Forth 2012: RECURSE

Interpretation:

Interpretation semantics for this word are undefined.

Compilation:

( -- )

Append the execution semantics of the current definition to the current definition. An ambiguous condition exists if RECURSE appears in a definition after DOES>.

See:

6.1.1250 DOES>, 6.1.2120 RECURSE, A.6.1.2120 RECURSE.

Rationale:

Typical use: : X ... RECURSE ... ;

This is Forth's recursion operator; in some implementations it is called MYSELF. The usual example is the coding of the factorial function.

: FACTORIAL ( +n1 -- +n2)
DUP 2 < IF DROP 1 EXIT THEN
DUP 1- RECURSE *
;

n₂ = n₁(n₁-1)(n₁-2)...(2)(1), the product of n₁ with all positive integers less than itself (as a special case, zero factorial equals one). While beloved of computer scientists, recursion makes unusually heavy use of both stacks and should therefore be used with caution. See alternate definition in A.6.1.2140 REPEAT.

Testing:

T{ : GI6 ( N -- 0,1,..N )
DUP IF DUP >R 1- RECURSE R> THEN ; -> }T
T{ 0 GI6 -> 0 }T
T{ 1 GI6 -> 0 1 }T
T{ 2 GI6 -> 0 1 2 }T
T{ 3 GI6 -> 0 1 2 3 }T
T{ 4 GI6 -> 0 1 2 3 4 }T

DECIMAL
T{ :NONAME ( n -- 0, 1, .., n )
     DUP IF DUP >R 1- RECURSE R> THEN
   ;
   CONSTANT rn1 -> }T
T{ 0 rn1 EXECUTE -> 0 }T
T{ 4 rn1 EXECUTE -> 0 1 2 3 4 }T

:NONAME ( n -- n1 )
   1- DUP
   CASE 0 OF EXIT ENDOF
     1 OF 11 SWAP RECURSE ENDOF
     2 OF 22 SWAP RECURSE ENDOF
     3 OF 33 SWAP RECURSE ENDOF
     DROP ABS RECURSE EXIT
   ENDCASE
; CONSTANT rn2

T{ 1 rn2 EXECUTE -> 0 }T
T{ 2 rn2 EXECUTE -> 11 0 }T
T{ 4 rn2 EXECUTE -> 33 22 11 0 }T
T{ 25 rn2 EXECUTE -> 33 22 11 0 }T