thunk

/thuhnk/ n. 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 to compute the expression and leave the address of the result in some standard location. 2. Later generalized into an expression, frozen together with its environment, for later evaluation if and when needed (similar to what in techspeak is called 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. 4. People and activities scheduled in a thunklike manner. "It occurred to me the other day that I am rather accurately modeled by a thunk --- I frequently need to be forced to completion." --- paraphrased from a plan file. Historical note 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 holds 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 realized (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".