Editing Forth (programming language) (section)

==Overview==
{{further|Reverse Polish notation}}

Forth emphasizes the use of small, simple functions called ''words''. Words for bigger tasks call upon many smaller words that each accomplish a distinct sub-task. A large Forth program is a hierarchy of words. These words, being distinct modules that communicate implicitly via a stack mechanism, can be prototyped, built and tested independently. The highest level of Forth code may resemble an English-language description of the application. Forth has been called a ''meta-application language'': a language that can be used to create [[Domain-specific language|problem-oriented languages]].<ref>{{cite book |last=Brodie |first=Leo |title=Starting Forth |edition=2nd |year=1987 |publisher=Prentice-Hall |isbn=978-0-13-843079-5 |url=https://www.forth.com/starting-forth/index.html}}</ref>

Forth relies on implicit use of a [[stack (abstract data type)|data stack]] and [[reverse Polish notation]] which is commonly used in calculators from [[Hewlett-Packard]]. In RPN, the operator is placed after its operands, as opposed to the more common [[infix notation]] where the operator is placed between its operands. Postfix notation makes the language easier to parse and extend; Forth's flexibility makes a static [[Backus-Naur form|BNF]] grammar inappropriate, and it does not have a monolithic compiler. Extending the compiler only requires writing a new word, instead of modifying a grammar and changing the underlying implementation.

Using RPN, one can compute the value of the arithmetic expression (25 × 10) + 50 in the following way:
<!-- https://pygments.org/docs/lexers/#pygments.lexers.forth.ForthLexer -->
<syntaxhighlight lang="forth">
 25 10 * 50 + CR .
 300 ok
</syntaxhighlight>

[[File:Stack1.svg|150px|left]]
First the numbers 25 and 10 are put on the stack.{{clear}}

[[File:Forthstack1 5.svg|150px|left]]
<br/>The word <code>*</code> takes the top two numbers from the stack, multiplies them, and puts the product back on the stack.{{clear}}

[[File:Forthstack2.svg|150px|left]]
Then the number 50 is placed on the stack.{{clear}}

[[File:Forthstack3.svg|150px|left]]
<br/>The word <code>+</code> adds the top two values, pushing the sum. <code>CR</code> ([[carriage return]]) starts the output on a new line. Finally, <code>.</code> prints the result. As everything has completed successfully, the Forth system prints <code>OK</code>.<ref name="3yqc9">{{harvnb|Brodie|1987|p=20}}</ref>{{clear}}

Even Forth's structural features are stack-based. For example:
<syntaxhighlight lang="forth">
 : FLOOR5 ( n -- n' )   DUP 6 < IF DROP 5 ELSE 1 - THEN ;
</syntaxhighlight>
The colon indicates the beginning of a new definition, in this case a new word (again, ''word'' is the term used for a subroutine) called <code>FLOOR5</code>. The text in parentheses is a comment, advising that this word expects a number on the stack and will return a possibly changed number (on the stack).

The subroutine uses the following commands: <code>DUP</code> duplicates the number on the stack; <code>6</code> pushes a 6 on top of the stack; <code>&lt;</code> compares the top two numbers on the stack (6 and the <code>DUP</code>ed input), and replaces them with a true-or-false value; <code>IF</code> takes a true-or-false value and chooses to execute commands immediately after it or to skip to the <code>ELSE</code>; <code>DROP</code> discards the value on the stack; <code>5</code> pushes a 5 on top of the stack; and <code>THEN</code> ends the conditional.

The <code>FLOOR5</code> word is equivalent to this function written in the [[C (programming language)|C programming language]] using the [[?:|conditional operator]] '?:'
<syntaxhighlight lang="c">
int floor5(int v) {
  return (v < 6) ? 5 : (v - 1);
}
</syntaxhighlight>
This function is written more succinctly as:
<syntaxhighlight lang="forth">
 : FLOOR5 ( n -- n' ) 1- 5 MAX ;
</syntaxhighlight>
This can be run as follows:
<syntaxhighlight lang="forth">
 1 FLOOR5 CR .
 5 ok
 8 FLOOR5 CR .
 7 ok
</syntaxhighlight>
First a number (1 or 8) is pushed onto the stack, <code>FLOOR5</code> is called, which pops the number again and pushes the result. <code>CR</code> moves the output to a new line (again, this is only here for readability). Finally, a call to <code>.</code> pops the result and prints.