thunk

Name

thunk — thunk — a computation boxed and shelved, evaluated only if forced

Synopsis

thunk [--lazy] [--memoize] [--never] EXPRESSION
force(thunk) -> value  // first call only
cost(ask twice) = cost(ask once) iff --memoize

Description

thunk wraps EXPRESSION in a zero-argument closure and returns immediately. Nothing in EXPRESSION runs. The wrapper is cheap; the work inside is deferred until something calls force() on it, an event that may occur once, many times, or never.

With --memoize the thunk overwrites itself with its result on first force and answers instantly thereafter. Without it, each force re-runs the whole expression, faithfully, at full price. thunk does not decide which is correct. thunk only defers.

A thunk over an infinite structure is legal and common. It represents work the machine could do but has declined to, pending demand that has not arrived.

Options

--lazy

Default. Do the work when asked and not before. If never asked, congratulations, you have saved the entire cost.

--memoize

Record the answer on first force. Subsequent forces return the note, not the labor. Call-by-need.

--again

Force a thunk already forced. Under --memoize, returns the memo. Without it, recomputes from scratch. On principle.

--never

Guarantee the thunk is not forced. Fully supported. This is the fastest possible evaluation of EXPRESSION.

--strict

Force immediately at construction. Defeats the purpose. Provided for those who cannot wait.

Exit Status

0

forced once, value returned, work filed

0

forced again under --memoize, memo returned, no work done

0

never forced, EXPRESSION unevaluated, nothing owed

137

a million tiny thunks nobody opened; heap exhausted by deferred intentions

History

"The term was coined around 1961 by P. Z. Ingerman's ALGOL 60 compiler group to name the closures generated for call-by-name arguments, which are re-evaluated on each mention. The name is the jocular past tense of think: the value had already been thunk of by the compiler. Chris Wadsworth's 1971 thesis added self-overwriting evaluation, giving call-by-need and the once-only thunk. Microsoft later reused the word for its 16-bit/32-bit interoperability stubs, unrelated machinery under the same name."

Bugs

Under call-by-name, asking twice costs twice. Reported as a performance defect since 1961. Reclassified as intended behavior.

A thunk chain can retain its entire environment in memory long after the value would have fit in a register. Known as a space leak. Not a leak, technically; nothing escaped.

--never has no observable output, which some users mistake for failure. It is the success case.

See Also

quiescence(8), backpressure(3), idempotent(1). The living exhibit demonstrates the word in motion:

▸ operate thunk

Author

"Maintained by The Deferrer, who has not computed your total and will not, unless asked, and possibly not then."