3.28 Interpretation and Compilation Semantics and Immediacy

When a word is compiled, it behaves differently from being interpreted. E.g., consider +:

     1 2 + .
     : foo + ;

These two behaviours are known as compilation and interpretation semantics. For normal words (e.g., +), the compilation semantics is to append the interpretation semantics to the currently defined word (foo in the example above). I.e., when foo is executed later, the interpretation semantics of + (i.e., adding two numbers) will be performed.

However, there are words with non-default compilation semantics, e.g., the control-flow words like if. You can use immediate to change the compilation semantics of the last defined word to be equal to the interpretation semantics:

     : [FOO] ( -- )
      5 . ; immediate
     
     [FOO]
     : bar ( -- )
       [FOO] ;
     bar
     see bar

Two conventions to mark words with non-default compilation semantics are names with brackets (more frequently used) and to write them all in upper case (less frequently used).

In Gforth (and many other systems) you can also remove the interpretation semantics with compile-only (the compilation semantics is derived from the original interpretation semantics):

     : flip ( -- )
      6 . ; compile-only \ but not immediate
     flip
     
     : flop ( -- )
      flip ;
     flop

In this example the interpretation semantics of flop is equal to the original interpretation semantics of flip.

The text interpreter has two states: in interpret state, it performs the interpretation semantics of words it encounters; in compile state, it performs the compilation semantics of these words.

Among other things, : switches into compile state, and ; switches back to interpret state. They contain the factors ] (switch to compile state) and [ (switch to interpret state), that do nothing but switch the state.

     : xxx ( -- )
       [ 5 . ]
     ;
     
     xxx
     see xxx

These brackets are also the source of the naming convention mentioned above.

Reference: Interpretation and Compilation Semantics.