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

Solution 1:

It is really simple. When you have some computation, like adding 3 to 5, in your program, then creating a thunk of it means not to calculate it directly, but instead create a function with zero arguments that will calculate it when the actual value is needed.

(let ((foo (+ 3 5))) ; the calculation is performed directly, foo is 8
  ;; some other things
  (display foo)) ; foo is evaluated to 8 and printed

(let ((foo (lambda () (+ 3 5)))) ; the calculation is delayed, foo is a
                                 ; function that will perform it when needed
  ;; some other things
  (display (foo))) ; foo is evaluated as a function, returns 8 which is printed

In the second case, foo would be called a thunk.

Lazy languages blur the line between binding a variable to a value and creating a function to return that value, so that writing something like the first form above is actually treated like the second, under the hood.

Solution 2:

A "thunk" is a procedure object with no formal arguments, e.g. from your SRFI link:

(lambda () (write '(b1)))

The b1 variable is bound in the enclosing block, and this gives us a clue to the etymology of the word "thunk," which relies on a joke about poor grammar.

A zero-argument function has no way to change its behavior based on parameters it is called with, since it has no parameters. Therefore the entire operation of the function is set -- it is just waiting to be executed. No more "thought" is required on the part of the computer, all of the "thinking" has been done -- the action is completely "thunk" through.

That's all a "thunk" is in this SRFI's context -- a procedure with no arguments.

Solution 3:

Wikipedia has the following answer:

In functional programming, "thunk" is another name for a nullary function — a function that takes no arguments. Thunks are frequently used in strict languages as a means of simulating lazy evaluation; the thunk itself delays the computation of a function's argument, and the function forces the thunk to obtain the actual value. In this context, a thunk is often called a suspension or (in Scheme) a promise.

Adding a lazy evaluation example in Scheme. Here, promise is another word for thunk.