Gforth allows you to define combined words – words that have an arbitrary combination of interpretation and compilation semantics.
interpret/compile:
( interp-xt comp-xt "name" – ) gforth-0.2 “interpret/compile:”
This feature was introduced for implementing TO
and S"
. I
recommend that you do not define such words, as cute as they may be:
they make it hard to get at both parts of the word in some contexts.
E.g., assume you want to get an execution token for the compilation
part. Instead, define two words, one that embodies the interpretation
part, and one that embodies the compilation part. Once you have done
that, you can define a combined word with interpret/compile:
for
the convenience of your users.
You might try to use this feature to provide an optimizing implementation of the default compilation semantics of a word. For example, by defining:
:noname foo bar ; :noname POSTPONE foo POSTPONE bar ; interpret/compile: opti-foobar
as an optimizing version of:
: foobar foo bar ;
Unfortunately, this does not work correctly with [compile]
,
because [compile]
assumes that the compilation semantics of all
interpret/compile:
words are non-default. I.e., [compile]
opti-foobar
would compile compilation semantics, whereas
[compile] foobar
would compile interpretation semantics.
Some people try to use state-smart words to emulate the feature provided
by interpret/compile:
(words are state-smart if they check
STATE
during execution). E.g., they would try to code
foobar
like this:
: foobar STATE @ IF ( compilation state ) POSTPONE foo POSTPONE bar ELSE foo bar ENDIF ; immediate
Although this works if foobar
is only processed by the text
interpreter, it does not work in other contexts (like '
or
POSTPONE
). E.g., ' foobar
will produce an execution token
for a state-smart word, not for the interpretation semantics of the
original foobar
; when you execute this execution token (directly
with EXECUTE
or indirectly through COMPILE,
) in compile
state, the result will not be what you expected (i.e., it will not
perform foo bar
). State-smart words are a bad idea. Simply don’t
write them20!
For a more detailed discussion of this topic, see
M. Anton Ertl,
State
-smartness—Why
it is Evil and How to Exorcise it, EuroForth ’98.