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Rationale

It can be difficult to manage resource lifetimes, especially when laziness is involved. Simply local scoping can be inadequate. Resource scopes can decouple resource lifetimes from creating scope.

Plan

  • Generalize from resource scopes to general scopes (succeed/fail/exit)
  • with-open et al make scopes
  • Some design work and code has been done

Use Cases

  • REPL usage: open stream and it closes before I lazily consume it. Doh!
  • Start an activity, and then have nested resource lifetime automatically tie to it
    • Task A calls Iib B which allocates resources
  • Prevent resource lifetimes from nesting
    • lib C knows it doesn't want its lifetime governed by Task A
  • All works across threads

Problems

  • Other than memory allocation, the proper freeing of resources (e.g. closing open files) is on the user
    • thus, easy to get wrong
  • 'He who acquires releases' is one strategy
    • can be automated, e.g. with-xxx
      • but with-xxx puts an envelope around the availability of the resource, tied to the acquirer
      • we trip over this when we create and return lazy seqs that reference resources
      • anywhere else?, could we limit the scope of the solution to lazy seqs?
        • even then, needs care
        • e.g. closing on seq exhaustion not triggered by partial consumption
  • piggybacking on GC gets around this
    • i.e. finalizers
    • but is bad, because we can't know when/if they will run
      • non-option
  • reference counting works in some langs
    • but not on the JVM where aliasing is common and undetectable
    • is it possible to wrap resources with a counter/tracker?
      • probably not
  • can tag things as closable and call close everywhere defensively
    • but everywhere is a big and dangerous place
  • Fundamentally - how do you know when something is a 'resource'
    • and how do you know when no one cares about it anymore?
      • and what does it mean to care?
      • could just fold down to  - care means would be unhappy if closed
    • and what needs to be done about it when that is the case?
      • who knows what that is?

Issues

  • Consider threads as well as nesting
  • How do scopes interact with io! and stm
  • Can we hydrate the fork/join tree where we need it?
  • How to prevent mess?
    • (when-scope :fails ...) et al are very declarative
    • but are setting up non-local effects
    • potentially leading to wtf when triggered
    • is this the right external API?
      • any way to ensure correctness
      • will we be beset with requests for order control
  • Is the nearest enclosing scope always the right one?
    • what to do if not
      • named scopes
      • scopes as args
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