Clojure – Are Square Brackets and Curly Braces S-Expressions?

clojurelisp

I am trying to learn Lisp and looking at all the Lisps out there and their differences.

I see that in some implementations of Scheme, you can use square brackets interchangeably with round brackets for readability, so because they are treated the same, I assume they are still just S-expressions like everything else.

However, in Clojure I see that square brackets and curly braces are used to represent different data types like vectors and so on. Are these also still valid S-expressions, or must they be considered atoms? Doesn't this "break" the whole concept of S-expressions, thereby making Clojure an "impure" Lisp?

Best Answer

Literal notation for vectors and maps (ie, square brackets and curly braces) are just read-time sugar, and anything you can represent with that notation can also be represented with S-expressions. As a result, there's no loss of power or homoiconicity, and indeed reader macros (which I understand are even more powerful) are available in a number of lisps.

Incidentally, note that Clojure now provides a mechanism for creating custom "tagged" literals, which, like other literals, are transformed to S-expressions before they are compiled and interpreted.

Related Solutions

Clojure – Understanding Vars, Atoms, and Refs

Use Refs for synchronous, coordinated and shared changes.
Use Agents for asynchronous, independent and shared changes.
Use Atoms for synchronous, independent and shared changes.
Use Vars for isolated changes.

EDIT
The links you posted are highly important to read, however I also recommends this: Software Transactional Memory

Keep in mind that aborted transactions will be replayed. What does this mean?
It means that the code inside dosync could be executed a couple of times, so you need to be careful the code should be side effects free.

One use of agents is to have a controlled side effects. Consider this:

(dosync
  (send log-agent message args)
  (alter reference function))

The STM will hold all of the actions needed to be sent to agents until succeed.

What is difference between atoms and agents?
Updates to agents will happen asynchronously at some point of the future, updates to atoms will happened immediately.

Atoms and agents don't need to run inside a transaction, refs should be (coordinated changes, remember?)

Design – Clojure state and persistence

Clojure's in-memory STM doesn't handle durable persistence directly (by definition, it is designed for in-memory use case....)

However the principles apply equally well to persisted storage. For example take a look at Datomic, which is effectively a scalable ACID database management system based on Clojure's data structures and state management philosophy.

The common principles are roughly:

Immutable persistent data structures
State transitions managed by pure functions
Unlimited read scalability (non-transactional reads never need locks)
"pluggable" transaction semantics to suit your application
Separation of identity and state
Emphasis on data separated from functions that process it (i.e. distinct from the OOP concept of encapsulating data and code)

Best Answer

Related Solutions

Clojure – Understanding Vars, Atoms, and Refs

Design – Clojure state and persistence

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