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5.10.1 Combined Words

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       “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 them1!

It is also possible to write defining words that define words with arbitrary combinations of interpretation and compilation semantics. In general, they look like this:

     : def-word
         create-interpret/compile
         code1
     interpretation>
         code2
     <interpretation
     compilation>
         code3
     <compilation ;

For a word defined with def-word, the interpretation semantics are to push the address of the body of word and perform code2, and the compilation semantics are to push the address of the body of word and perform code3. E.g., constant can also be defined like this (except that the defined constants don't behave correctly when [compile]d):

     : constant ( n "name" -- )
         create-interpret/compile
         ,
     interpretation> ( -- n )
         @
     <interpretation
     compilation> ( compilation. -- ; run-time. -- n )
         @ postpone literal
     <compilation ;

doc-create-interpret/compile doc-interpretation> doc-<interpretation doc-compilation> doc-<compilation

Words defined with interpret/compile: and create-interpret/compile have an extended header structure that differs from other words; however, unless you try to access them with plain address arithmetic, you should not notice this. Words for accessing the header structure usually know how to deal with this; e.g., ' word >body also gives you the body of a word created with create-interpret/compile.


Fußnoten

[1] 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.