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

I come across the word ‘thunk’ at a lot of places in code and documentation related to Scheme, and similar territories. I am guessing that it is a generic name for a procedure, which has a single formal argument. Is that correct? If yes, is there more to it? If no, please?

For eg. in SRFI 18, in the ‘Procedures’ section.


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.

Source : Link , Question Author : Community , Answer Author : Svante

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