Forth Programmer’s Handbook
4.1.6 Text Interpreter Directives
It is useful to control the logical flow when compiling an application. You may wish, for example, to load a certain source code file only if a flag indicates the need for that file. A number of Forth words provide this kind of control. Although these words are almost always used outside of definitions, they are all IMMEDIATE (i.e., they will execute when encountered in compiling state) and so may be used in definitions, if needed.
Here are two examples:
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<flag> [IF] |
INCLUDE <file1> |
[THEN] |
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[DEFINED] <word in file1> [IF] |
INCLUDE <file2> |
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[ELSE] INCLUDE <file3> [THEN] |
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Glossary |
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[DEFINED] <name> |
( — flag ) |
common usage |
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Search the dictionary for name. If name is found, return true; otherwise, return |
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false. “bracket-defined” |
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[UNDEFINED] <name> |
( — flag ) |
common usage |
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Search the dictionary for name. If the word is found, return false; otherwise, |
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return true. “bracket-undefined” |
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[IF] |
( flag — ) |
Tools Ext |
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Begin an interpretive branch. If the flag is true, do nothing (i.e., continue interpretation). If the flag is false, parse and discard words from the parse area (including nested occurrences of [IF] … [THEN] clauses) until either the word [ELSE] or the word [THEN] has been parsed and discarded. If the parse area becomes exhausted, it is refilled as with REFILL. Because [IF] discards [ELSE] (if it is present) when flag is false, interpretation will continue after [ELSE], thus interpreting the contents of the [ELSE] clause. “bracket-if”
[ELSE] ( — ) Tools Ext
Parse and discard words from the parse area (including nested occurrences of [IF] … [THEN] clauses) until the word [THEN] has been parsed and discarded. If the parse area becomes exhausted, it is refilled as with REFILL.
The Forth Interpreter and Compiler 131