What is a 'thunk'?

Question

I\'ve seen it used in programming (specifically in the C++ domain) and have no idea what it is. Presumably it is a design pattern, but I could be wrong. Can anyone give a go

Answer 1

The term thunk originally referred to the mechanism used by the Royal Radar Establishment implementation of pass-by-name in their Algol60 compiler. In general it refers to any way to induce dynamic behavior when referencing an apparently static object. The term was invented by Brian Wichmann, who when asked to explain pass-by-name said "Well you go out to load the value from memory and then suddenly - thunk - there you are evaluating an expression."

Thunks have been put in hardware (cf. KDF9, Burroughs mainframes). There are several ways to implement them in software, all very machine, language and compiler specific.

The term has come to be generalized beyond pass-by-name, to include any situation in which an apparently or nominally static data reference induces dynamic behavior. Related terms include "trampoline" and "future".

Answer 2

I was distressed to find no general 'computer science' definition of this term matching its de-facto usage as known historically to me. The first real-life encounter I can recall where it was actually called that was in the OS/2 days and the 16-32 bit transition. It appears "thunking" is like irony in its application today.

My rough general understanding is that the thunk is a stub routine that just does nothing or routes across some fundamental boundary in kind between systems as in the mentioned historical cases.

So the sense is like a synesthesia of being dropped from the one environment to the other making (metaphorically/as a simile) a "thunk" sound.

Answer 3

There's considerable variation in use. Almost universally, a thunk is a function that's (at least conceptually) unusually small and simple. It's usually some sort of adapter that gives you the correct interface to something or other (some data, another function, etc.) but is at least seen as doing little else.

It's almost like a form of syntactic sugar, except that (at least as usually used) syntactic sugar is supposed to make things look the way the human reader wants to see them, and a thunk is to make something look the way the compiler wants to see it.

Answer 4

This question has already been asked on SO, see:

What is a 'thunk', as used in Scheme or in general?

From what I can tell, it's akin to a lambda statement, where you may not want to return the value until you need to evaluate it; or it can also be compared to a property getter which by design executes some code in order to return a value while yet having the interface form that comes across more like a variable, but also has polymorphic behavior that can be swapped out whether by inheritance or by swapping out the function pointer that would evaluate and return a value at runtime based on compile-time or environmental characteristics.