Call-by-push-value: values and commands

ral’s organising principle is the call-by-push-value separation of values from commands:

• values are inert data: strings, integers, lists, records, thunks;
• commands are computations that read, write, fail, or return;
• a thunk {M} suspends a command as a value; ! forces it back into a command.

Two sigils keep retrieval and forcing distinct:

• $name dereferences a binding to its value without side effect;
• a bare word in head position is looked up and, if it names a thunk, forced.

Once captured, data is never re-lexed, split, or globbed. So the shell collapse — every datum at once a string, a command name, and re-lexable source — never arises, and neither do the escaping bugs that follow from it.

The parameterised block { |x| M } is the language’s single abstraction mechanism. It subsumes what conventional shells splinter across functions, aliases, eval, subshells, and trap handlers: a block can be bound, passed, returned, or installed as an effect handler. Currying desugars to nested lambdas, so partial and under-application are uniform.

Bindings are immutable. Re-let introduces a fresh binding that shadows the outer one within its lexical scope; closures capture at definition time. This buys equational reasoning in the pure fragment and makes spawn safe — a spawned block shares nothing mutable.

Invariants.

• $ consults only the value namespace and never triggers command lookup or handler dispatch.
• Captured external output is decoded UTF-8 strictly — invalid bytes fail with a hint to keep them via | from-bytes — and the decoded text is never re-lexed.
• Closures observe the bindings in force where they were defined, not where they run.

See also syscalls-are-effects (commands are the effect half this splits off), scoping, control-operators, fixed-arity, system-c (box = thunk, unbox = force), call-by-push-value (the substrate at its source).

Realised in compilation-ladder and evaluator-machine.

Cite: RATIONALE §“Values and commands”, §“Two sigils”, §“Blocks as the single abstraction mechanism”; docs/SPEC.md §0, §3, §4.