thunk

A thunk is a programming construct that represents a parameterless function which encapsulates a computation to be performed later. By capturing the necessary environment or arguments, a thunk delays evaluation and allows code to request the result at an indeterminate time. The term originated in early compiler literature, and its exact origin is informal, appearing in the 1960s and 1970s in discussions of delayed computation and call mechanics.

In lazy evaluation and functional languages, thunks implement deferred computation. A thunk wraps an expression so that evaluating the thunk triggers the evaluation of the expression, sometimes with the result memoized for repeated use. Thunks also appear in continuation-passing style and in call-by-need implementations, where computation is postponed until its value is actually required.

Thunks are used in compiler design and foreign-function interfaces to adapt calling conventions or environments. A thunk can be a small wrapper that adjusts the this pointer, bridges differences between languages (for example, C and C++), or supplies missing arguments, allowing a function to be invoked through a uniform, generic entry point.

In contemporary software development, the term appears in other contexts as well. A notable modern example is the Redux thunk middleware in JavaScript, where a thunk is a function returned by an action creator that can perform asynchronous work and dispatch actions later.

Overall, thunks provide a simple, flexible mechanism to encapsulate and control the timing of computations, offering a bridge between eager execution, lazy evaluation, and cross-language calling.