Recursion

[wboeke]

Recursion does'nt work. E.g:
: fact dup 2 > if dup 1- fact * then ;
5 fact .
allways gives 2
However: very simple recursion is okay:
variable n
0 n !
: tst n @ 10 < if n @ . 1 n +! tst then ;
tst
Any suggestion?

[tcoram]

Right now, toolboxforth only supports tail recursion. Effectively, for a tail recursive call to work it has to be the last word in the flow of execution. It essential compiles to a "goto" (jmp), so * is never executed. I've seen an implementation of Forth that looks ahead to see if there is a terminating word (e.g. ";" "then", etc) and does the optimization, otherwise does a normally recursive call. I've never had a use for normal recursion in Forth, but tail calls can actually be faster than begin again loops.

[tcoram]

One way to do normal recursion would be:
: fact dup 2 > if dup 1- ['] fact exec * then ;

[tcoram]

Added a more traditional recursion capability to util.f:
: recurse lwa @ 1+ count 63 and 2/ + [compile] lit , [compile] exec ; immediate

So, you can now
: fact dup 2 > if dup 1- recurse * then ;

[wboeke]

Sorry to reopen, but I tried it:
: fact dup 2 > if dup 1- recurse * then ;
5 fact .
120 ok
Hurrah!
Then double recursion:
: fib dup 1 > if dup 1 - recurse swap 2 - recurse + else drop 1 then ;
10 fib .
line: 1: Huh? >>> fib <<< .
ok

I am using recursion often in forth, but seldom double recursion. So: thank you!

[tcoram]

Ah! My bad. The problem is in my skipping of character packed words. That "fact" worked and "fib" didn't is due to my bad character skipping math (3 vs 4 character words).
So...
: recurse lwa @ 1+ count 63 and 2 /mod + + [compile] lit , [compile] exec ; immediate
does the right thing and should work!

Fix checked into util.f