retroforth/doc/book/techniques/quotes

# Quotations

Quotes are anonymous functions. RETRO uses these as the basis for
executable flow control and combinatorial logic.

## Using Quotations

To make a quotation, surround the code with square brackets. E.g.,

    #1 #2 eq? [ 'No_match s:put nl ] -if

Quotes can be nested:

    [ #3 [ #4 ] dip ] call

After creation, a pointer to the quotation is left on the stack
(or is compiled into the current definition).

## Combinators

Words operating on quotations are called combinators; these are
discussed in *Using Combinators*.

## Implementation

A quotation is compiled as:

    ... code before quotation ...
    i liju....                    (if_compiling_only)
    d address after quotation     (if_compiling_only)
    ... code for quotation
    i re......                    (this_is_where_the_quote_ends)
    i li......
    d address of code for quotation
    ... code after quotation ....

## Other Notes

Quotations are used heavily in RETRO. They give the source a
feel that's different from traditional Forth, and allow for
a more consistent syntax.

For instance, in a traditional Forth, you might have some
conditionals:

    IF ... THEN
    IF ... ELSE ... THEN
    IF ... EXIT THEN

RETRO uses conditional combinators for these:

    [ ... ] if
    [ ... ] [ ... ] choose
    [ ... ] if;

Or loops:

    FOR ... NEXT

Is replaced by:

    [ ... ] times

This can also extend to stack flow. Sequences like:

    >R ... >R
    DUP >R ... >R

Become:

    [ ... ] dip
    [ ... ] sip

And forms like:

    1 2 3 * swap 3 * swap

Can be replaced with a combinator like:

    #1 #2 [ #3 * ] bi@

While there is a different set of words to learn, I find that
overall there's less noise from low level stack shuffling words
and the added consistency with regards to overall syntax has
been nice as I was never fond of the multiple forms that existed
in traditional Forth.