Editing Stack-oriented programming (section)

== Stack effect diagrams ==
As an aid to understanding the effect of the statement, a short comment is used showing the top of the stack before and after the statement. The top of the stack is rightmost if there are multiple items. This notation is commonly used in the Forth language, where comments are enclosed in parentheses.

<syntaxhighlight lang="forth">( before -- after )</syntaxhighlight>

For example, the basic Forth stack operators are described:

<syntaxhighlight lang="forth">
dup  ( a -- a a )
drop ( a -- )
swap ( a b -- b a )
over ( a b -- a b a )
rot  ( a b c -- b c a )
</syntaxhighlight>

The <code>fib</code> function below is described:

<syntaxhighlight lang="forth">fib  ( n -- n' )</syntaxhighlight>

It is equivalent to [[precondition]]s and [[postcondition]]s in [[Hoare logic]]. Both comments may also be referenced as [[assertion (computing)|assertions]], though not necessarily in the context of Stack-based languages.