Sometimes you face a Forth system, that does not support a specific feature that you might want to use. For example you might want to compare two numbers (first and second) to find out whether or not the first number is less or equal than the second number. You would like to write
first second <=
If the the system you use does not define the word
<= you have several choices to deal with this:
<=you can write the substitution phrase
> 0=as less or equal is the same as not greater.
: <= ( n1 n2 -- f ) > 0= ;
It is not always reasonable to make a new definition. Sometimes it is better to just use the substitution phrase as is in the program.
Anyway it is good to know about substitution phrases.
So here is the definition of a substition phrase
A substitution phrase is a (typically short) sequence of words that replace an undefined word. Executing the phrase and executing the replaced word would have the same effect (if the word would be defined).
Here are some substitution phrases that you can use and that might be used in programs that you encounter:
|You can write||instead of||if|
|no proper flag is necessary, i.e. for
Some Forth systems do not support
DO LOOP or don't have
+LOOP. You can substitute their uses with appropriate
BEGIN UNTIL ( or
BEGIN WHILE REPEAT) loops.
Please have a look at the files
for a discussion how substitute these.
Sometimes you cannot just use a substitution phrase and pour them into a definition. When invoking a word it uses the return stack to store the return address of the caller, so execution can continue in the caller when the current word is done. So, if your execution phrase contains return stack operators (as the substitution phrase for
2r> above), you cannot use that phrase to define a substitution word.
\ INCORRECT : 2r> ( -- x1 x2 ) r> r> swap ; \ INCORRECT
You would need to define a compiling word and compile the appropriate words as if the substition phrase would have been used directly:
\ CORRECT : 2r> ( -- x1 x2 ) postpone r> postpone r> postpone swap ; immediate
If this in the end is simpler than using the substitution phrase directly, depends on the situation.