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## primrec

Combinator

From the ["Overview of the language JOY"](https://www.kevinalbrecht.com/code/joy-mirror/j00ovr.html)

> The primrec combinator expects two quoted programs in addition to a
> data parameter. For an integer data parameter it works like this: If
> the data parameter is zero, then the first quotation has to produce the
> value to be returned. If the data parameter is positive then the second
> has to combine the data parameter with the result of applying the
> function to its predecessor.

> 5  \[1\]  \[\*\]  primrec

> Then primrec tests whether the top element on the stack (initially the
> 5) is equal to zero. If it is, it pops it off and executes one of the
> quotations, the \[1\] which leaves 1 on the stack as the result.
> Otherwise it pushes a decremented copy of the top element and recurses.
> On the way back from the recursion it uses the other quotation, \[\*\],
> to multiply what is now a factorial on top of the stack by the second
> element on the stack.


       0 [Base] [Recur] primrec
    ------------------------------
          Base

             n [Base] [Recur] primrec
    ------------------------------------------ n > 0
       n (n-1) [Base] [Recur] primrec Recur

### Discussion

Simple and useful specialization of the [genrec] combinator from the
[original Joy system](https://www.kevinalbrecht.com/code/joy-mirror/index.html).

### Crosslinks

[genrec]
[tailrec]