The magic of boo - Flexible syntax
time to read 3 min | 526 words

when I am writing DSL, I keep hitting one pain point. The CLR naming conventions, which are more or less imprinted on my eyelids, are not really conductive to clear reading in a DSL.

Let us take these entities, and see what we get when we try to build a DSL from them:

image

The DSL is for defining business rules, and it looks like this:

when User.IsPreferred and Order.TotalCost > 1000:
	AddDiscountPrecentage  5
	ApplyFreeShipping
when not User.IsPreferred and Order.TotalCost > 1000:
	SuggestUpgradeToPreferred 
	ApplyFreeShipping
when User.IsNotPreferred and Order.TotalCost > 500:
	ApplyFreeShipping

The main problem with this style of writing is that it is visually condense. I can read it pretty much as easily as I read natural English, but anyone who is not a developer really have to make an effort, and even for me, trying to read ruby styled code is easier. Here is how this would look like when using the ruby style conventions:

when User.is_preferred and Order.total_cost > 1000:
    add_discount_precentage 5
    apply_free_shipping
when not User.is_preferred and Order.total_cost > 1000:
   suggest_upgrade_to_preferred 
    apply_free_shipping
when User.is_not_preferred and Order.total_cost > 500:
   apply_free_shipping

This is much easier to read, in my opinion. The problem is that I consider this extremely ugly.

image

Obviously a different solution is needed...

Wait a minute! Boo has an open compiler. Why not just change the way it handle references? And that is what I did:

///<summary>
/// Allow to use underscore separated names, which will be translated to pascal case names.
/// pascal_case -> PascalCase.
/// All names that contains an underscores will go through this treatment.
///</summary>
/// <example>
/// You can  enable this behavior using the following statement
/// <code>
/// compiler.Parameters.Pipeline
///		.Replace(typeof (ProcessMethodBodiesWithDuckTyping),
/// 				 new ProcessMethodBodiesWithDslNamesAndDuckTyping());
/// </code>
/// </example>
public class ProcessMethodBodiesWithDslNamesAndDuckTyping : ProcessMethodBodiesWithDuckTyping
{
	/// <summary>
	/// Called when we encounter a reference expression
	/// </summary>
	/// <param name="node">The node.</param>
	public override void OnReferenceExpression(ReferenceExpression node)
	{
		if(node.Name.Contains("_"))
			SetNodeNameToPascalCase(node);
		base.OnReferenceExpression(node);
	}

	/// <summary>
	/// Called when we encounters a member reference expression
	/// </summary>
	/// <param name="node">The node.</param>
	public override void OnMemberReferenceExpression(MemberReferenceExpression node)
	{
		if (node.Name.Contains("_"))
			SetNodeNameToPascalCase(node);
		base.OnMemberReferenceExpression(node);
	}

	/// <summary>
	/// Sets the node name to pascal case.
	/// </summary>
	/// <param name="node">The node.</param>
	private static void SetNodeNameToPascalCase(ReferenceExpression node)
	{
		string[] parts = node.Name.Split(new char[] { '_' },StringSplitOptions.RemoveEmptyEntries);
		StringBuilder name = new StringBuilder();
		foreach (var part in parts)
		{
			name.Append(char.ToUpperInvariant(part[0]))
				.Append(part.Substring(1));
		}
		node.Name = name.ToString();
	}
}

I love Boo, with cause.

Tweet Share Share 12 comments
Tags: