Exploring Fantom DSLs

Brief intro to DSLs

Fandoc says:

Technically speaking, DSLs are presented as DSL expressions, and it's result type is defined by the anchor type, so that

  str := Str<|hello, world!|>  // str has type Str
  foo := Foo<|bar|> // foo has type Foo

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:

 a := Node("a")
 b := Node("b")
 c := Node("c")
 d := Node("d")
 ab := Edge(a, b)
 bc := Edge(b, c)
 bd := Edge(b, d)
 graph := Graph([a,b,c,d], [ab, bc, bd])

What a lot of code! Let's use some DSL magic:

  graph := Graph<|a -> b
                  b -> c
                  b -> d|>

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:

Graph(
  ["a","b","c","d"].map { 
    Node(it) 
  }, 
  [
    ["a","b"], 
    ["b", "c"], 
    ["b", "d"]].map { 
      Edge(
        Node(it.first), 
        Node(it.last)
      ) 
    }
  )

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.

Beyond the DSL src