Clojure differences between Ref, Var, Agent, Atom, with examples

clojure var agent
user472557 picture user472557 · Feb 3, 2012 · Viewed 21.4k times · Source

I'm very new to Clojure, Can you guys give me explanation with real world scenarios. I mean, where to use Ref, Var, Agent, Atom. I read book, but, still couldn't understand the real world examples.

Answer

Arthur Ulfeldt picture Arthur Ulfeldt · Feb 4, 2012

I highly recommend "The Joy of Clojure" or "programming Clojure" for a real answer to this question, I can reproduce a short snip-it of the motivations for each:

start by watching this video on the notion of Identity and/or studying here.

  • Refs are for Coordinated Synchronous access to "Many Identities".
  • Atoms are for Uncoordinated synchronous access to a single Identity.
  • Agents are for Uncoordinated asynchronous access to a single Identity.
  • Vars are for thread local isolated identities with a shared default value.

Coordinated access is used when two Identities needs to change together, the classic example is moving money from one bank account to another, it needs to either move completely or not at all.

Uncoordinated access is used when only one Identity needs to update, this is a very common case.

Synchronous access is used when the call is expected to wait until all Identities have settled before continuing.

Asynchronous access is "fire and forget" and let the Identity reach its new state in its own time.

Related questions

Scala vs. Groovy vs. Clojure

Can someone please explain the major differences between Scala, Groovy and Clojure. I know each of these compiles to run on the JVM but I'd like a simple comparison between them.

Read More
A regex to match a substring that isn't followed by a certain other substring

I need a regex that will match blahfooblah but not blahfoobarblah I want it to match only foo and everything around foo, as long as it isn't followed by bar. I tried using this: foo.*(?<!bar) which is fairly …

Read More
How to bundle a native library and a JNI library inside a JAR?

The library in question is Tokyo Cabinet. I want is to have the native library, JNI library, and all Java API classes in one JAR file to avoid redistribution headaches. There seems to be an attempt at this at GitHub, …

Read More