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FScript

Defer and Laziness

FScript is eager by default for effectful work. defer is the explicit way to delay that start.

lazyConfig = defer FileSystem.readFile('./config.json')

That means:

  • FileSystem.readFile(path) starts when execution reaches it
  • defer FileSystem.readFile(path) captures the work without starting it yet

This page is the short guide. For the full explanation, examples, and design rationale, see Detailed Defer and Laziness.

Why defer exists

FScript separates two ideas:

  • starting effectful work
  • consuming the result of effectful work

By default, effectful calls start eagerly. defer exists for the cases where that is not what you want.

Use defer when:

  • work is optional
  • work is expensive
  • work should begin only if a later branch needs it
  • you want laziness to be clearly visible in the source

Runtime behavior

Draft 0.1 prefers memoized single-start semantics:

  • creating a deferred value captures the expression and its environment
  • forcing it starts the work
  • repeated force observes the same eventual result