5.21.1.3 Locals programming style

The freedom to define locals anywhere has the potential to change programming styles dramatically. In particular, the need to use the return stack for intermediate storage vanishes. Moreover, all stack manipulations (except PICKs and ROLLs with run-time determined arguments) can be eliminated: If the stack items are in the wrong order, just write a locals definition for all of them; then write the items in the order you want.

This seems a little far-fetched and eliminating stack manipulations is unlikely to become a conscious programming objective. Still, the number of stack manipulations will be reduced dramatically if local variables are used liberally (e.g., compare max (see Gforth locals) with a traditional implementation of max).

This shows one potential benefit of locals: making Forth programs more readable. Of course, this benefit will only be realized if the programmers continue to honour the principle of factoring instead of using the added latitude to make the words longer.

Using TO can and should be avoided. Without TO, every value-flavoured local has only a single assignment and many advantages of functional languages apply to Forth. I.e., programs are easier to analyse, to optimize and to read: It is clear from the definition what the local stands for, it does not turn into something different later.

E.g., a definition using TO might look like this:

     : strcmp { addr1 u1 addr2 u2 -- n }
      u1 u2 min 0
      ?do
        addr1 c@ addr2 c@ -
        ?dup-if
          unloop exit
        then
        addr1 char+ TO addr1
        addr2 char+ TO addr2
      loop
      u1 u2 - ;

Here, TO is used to update addr1 and addr2 at every loop iteration. strcmp is a typical example of the readability problems of using TO. When you start reading strcmp, you think that addr1 refers to the start of the string. Only near the end of the loop you realize that it is something else.

This can be avoided by defining two locals at the start of the loop that are initialized with the right value for the current iteration.

     : strcmp { addr1 u1 addr2 u2 -- n }
      addr1 addr2
      u1 u2 min 0
      ?do { s1 s2 }
        s1 c@ s2 c@ -
        ?dup-if
          unloop exit
        then
        s1 char+ s2 char+
      loop
      2drop
      u1 u2 - ;

Here it is clear from the start that s1 has a different value in every loop iteration.