Stack Effect Diagrams
As an aid to understanding the effect of 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.
( before -- after )For example, the basic Forth stack operators are described:
dup ( a -- a a ) drop ( a -- ) swap ( a b -- b a ) over ( a b -- a b a ) rot ( a b c -- b c a )And the fib function below is described:
fib ( n -- fib )It is equivalent to preconditions and postconditions in Hoare's logic. Both comments may also be referenced as assertions, thought not necessarily in context of Stack-based languages.
Read more about this topic: Stack-oriented Programming Language
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