https://en.wikipedia.org/wiki/Tacit_programming

Tacit programming basically means constructing new function/routines without introducing explicitly named parameters.

In an ML-family language, you can construct `double` with something like:

  double = 2*

Partial application of the binary function * means we don't need to be explicit like this:

  double x = 2 * x

The linked Prolog is implementing an interpreter for a Forth-like language. Forths let you do tacit programming via operations on the stack. The above example, in Forth, would be:

  : double dup + ;
  : double 2 * ;

Neither of which has an explicit variable. The first duplicates the top of the stack then adds the top two items on the stack, the second pushes 2 onto the stack and then multiplies the top two items.

If you copy/paste the gist into the site below you can play around with the program stored in `main`. Change `main` to just:

  main --> mul, add.

With the query `main(R).` you'll get a result of `R = 1*2.`. Add each instruction one at a time and re-execute the query. The result will always be the top of the stack, before the last `is_` you'll get this:

  R = abs(5^(pi-5)).

Which is then processed by the last `is_` into the result, 0.05023...

https://play.scryer.pl/