将Clojure宏用于DSL

Question

I am working on a Clojure project and I often find myself writing Clojure macros for DSLs, but I was watching a Clojure video of how a company uses Clojure in their real work and the speaker said that in practical use they do not use macros for their DSLs, they only use macros to add a little syntactic suger. Does this mean I should write my DSL in using standard functions and then add a few macros at the end?

Update : After reading the many varied (and entertaining) responses to this question I have realized that the answer is not as clear cut as I first thought, for many reasons:

1. There are many different types of API in an application (internal, external)

2. There are many types of user of the API (business user who just wants to get something done fast, Clojure expert)

3. Is there macro there to hide boiler plate code?

I will go away and think about the question more deeply, but thanks for your answers as they have given me lots to think about. Also I noticed that Paul Graham thinks the opposite of the Christophe video and thinks macros should be a large part of the codebase (25%):

http://www.paulgraham.com/avg.html

Answer 1

To some extent I believe this depends on the use / purpose of your DSL.

If you are writing a library-like DSL to be used in Clojure code and want it to be used in a functional way, then I would prefer functions over macros. Functions are "nice" for Clojure users because they can be composed dynamically into higher order functions etc. For example, you are writing a functional web framework like Ring .

If you are writing a imperative DSL that will be used pretty independently of other Clojure code and you have decided that you definitely don't need higher order functions, then the usage will be pretty similar and you can chose whichever makes most sense. For example, you might be creating some kind of business rules engine.

If you are writing a specialised DSL that needs to produce highly performant code , then you will probably want to use macros most of the time since they will be expanded at compile time for maximum efficiency. For example, you're writing some graphics code that needs to expand to exactly the right sequence of OpenGL calls......

Answer 2

Yes!

Write functions whenever possible. Never write a macro when a function will do. If you write to many macros you end up with somthing that is much harder to extend. Macros for example cant be applied or passed around.

Christophe Grand: (not= DSL macros)

http://clojure.blip.tv/file/4522250/

Answer 3

No!

Don't be afraid of using macros extensively. Always write a macro when in doubt. Functions are inferior for implementing DSLs - they're taking the burden onto the runtime, whereas macros allows to do many heavyweight computations in a compilation time. Just think of a difference of implementing, say, an embedded Prolog as an interpreter function and as a macro which compiles Prolog into some form of a WAM.

And do not listen to those who say that "macros cant be applied or passed around", this argument is entirely a strawman. Those people are advocating interpreters over compilers, which is simply ridiculous.

A couple of tips on how to implement DSLs using macros:

• Do it in stages. Define a long chain of languages from your DSL to the underlying Clojure. Keep each transform as simple as possible - this way you'd be able to easily maintain and debug your DSL compiler.
• Prepare a toolbox of DSL components that you will reuse when implementing your DSLs. It should include target languages of different semantics (eg, untyped eager functional - it is Clojure itself, untyped lazy functional, first order logic, typed imperative, Hindley-Millner typed eager functional, dataflow, etc.). With macros it is trivial to combine properties of all that target semantics seamlessly.
• Maintain a set of compiler-building tools. It should include parser generators (useful even if your DSLs are entirely in S-expressions), term rewriting engines, pattern matching engines, implementations for some common algorithms on graphs (eg, graph colouring), etc.

Answer 4

Here's an example of a DSL in Haskell that uses functions rather than macros:

http://contracts.scheming.org/

Here is a video of Simon Peyton Jones giving a talk about this implementation:

http://ulf.wiger.net/weblog/2008/02/29/simon-peyton-jones-composing-contracts-an-adventure-in-financial-engineering/

Leverage the characteristics of Clojure and FP before going down the path of implementing your own language. I think SK-logic's tips give you a good indication of what is needed to implement a full blown language. There are times when it's worth the effort, but those are rare.