6.14.5 How to define immediate words ¶

You can change the compilation semantics of a word to be the same as the interpretation semantics with

immediate ( –  ) core

Change the compilation semantics of a word to be the same as its interpretation semantics.

A contrived example:

: [foo]
  ." foo" ; immediate

: bar
  [foo] ; \ prints "foo"
bar \ no output

The immediate causes [foo] to perform the interpretation semantics during the definition of bar rather than compiling them. A convention sometimes (but not always) used for immediate words is to have their names in brackets, e.g. ['].

A common use of immediate is to define macros (see Macros).

The text interpretation of a macro in interpret state is often a mistake, so you can turn the macro into a compile-only word with

compile-only ( –  ) gforth-0.2

Mark the last definition as compile-only; as a result, the text interpreter and ' will warn when they encounter such a word.

Example:

: endif
  postpone then ; immediate compile-only

: foo
  if ." true" endif ;

endif \ "warning: endif is compile-only"

The warning is followed by a stack underflow error because then wants to consume an orig (see Arbitrary control structures).

Note that compiling code while the text interpreter is in interpret state is not a problem in itself, even if a number of words are marked compile-only. A more serious problem is compiling code if the current definition is not an unfinished colon definition: there is no way to run the resulting code. Gforth warns about that even if a word is not marked compile-only or if you text-interpret it in compile state:

: compile-+
  postpone + ;

: foo [ compile-+ ] ; \ no warning; interpretation semantics of compile-+
                      
compile-+ \ warning: Compiling outside a definition
if        \ warning: IF is compile-only
          \ warning: Compiling outside a definition
compile-+ \ warning: Compiling outside a definition
then      \ warning: THEN is compile-only
] if      \ warning: Compiling outside a definition
  +       \ warning: Compiling outside a definition
  then
[

Note that switching to compile state in the last four lines silences the “is compile-only” warnings, because in these lines the compilation semantics of the words is performed.

Why does then not produce “Compiling outside a definition” warnings in the example above? Then does not generate any code by itself, it just changes the target of the code compiled by the matching if or ahead.

restrict ( –  ) gforth-0.2

A synonym for compile-only