6.9.5 General control structures with case

Gforth provides an extended case that solves the problems of the multi-exit loops discussed above, and offers additional options. You can find a portable implementation of this extended case in compat/caseext.fs.

There are three additional words in the extension. The first is ?of which allows general tests (rather than just testing for equality) in a case; e.g.,

     : sgn ( n -- -1|0|1 )
       ( n ) case
         dup 0 < ?of drop -1 endof
         dup 0 > ?of drop 1  endof
         \ otherwise leave the 0 on the stack
       0 endcase ;

Note that endcase drops a value, which works fine much of the time with of, but usually not with ?of, so we leave a 0 on the stack for endcase to drop. The n that is passed into sgn is also 0 if neither ?of triggers, and that is then passed out.

The second additional word is next-case, which allows turning case into a loop. Our triple-exit loop becomes:

     case
       condition1 ?of exit-code1 endof
       condition2 ?of exit-code2 endof
       condition3 ?of exit-code3 endof
       ...
     next-case
     common code afterwards

As you can see, this solves both problems of the variants discussed above (see BEGIN loops with multiple exits). Note that next-case does not drop a value, unlike endcase.¹

The last additional word is contof, which is used instead of endof and starts the next iteration instead of leaving the loop. This can be used in ways similar to Dijkstra's guarded command do, e.g.:

     : gcd ( n1 n2 -- n )
         case
             2dup > ?of tuck - contof
             2dup < ?of over - contof
         endcase ;

Here the two ?ofs have different ways of continuing the loop; when neither ?of triggers, the two numbers are equal and are the gcd. Endcase drops one of them, leaving the other as n.

You can also combine these words. Here's an example that uses each of the case words once, except endcase:

     : collatz ( u -- )
         \ print the 3n+1 sequence starting at u until we reach 1
         case
             dup .
             1 of endof
             dup 1 and ?of 3 * 1+ contof
             2/
         next-case ;

This example keeps the current value of the sequence on the stack. If it is 1, the of triggers, drops the value, and leaves the case structure. For odd numbers, the ?of triggers, computes 3n+1, and starts the next iteration with contof. Otherwise, if the number is even, it is divided by 2, and the loop is restarted with next-case.