Technically speaking, DSLs are presented as DSL expressions, and it's result type is defined by the anchor type, so that
Therefore, the most obvious way to use DSL expressions is instantiation of some complex objects. For example, imagine we are writing a library for graph manipulation, so we define classes like Graph, Node, and Edge. See Graphs.fan
And imagine that we need to instantiate some graphs for our tests, so we write the code like this:
What a lot of code! Let's use some DSL magic:
The DSL plugin for Graph just parses the code between <| and |> (which is accessed via compiler::DslExpr.src) and generates appropriate object creation.
However even such a simple example is quite tricky - to allow our DSL to be used as field initializer, or default parameter value, we need to convert our DSL code to a single expression.
So, before implementing the DSL plugin itself, we need to understand how we can replace our code with a single expression. In this particular example, assuming that Node and Edge classes override equals and hash correctly, it can be done like this:
The source code of DSL plugin can be found here.
Uh, after looking at the source of GraphDsl, the question is - why do we want to write DSL plugins? The same task can be fairly easy implemented in simple static method like Graph.fromStr. Why anyone want to use heavy low-level Compiler API? The benefit like compile-time validation and generation of compile error on a bad line seems to be too small, almost negligible.
That's what I thought when saw Fantom DSLs for a first time, and then forgot about them almost for a year.
Last week, being tired from code like this:
I thought it'd be cool to have multiple dispatch in Fantom. The initial implementation supposed to be quite simple - we have Dispatcher class which looks like this:
Using this, the code above can be written like this:
Slightly better, but still a lot of boilerplate code. What if we'd use DSLs + Symbols here? Using that, we can write code like this:
So, all we need to do is to write simple marker facet:
And DSL plugin which will make everything for us - find all methods annotated with @Dispatch and starting with a given prefix, take functions from them and then pass list of functions to Dispatcher constructor. Sounds fairly easy, but when I started implementing it, I found the first problem - from a DslPlugin.compile we don't know where we are - I mean, we don't know anything about enclosing type or method.
Luckily for me, DslPlugin extends CompilerSupport, which means we have full access to all compilation units and type definitions for them! So we can take location of our DSL expression, and then by iterating through all compilation units and comparing location, we can find our compilation unit. Using the same way, we can find enclosing type definition.
The rest is simple - iterate through all slots of our type, find all methods with given prefix and facet and construct expression for Dispatcher creation.
Nice! But wait for a second - this means that new instance of Dispatcher will be created per each method invocation. That's not exactly what we want. What if we could inject a private static const Dispatcher selectDispatcher? And yes, we can! So, right inside our DSL plugin, we can write something like this:
However, there's one more thing we have to do - modify static initializer for our type. Static initializer is a special method generated by compiler and it contains all assignments made to static field definitions:
At the stage when DSL plugins are called, this static method is already generated (if there are other static fields in this class), so we need to manually find this method and modify it's code (if there are no static fields, we also need to create this method):
And voila! Everything compiles and runs smoothly now. However it already smells like black magic. Let's go further!