Concatenative programming language
A concatenative programming language is a point-free computer programming language in which all expressions denote functions and the juxtaposition of expressions denotes function composition. Concatenative programming replaces function application, which is common in other programming styles, with function composition as the default way to build subroutines. Functions and procedures written in concatenative style are not value level, i.e. they typically don't represent the data structures they operate on with explicit names or identifiers; instead they are function level - a function is defined as a pipeline, a sequence of operations that take parameters from an implicit data structure on which all functions operate, and return the function results to that shared structure so that it will be used by the next operator.
For example, a sequence of operations in an applicative language like the following:
y = foo(x) z = bar(y) w = baz(z)
...is written in a concatenative language as a sequence of functions, without parameters:
foo bar baz
The combination of a compositional semantics with a syntax that mirrors such a semantics makes concatenative languages highly amenable to algebraic manipulation of programs; although it may be difficult to write mathematical expressions directly in them. Concatenative languages can be implemented in an efficient way with a stack machine, and are a common strategy to program virtual machines. 
Much of the original work on concatenative language theory was carried out by Manfred von Thun.
The properties of concatenative languages are the result of their compositional syntax and semantics:
- The reduction of any expression is the simplification of one function to another function; it is never necessary to deal with the application of functions to objects.
- Any subexpression can be replaced with a name that represents the same subexpression. This is referred to in the concatenative community as factoring and is used extensively to simplify programs into smaller parts.
- The syntax and semantics of concatenative languages form the algebraic structure of a monoid.
- Concatenative languages can be made well-suited to an implementation inspired by linear logic where no garbage is ever generated.
The first concatenative programming language was Forth, although Joy was the first language to call itself concatenative. Other concatenative languages are Cat, Enchilada, Factor, Onyx, PostScript, RPL, Staapl, Trith, XY, Kitten, and Om.
Most existing concatenative languages are stack-based; this is not a requirement and other models have been proposed. Concatenative languages are currently used for embedded, desktop, and web programming, as target languages, and for research purposes.
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- concatenative.org: A wiki about concatenative languages
- Staapl: Forth on Scheme for Embedded Controllers
- The Enchilada Programming Language
- The Concatenative Language XY
- The Om Programming Language