Variable

Wikipedia

• In computer programming, a variable or scalar is a storage location (identified by a memory address) paired with an associated symbolic name, which contains some known or unknown quantity of information referred to as a value.

Variable properties

• Lot of unknowns
  • type inference
  • lifetime
  • visibility (threads?)
  • update - does it happen in all times (HW pins…)
  • update - is visible from other threads?
  • update - is atomic?
  • multiple updates - is sequence of operation conserved?
  • hazards
    • RAW, WAR, WAW

Clojure approach

• Variables are not the best technology (too many properties/unknowns)
  • dtto for objects :)
• Be immutable whenever possible
  • (Rust mimicks this approach btw)
• Let’s use four distinct mechanisms instead
  • vars
  • refs
  • atoms
  • agents
• Mutate via functions
• Validators
• Watchers

Vars/Refs/Atoms/Agents

Vars   thread local   sychronous    single identity
Refs   transactional  synchronous   multiple identities
Agents uncoordinated  asynchronous  single identity
Atoms  uncoordinated  synchronous   single identity

Vars

• Mutable storage location
• Mutation on per-thread basis
  • thread isolation

Refs

• Shared mutable storage
• STM - Software Transactional Memory
• Mutation withing transaction
• Coordinated, synchronous change of multiple locations

(dosync
  (alter (ref1 ...))
  (alter (ref2 ...)))

Agents

• Independent, asynchronous change of individual locations
• Mutation done asynchronously (in new thread/goroutine/…)
• It’s possible to wait for mutation to finish

Atoms

• Shared, synchronous, independent state
• All changes are visible to other threads
  • w/o the need to locking etc.

Validators

• Function called before var/ref/agent/atom is mutated
• Can return true/false or throw an exception if needed

Watcher

• Function called automatically with key, reference, old state and new state
• Might be called from multiple threads simultaneously