Editing Forth (programming language) (section)

=== Mixing states of compiling and interpreting ===
Here is the definition of a word <code>EMIT-Q</code> which when executed emits the single character <code>Q</code>:
<syntaxhighlight lang="forth">
 : EMIT-Q   81 ( the ASCII value for the character 'Q' ) EMIT ;
</syntaxhighlight>
This definition was written to use the [[ASCII]] value of the <code>Q</code> character (81) directly. The text between the parentheses is a comment and is ignored by the compiler. The word <code>EMIT</code> takes a value from the data stack and displays the corresponding character.

The following redefinition of <code>EMIT-Q</code> uses the words <code><nowiki>[</nowiki></code> (left-bracket), <code><nowiki>]</nowiki></code> (right-bracket), <code>CHAR</code> and <code>LITERAL</code> to temporarily switch to interpreter state, calculate the ASCII value of the <code>Q</code> character, return to compilation state and append the calculated value to the current colon definition:
<syntaxhighlight lang="forth">
 : EMIT-Q   [ CHAR Q ]  LITERAL  EMIT ;
</syntaxhighlight>
The parsing word <code>CHAR</code> takes a space-delimited word as parameter and places the value of its first character on the data stack. The word <code><nowiki>[CHAR]</nowiki></code> is an immediate version of <code>CHAR</code>. Using <code><nowiki>[CHAR]</nowiki></code>, the example definition for <code>EMIT-Q</code> could be rewritten like this:
<syntaxhighlight lang="forth">
 : EMIT-Q   [CHAR] Q  EMIT ; \ Emit the single character 'Q'
</syntaxhighlight>
This definition used <code>\</code> (backslash) for the describing comment.

Both <code>CHAR</code> and <code><nowiki>[CHAR]</nowiki></code> are predefined in {{Not a typo|ANS}} <!-- not a misspelling --> Forth. Using <code>IMMEDIATE</code> and <code>POSTPONE</code>, <code><nowiki>[CHAR]</nowiki></code> could have been defined like this:
<syntaxhighlight lang="forth">
 : [CHAR]   CHAR  POSTPONE LITERAL ; IMMEDIATE
</syntaxhighlight>