Continuations in Clojure.

Question

I read somewhere where rich hickey said:

"I think continuations might be neat in theory, but not in practice"

I am not familiar with clojure.
1. Does clojure have continuations?
2. If no, dont you need continuations? I have seen a lot of good examples especially from this guy. What is the alternative?
3. If yes, is there a documentation?

Answer 1

Is continuation a necessary feature in a language?

No. Plenty of languages don't have continuations.

If no, dont you need continuations? I have seen a lot of good examples especially from this guy. What is the alternative?

A call stack

Answer 2

When talking about continuations, you'll have to distinguish between two different kinds of them:

• First-class continuations - Continuation-support that is deeply integrated in the language (Scheme or Ruby). I doubt that Clojure supports them.

• Continuation-passing-style (CPS) - CPS is just a style of coding and any language supporting anonymous function will allow this style (which applies to Clojure too). Example:

  -- Standard function
  double :: Int -> Int
  double x = 2 * x
  
  
  -- CPS-function - We pass the continuation explicitly 
  doubleCPS :: Int -> (Int -> res) -> res
  doubleCPS x cont = cont (2 * x)
  
  
  -- Call
  print (double 2)
  
  
  -- Call CPS: Contintue execution with specified anonymous function
  double 2 (\res -> print res)
  

Read continuation on wikipedia.

I don't think that continuations are necessary for a good language but especially first-class continuations and CPS in functional languages like Haskell can be quite useful (Intelligent backtracking example)

Answer 3

I've written a Clojure port of cl-cont which adds continuations to Common Lisp.

http://github.com/swannodette/clj-cont/tree/master

Answer 4

A common use of continuations is in the implementation of control structures for: returning from a function, breaking from a loop, exception handling etc. Most languages (like Java, C++ etc) provide these features as part of the core language. Some languages don't (e.g: Scheme). Instead, these languages expose continuatiions as first class objects and let the programmer define new control structures. Thus Scheme should be looked upon as a programming language toolkit, not a complete language in itself.

In Clojure, we almost never need to use continuations directly, because almost all the control structures are provided by the language/VM combination. Still, first class continuations can be a powerful tool in the hands of the competent programmer. Especially in Scheme, continuations are better than the equivalent counterparts in other languages (like the setjmp/longjmp pair in C). This article has more details on this.

BTW, it will be interesting to know how Rich Hickey justifies his opinion about continuations. Any links for that?

Answer 5

Clojure (or rather clojure.contrib.monads) has a continuation monad; here's an article that describes its usage and motivation.