thunk

programming /thuhnk/ 1. "A piece of coding which provides an

address", according to P. Z. Ingerman, who invented thunks in

1961 as a way of binding actual parameters to their formal

definitions in ALGOL 60 procedure calls. If a procedure

is called with an expression in the place of a formalparameter, the compiler generates a thunk which computes the

expression and leaves the address of the result in some

standard location.

2. The term was later generalised to mean an expression,

frozen together with its environment (variable values), for

later evaluation if and when needed (similar to a

"closure"). The process of unfreezing these thunks is

called "forcing".

3. A stubroutine, in an overlay programming environment,

that loads and jumps to the correct overlay.

Compare trampoline.

There are a couple of onomatopoeic myths circulating about the

origin of this term. The most common is that it is the sound

made by data hitting the stack; another holds that the sound

is that of the data hitting an accumulator. Yet another

suggests that it is the sound of the expression being unfrozen

at argument-evaluation time. In fact, according to the

inventors, it was coined after they realised (in the wee hours

after hours of discussion) that the type of an argument in

ALGOL 60 could be figured out in advance with a little

compile-time thought, simplifying the evaluation machinery.

In other words, it had "already been thought of"; thus it was

christened a "thunk", which is "the past tense of "think" at

two in the morning".

4. (Microsoft Windows programming) universal thunk,

generic thunk, flat thunk.

[Jargon File]

(1997-10-11)