Rake, a popular Ruby DSL
Rake, like make or ant, is a DSL for defining tasks.
Tasks definitions above are written using Rake DSL. In particular, they define two dependent tasks. They state that the :world task is dependent from the :hello task. In other words, when the :world task is executed, the :hello task will be executed first. This will result in the string "Hello World!" printed on the console. The neat fact about the code above is that it is perfectly legal ruby code. In particular,
- task is a ruby method that accepts an hash argument
- :hello and :world are ruby symbols
- :hello => :world is a ruby hash
- the do ... end part is a ruby block
Makengo: an internal DSL in Go
When I approached to Go the first time I was intrigued by what is stated in its homepage:
"It feels like a dynamic language but has the speed and safety of a static language. It's a joy to use."
Belonging from years of experience with dynamic languages such as ruby and enjoying its capability to build DSL, I was curious to see how much the compiled and strongly typed nature of Go can be bended in this sense. So, I started writing a small experimental project named makengo.
Using makengo DSL, the previous tasks definitions become:
How does it look compared to the ruby version? First of all, the Go version takes 117 chars against the 75 chars of the Ruby version. So, it's ~56% more verbose. In particular
- There are too many parentheses
- String names are longer than symbol names (:hello vs "hello", :world vs "world")
- The block syntax is burdened by the func keyword
- In order to express the dependency we need the long named DependsOn function rather than the concise key => value hash syntax
Go DSL capabilities
The retrospection on Makengo raises the main question of this post: is Go suitable for building internal DSLs? Let's try answering, exploring Go's DSL capability in more detail.
Go has many features that help building DSL. The first thing that comes to mind are anonymous functions that act as closures. In makengo, I use anonymous an function to define what a task does.
Also, anonymous functions can take arguments in order to push values in the block. For example:
Compared with C, this a great step towards expressivity. However, compared with Ruby, Go's anonymous functions syntax appears to be a lot more verbose. This is due to the necessity of a func keyword and to the use of many parentheses. Also, considering that since Go is a strongly typed language, anonymous function arguments need their own type declaration.
In Go, types can receive methods. In Makengo, this fact is exploited to express dependencies among tasks. Actually, the Task function returns a task object and task object can receive the DependsOn method allowing for method chaining:
Dynamic reception and metaprogramming
Go lacks of a ruby-like method_missing method. In ruby, when an unknown method is sent to an object the object responds executing method_missing. This language feature is known as Dynamic Reception. Dynamic Reception, combined with the metaprogramming magic of ruby, allows for very readable DSL (taken from Machinist):
Regarding metaprogramming, Go has an interesting reflect package that allows the manipulation of objects with arbitrary types. This comes in handy, for example, in variadic functions (see below).
In Ruby, a literal map could be passed as argument to a function allowing for more expressive function calls inside the DSL:
Moreover, Ruby allows you omit the delimiters for a literal map. So you can shorten it to:
In Go, a literal map looks like this:
but there is no way to shorten it.
Literal lists and varargs
Like maps, literal lists in Go have not optional delimiters so it's not convenient to use them as function arguments inside DSL. BTW, Go has good support for varargs that combined with types reflection, types assertion and interfaces, are good substitutes for literal lists. For example, in Makengo I can define tasks that depend on more than a single task passing multiple arguments to DependsOn:
A comparison table
Let's try summarize in a table what we discussed until this point.
|Optional delimiters in maps||No||Yes|
|Optional delimiters in lists||No but you can use varargs||Yes|
|Short signature for blocks||No||Yes|
So what about the answer to the original question? Is Go suitable for building internal DSL? The answer of course is ... it depends!
As a compiled language, Go can't compete with dynamic languages in building complex and general purpose DSL. However, it's enough flexible to build simple DSL. Moreover, as said at the beginning of this post, the mean of expressivity is relative to the problem domain. Go can be very expressive when dealing with problems that involve network and concurrency issues.
Another road to explore is the implementation of a light VM in Go exploiting some of its advantages against C: built-in concurrency and GC. On top of this VM could live a set of dynamic languages more suited to implement external DSLs. It seems that Eleanor McHugh is on the right track with her GoLightly VM.
Go DSL capability need more investigation, of course. But a fact is clear to me: I should not think at Go as a Panacea trying to force its nature: Go it's not a scripting language. I should rather taking advantage of its peculiarities in order to fully exploit all of its expressivity.