enhanced C# — can make you more productive.

Design patterns are a valuable conceptual tool for developers, but some of them - especially complex ones like the Visitor Pattern, or ones that require lots of boilerplate like Decorator - arguably demonstrate that the language being used isn’t powerful enough. When used in conventional languages, many design patterns can only work by convention and cannot be encapsulated in a library or component, so they involve repetition and thus violate the DRY principle (don’t repeat yourself).

A LISP-style macro processor helps you solve the repetition-of-boilerplate problem, and it also provides a framework in which you can run sophisticated algorithms at compile-time (for example, have a look at LLLPG, just one of many macros included with LeMP.)


It’s not just design patterns. Any code pattern that involves unnecessary repetition is a sign of weakness in your programming language.

Example: using

A really simple example is ‘using’ statements:

using System;
using System.Linq;
using System.Text;
using System.Collections;
using System.Collections.Generic;
using System.IO;
using Loyc.Collections;
using Loyc.MiniTest;
using Loyc.Syntax;

Luckily, Visual Studio can add these for us. But wouldn’t it be nice if half the screen wasn’t ‘using’ statements every time you open a file? There is a LeMP macro that lets you collapse these onto a couple of lines:

using System(.Linq, .Text, .Collections(, .Generic), .IO, );
using Loyc(.Collections, .MiniTest, .Syntax);

The comma , before the closing ) adds an “empty” parameter to the list, which indicates that using System itself is one of the outputs you want to produce.

Example: Null checking

As long as there is no such thing as non-nullable reference types, we’ll be checking if our method parameters are null, and if we’re extra careful, we might check our return value, too. This can be done in the traditional way,

static string Twice(string s)
	if (s != null)
		throw new ArgumentNullException("s");
	return s + s;

Or in the new way, using Microsoft Code Contracts:

static string Twice(string s)
	Contract.Requires(s != null)
	return s + s;

But with LeMP, it’s a one-liner:

static string Twice(notnull string s) => s + s;

Your output file will say

static string Twice(string s)
	Contract.Assert(s != null, "Precondition failed: s != null")
	return s + s;

Note: This feature does not require the MS Code Contracts rewriter to be installed in Visual Studio, since LeMP has a built-in “rewriter” of its own, and it relies on Contract.Assert, one of the only methods of the Contracts class that does not require the rewriter. This behavior is customizable, e.g. LeMP can be told to use the standard methods instead, such as Contract.Requires and Contract.Ènsures.)

The notnull attribute can be applied to the return value, as well, to check at run-time that a method does not return null. However, notnull is not supported on ordinary variables. LeMP also includes other “code contract” attributes. For example, the notnull modifier actually equivalent to either [requires(_ != null)] or [ensures(_ != null)], depending on whether you use it on an argument or return value, respectively. The underscore _ represents the value of a parameter, or a return value, depending on where you have used the contract attribute.

Example: Small data types

I like to create a lot of small data types, rather than using a few huge ones. And when you’re making small data types, C# is annoying. A simple type isn’t hard:

public class Person {
	public string Name;
	public DateTime DateOfBirth;
	public List<Person> Children;

But this simplicity has a big price:

So, you probably need a constructor. But adding a constructor is a pain!

public class Person
	public string Name           { get; private set; }
	public DateTime DateOfBirth  { get; private set; }
	public List<Person> Children { get; private set; }
	public Person(string name, DateTime dateOfBirth, List<Person> children)
		Name = name;
		DateOfBirth = dateOfBirth;
		Children = children;
		// TODO: Add validation code

It’s too much repetition!

LeMP solves these problems with a combination of (1) a macro, and (2) a little syntactical “makeover” of C#. In LeMP you’d write this:

public class Person
	public this(
		public string Name           { get; private set; },
		public DateTime DateOfBirth  { get; private set; },
		public List<Person> Children { get; private set; })
		// TODO: Add validation code

Your output file will contain exactly the code listed above, and there is no repetition except for public .. { get; private set; } (but you might not want everything to be a public property anyway, and if you’re using C# 6.0 / VS2015 you can drop the private set part). Great!

What’s going on? Enhanced C# includes two syntax changes to support this, each with a supporting macro:

  1. To reduce repetition and ambiguity, Enhanced C# allows this as a constructor name (a feature borrowed from the D language). A macro changes this into Person so that plain C# understands it.
  2. Enhanced C# allows property definitions as method parameters (or wherever an expression is allowed). A macro is programmed to notice properties, and visibility attributes (like public) on variables. When it notices one of those, it responds by transferring it out to the class, and putting a normal argument in the constructor. Finally, it adds a statement at the beginning of the constructor, to assign the value of the argument to the property or field.

Learn more

Learn more about LeMP in these published articles:

Macro reference manual

Help wanted

Do you have time to make LeMP better? I can’t pay you, since this is all non-profit. However, if you’re an employer maybe you could hire me for consulting work. Seriously: if you don’t hire me I might run out of things to do with C# soon, and then who’s going to maintain LeMP? Sorry, I hope that didn’t sound like a threat. Just sayin’.