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Programming paradigm that relies on a stack machine model

Stack oriented programming is a programming paradigm that relies on a stack (or multiple stacks) to manipulate data and/or pass parameters. Several programming languages fit this description, notably Forth, RPL, and PostScript. Stack oriented programming languages operate on one or more stacks, each of which may serve a different purpose. Programming constructs in other programming languages need to be modified for use in a stack oriented system. Most stack oriented languages operate in postfix or Reverse Polish notation. Any arguments or parameters for a command are stated before that command. For example, postfix notation would be written 2, 3, multiply instead of multiply, 2, 3 (prefix or Polish notation), or 2 multiply 3 (infix notation). The programming languages Forth, Factor, RPL, PostScript, BibTeX style design language and many assembly languages fit this paradigm. Stack based algorithms consider data, by utilising one piece of data from atop the stack, and returning data back atop the stack. The need for stack manipulation operators, allow for the stack to manipulate data. To emphasise the effect of a statement, a comment is used showing the top of the stack before and after the statement. This is known as the stack effect diagram. PostScript stacks consider separate stacks for additional purposes. This considers variables, dictionaries, procedures, anatomy of some typical procedures, control and flow. The analysis of the language model allows expressions and programs to be interpreted simply and theoretically.

Since the stack is the key means to manipulate data in a stack oriented language, such languages often provide some sort of stack manipulation operators. Commonly provided are dup, to duplicate the element atop the stack, exch (or swap), to exchange elements atop the stack (the first becomes second and the second becomes first), roll, to cyclically permute elements in the stack or on part of the stack, pop (or drop), to discard the element atop the stack (push is implicit), and others. These become key in studying procedures.

PostScript and some other stack languages have other separate stacks for other purposes.

The simple model provided in a stack oriented language allows expressions and programs to be interpreted simply and theoretically evaluated much faster, since no syntax analysis need be done, only lexical analysis. The way such programs are written facilitates being interpreted by machines, which is why PostScript suits printers well for its use. However, the slightly artificial way of writing PostScript programs can form an initial barrier to understanding stack oriented languages such as PostScript. While the ability to shadow by overriding inbuilt and other definitions can make programs hard to debug, and irresponsible use of this feature can cause unpredictable behaviour, it can simplify some functions greatly. For example, in PostScript use, the showpage operator can be overridden with a custom one that applies a certain style to the page, instead of having to define a custom operator or to repeat code to generate the style.