A Path Less Travelled

Mar 2014

Monads as EDSL

DSLs are mostly standalone scripts or snippets, with a syntax different any other general purpose languages (otherwise they're not in DSL, but just this language). It's easy to tell what is a DSL: not C++, but can be loaded and executed by a C++ program; and what's not: C++.

However, for EDSLs, the water is not this clear. At one extreme of the spectrum, EDSLs in Lisp is undeniably a DSL, but embedded in Lisp. On the other end, Ruby EDSLs are just ruby code that needs more explanation to clarify its meaning, and looks a little bit cooler than other code.

I'm gonna talk about Ruby-ish "EDSL"s today. To give non-Haskellers a feeling of the language, and hopefully may bring some of them to this wonderful language.

If you know Ruby, especially Ruby on Rails, you must have seen EDSLs, or just DSLs as they're called by Rubyists. Say the query building in ActiveRecord, with select, joins, where, group, and other keywords from SQL used in Ruby code as if they're valid Ruby. And they are. Or a more alien looking HTML building DSL, without dots to chain all keywords, but organized just like a HTML, or rather, a Jade file. Like this:

Now, doing the same in Haskell is the most widespread way to build HTMLs. Blaze HTML has 91 reverse dependencies on hackage (not counting many blog sites which are not on it), while its rivals all have fewer than 40 (and half of Hamlet's reverse dependencies start with Yesod, half of heist's contain snap). To quote its reference:

Neat, isn't it? And it's better than Ruby EDSLs, as it's simpler than the safe ones, and safer than the clean ones. For the Ruby snippet I show you previously, it uses instance_eval to make ul & li callable from inside the block of code. instance_eval is considered eval, well, evil, sorry about the typo. It changes the context of the executing block (and then you're afraid, Ruby users? JavaScript will scare you to death by letting you guess what this is inside a function call), whatever it means. For cleaner versions, like in the FormHelper from RoR, a f. prefixes every EDSL verb, and making it less cryptic, and more Ruby like.

The Haskell version is possible thanks to something called "Monads)". And thanks to Haskell's do notation to avoid strange arrow like symbols (>>=). Reading the latter link, you can find out how Haskellers think of monads: as a composable computation descriptions. Well, I always find this line vague, so let's just say it's an evaluation context, like the this or self in various mainstream languages, in a very stretched sense. Say in the example below:

You can save it in stack.hs, and run it with runghc stack.hs. It prints [2,1], as you have seen, with 1, 2, and 3 pushed one after another, and the last number popped. The testStack has a type of Stack Int (), which is its return type, making it a constant with no arguments. Wait, it's a constant? but it's all actions inside, ow can it be a constant? Say I am writing in Ruby or C++, can I have a constant with 3 push statements and 1 pop? I can't even have a variable with such a value, which is not even a value. testStack doesn't make a stack and return it, as you can see from the code, it returns a string saying it's not a stack builder!

Well, keep reading the HaskellWiki, you will find:

The essence of monad is thus separation of composition timeline from the composed computation's execution timeline, as well as the ability of computation to implicitly carry extra data, as pertaining to the computation itself, in addition to its one (hence the name) output, that it will produce when run (or queried, or called upon).

In mortal language, it basically means 2 things:

  1. calling testStack doesn't run it immediately. Given a stack, it can manipulate it and return a human readable explanation, but it doesn't produce a stack by itself. It's like a promise (as in concurrent programming) of stack operations, but pending and not resolved;
  2. when you actually goes on to resolve the pending promise, with a real stack, by calling execState, the operations will happen, and the string will be returned. And the stack magically makes its appearance in the Stack monad to testStack.

Now, if you look at State's definition, it's newtype State s a = {runState :: s -> (a, s)}, meaning it's a "structure" with 1 field called runState, which is a function type, taking a s (or [Int] in our example), and returns (a, s), which is a tuple ((String, [Int]) in testStack's case). This function is the "computation description" mentioned in HaskellWiki: push pushes something to the stack, and pop pops something out. The structure is thus a Monad.

Though the stack operations are simplistic, they look like Ruby EDSLs, or even better. Unlike Ruby, the EDSL can be easily extended, even in other modules, or even other projects. You don't need to inject or monkey patch anything, you just define a function returning a Stack monad. Actually, the testStack function, despite its name and the useless deed it does, is a valid verb for stack operation. You can define pushTwice or popTillEmpty or various other interesting operations.

For serious, useful and understandable monad starters, you can take a look at parsec. Maybe is a monad with no EDSL like verbs, but very useful. You can consider it as a safe nullable pointer, which will never throw a NullPointerException in your face. Either is also interesting, as it makes skipping computation on exception possible without goto or stack unwinding. Lists ([]) are monads as well, and list comprehension is just a syntax sugar for monadic operations. The day I first ran into this pearl was made awesome by it.

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