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 formal parameter, 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]

Last updated: 1997-10-11

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Copyright Denis Howe 1985

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