3.33 POSTPONE

You can compile the compilation semantics (instead of compiling the interpretation semantics) of a word with POSTPONE:

     : MY-+ ( Compilation: -- ; Run-time of compiled code: n1 n2 -- n )
      POSTPONE + ; immediate
     : foo ( n1 n2 -- n )
      MY-+ ;
     1 2 foo .
     see foo

During the definition of foo the text interpreter performs the compilation semantics of MY-+, which performs the compilation semantics of +, i.e., it compiles + into foo.

This example also displays separate stack comments for the compilation semantics and for the stack effect of the compiled code. For words with default compilation semantics these stack effects are usually not displayed; the stack effect of the compilation semantics is always ( -- ) for these words, the stack effect for the compiled code is the stack effect of the interpretation semantics.

Note that the state of the interpreter does not come into play when performing the compilation semantics in this way. You can also perform it interpretively, e.g.:

     : foo2 ( n1 n2 -- n )
      [ MY-+ ] ;
     1 2 foo .
     see foo

However, there are some broken Forth systems where this does not always work, and therefore this practice was been declared non-standard in 1999.

Here is another example for using POSTPONE:

     : MY-- ( Compilation: -- ; Run-time of compiled code: n1 n2 -- n )
      POSTPONE negate POSTPONE + ; immediate compile-only
     : bar ( n1 n2 -- n )
       MY-- ;
     2 1 bar .
     see bar

You can define ENDIF in this way:

     : ENDIF ( Compilation: orig -- )
       POSTPONE then ; immediate

Assignment: Write MY-2DUP that has compilation semantics equivalent to 2dup, but compiles over over.