Thun/docs/reference/ii.md

Combinator Take a quoted program from the stack and run it twice, first under the top item, then again with the top item. ... a [Q] ii

## ii Combinator Take a quoted program from the stack and run it twice, first under the top item, then again with the top item. ... a [Q] ii

 ... Q a Q

Definition

[dip]

Example

It's a little tricky to understand how this works so here's an example trace:

      1 2 3 4 [++] • [dip] dupdip i
1 2 3 4 [++] [dip] • dupdip i
      1 2 3 4 [++] • dip [++] i
             1 2 3 • ++ 4 [++] i
             1 2 4 • 4 [++] i
           1 2 4 4 • [++] i
      1 2 4 4 [++] • i
           1 2 4 4 • ++
           1 2 4 5 •

Discussion

In some cases (like the example above) this is the same effect as using [app2] but most of the time it's not:

   1 2 3 4 [+] ii
--------------------
        1 9

   1 2 3 4 [+] app2
----------------------
       1 2 5 6

Crosslinks

[app2] [b]