# Conditional Interpret¶

It is often desirable to do actions conditionally. E.g. define a word if it’s not there. For that, the words [defined] and [undefined] can be used. Amforth lacks an [if] to really make use of them. A real [if] is not that easy and a huge piece of code since it has to support nested [if] too. A way simpler solution is the following. It is restricted to the current SOURCE content, which is usually the current command line.

The basic idea is a conditional comment: ?\. It takes a flag and works like \ if the flag is true. if the flag is false, the remaining line is interpreted as if nothing has happened.

: ?\ ( f -- )
if postpone \ then
; immediate


The use is straigt forward:

\ define foo unless it already exists
[undefined] foo ?\ : foo ." I'm foo " ;

\ call a word if defined
[defined] ver 0= ?\ ver


This recipe is based on a usenet posting of Bruce McFarling, 13.7.2014, on comp.lang.forth.