One-time file loading

[Other proposals]


A library is needed by several parts of the source code, but it should be loaded only once.


REQUIRED ( i*x c-addr u -- i*x )
If the file specified by c-addr u has been INCLUDED or REQUIRED already, discard c-addr u; otherwise, perform INCLUDED.

If the same file is REQUIRED twice using different names (e.g., through symbolic links), or different files with the same name are REQUIRED (by doing some renaming between the invocations of REQUIRED), the file will be INCLUDED the first time, but not necessarily several times.

An ambiguous conditionm exists if the stack effect of including the file is not ( i*x -- i*x ).

Typical Use

... s" filename" required ...


The syntax follows the good example of INCLUDED in being non-parsing. A parsing version like NEEDS can be easily defined from it with:
Some might say: Why not use the C solution: The C solution to this problem is putting a wrapper like
#ifndef FILE_H
#define FILE_H
around every source file. This is inefficient (the whole file has to be read again, unless the compiler does some pretty sophisticated stuff), requires cooperation from the author of the file (which is problematic, because not the author, but the users of the file have the trouble), and is cumbersome to express in standard Forth (there is no direct equivalent to #ifdef).


Gforth has REQUIRED. Many systems, including F-PC, Gforth, and Win32Forth, have NEEDS, a parsing version of REQUIRED. I use it extensively, and so do others.

Change History

Aug 14,2001: Changed stack effect from ( i*x c-addr u -- j*x ) to ( i*x c-addr u -- i*x ), following a suggestion from Guido Draheim, and related changes in the text.


Michael L. Gassanenko:
Yes. I do use NEEDS.

> (there is no direct equivalent to #ifdef).

I often invent analogs to #ifdef , namely,
if a name is defined, interpret the following line.
AFAIK, both F-PC and WIN32FOR have such words,
but the only name I can remember is #ifdef / #ifndef (it seems, I myself
used this name, because the others were not much meaningful).
Peter Knaggs:
introduced a version of REQUIRED some time ago that works exactly in
this manner.  I agree very much with its usefulness, indeed it would
allow a standard "library" model.  Having said that, it can be defined
using standard ANS.
Guido Draheim:
* Both's Swiftforth and MPE's ProforthVFX have defined `requires`
as the selfparsing version of `required`, and they use it. PFE will adopt
this in the next version (> 0.30.30)

* The specification is non-deterministic in its stack effect - the user of
`required` has no way to check in advance if a file will get `included`.
Many systems use a word like `loaded?` but it can not as easily specified
for many systems just as it is done for `required` itself, where the form of
`requires` has found wide acceptance. It may be useful to *allow* the use 
CSP-like techniques to ensure a deterministic stack-effect, and it may be
useful to *recommend* that atleast a warning message is shown for the case
of stack-depth differences.

Anton Ertl