> Supporting random numbers of local variables in Forth makes for > complex handling of the return stack at run-time, or a complex > compiler, or both. > > Reforth supports precisely two local variables: at and my > > at is designed to hold an address and is set with at!. It is > like the A register from Chuck's later work, except it does > not have to be saved and restored when calling words that also > use it for their own purposes. The words @+ and !+ use and > increment it automatically. It also works well (and importantly, > reads well) as a pointer for relative addressing. > > my is a general purpose bucket set with my!. It fills the same > sort of role as >r, r@, and r>, but is faster, uses less code, > and doesn't need to be cleaned up.

I haven't implemented anything using these yet, but here's a quick implementation. (The actual preseve/restore using the v:preserve combinator is done a bit later)

~~~'At var 'My var :at! !At ; :at @At ; :my! !My ; :my @My ; ~~~

Subwords

> Word definitions may be nested. > > : hiphip ( -- ) > : cheer "hip hip, hooray!" type ; > cheer cheer cheer > ; > Ok, so one could do the above with a loop. But there is more to it: > > Sub-words are private and scoped so they are only visible in the > current definition, avoiding namespace pollution. Above, one could > not call cheer outside the definition of hiphip. > > Smaller definitions are a good thing for code clarity and maintenance. > Go ahead and break up those long definitions into sub-words. Try using > a sub-words instead of a nested code blocks. > > Private sub-words solve half the problem with vocabularies. More on > this later.

I'm not comfortable with this as it does add some overhead. The colon sigil is extended to add a jump around word entries, so that nesting can work properly. This also extends the colon and semicolon words to add support for preserving the my and at variables.

~~~{{ 'Scope var 'Nest var :def here n:inc #0 compile:jump swap d:create &class:word reclass @Nest n:zero? [ @Dictionary !Scope ] if &Nest v:inc @Compiler &Compiler v:on '&At_[_&My_[ s:evaluate ; :-def ']_v:preserve_]_v:preserve s:evaluate compile:ret !Compiler here swap store &Nest v:dec @Nest n:zero? [ @Scope !Dictionary ] if ; immediate ---reveal--- :sigil:: def ; immediate :; |-def -def immediate #0 !Nest }} ~~~

Sean also extends this further, adding in support for using the subwords as a vocabulary of sorts. I'm not implementing this in RETRO as I rather dislike it. On the whole, I'd rather throw the words into a namespace, such that:

> :hiphip:cheer 'hiphip,hooray! s:put nl ; > > :hiphip (-) > hiphip:cheer hiphip:cheer hiphip:cheer ;

> {{ > :cheer 'hiphip,hooray! s:put nl ; > ---reveal--- > :hiphip (-) cheer cheer cheer ; > }}