Algebraic effects and handlers

Handlers are the orthogonal layer that reinterprets an operation. That system calls are algebraic effects is the principle; whether to install a handler over an operation — and whether that handler reifies a continuation — are two independent choices layered on top, and ral takes the smallest on both: a restricted, continuation-free fragment.

ral installs effect handlers over a dynamic frame stack with within [handlers: [name: handler], handler: fallback] { body }. A handler maps an operation name to a lambda; the catch-all handler: applies when no per-name entry matches. Handlers intercept external commands only — lexical bindings, builtins, and the prelude are not shell aliases and cannot be overridden.

A per-name handler (and every alias) must be a unary lambda { |args| … }, applied to the intercepted command’s argument list; the catch-all must be a binary lambda { |name args| … }, applied to the command’s name and its argument list. The calling convention is fixed by the surface position, not the value’s runtime shape: a bare block { … }, a non-lambda value, or a lambda of the wrong arity is rejected at install time.

ral’s handlers are the tail-resumptive fragment of algebraic-effect handlers, along three axes — the first two recorded in handlers-deep-self-masking:

• Deep. A handler persists across the dynamic extent of the body, re-wrapping each continuation, so successive operations all reach it.
• Self-masking. While a handler’s own body runs, its frame is lifted from the stack, so a same-name call inside the body reaches the next outer handler (or the OS), never itself.
• Tail-resumptive. A handler receives the command name and arguments and either returns a value, forwards to the next enclosing frame, or fails. A returned value is substituted for the intercepted operation and the body resumes — that substitution is the resumption.

This is why there is no first-class resume: resumption is implicit, exactly-once, and in tail position. What ral omits is the reified continuation that non-tail or multi-shot resumption would require. The natural idiom is wrap-and-forward, within [handlers: [git: { |args| my-git ...$args }]] { ... }, which self-masking keeps from diverging; the live semantic question is therefore the masking discipline — which handler is in scope at each point — not how a captured continuation is re-entered.

See also syscalls-are-effects, control-operators, scoping, grant, system-c (self-masking as capability-set subtraction), handlers-of-algebraic-effects (the calculus this fragment restricts).

Realised in handler-dispatch.

Cite: RATIONALE §“Effect handlers: deep with self-masking”; docs/SPEC.md §3.2.