OpenSCAD User Manual/Functional Descriptive Language

OpenSCAD is a Functional programming language, as such variables are bound to expressions and keep a single value during their entire lifetime due to the requirements of referential transparency. In imperative languages, such as C, the same behavior is seen as constants, which are typically contrasted with normal variables.

In other words OpenSCAD variables are more like constants, but with an important difference. If variables are assigned a value multiple times, only the last assigned value is used in all places in the code. See further discussion at #Variables cannot be changed. This behavior is due to the need to supply variable input on the command line, via the use of -D variable=value option. OpenSCAD currently places that assignment at the end of the source code, and thus must allow a variable's value to be changed for this purpose.

Values cannot be modified during run time; all variables are effectively constants that do not change once created. Each variable retains its last assigned value, in line with Functional programming languages. Unlike Imperative languages, such as C, OpenSCAD is not an iterative language, and as such the concept of x = x + 1 is not valid. The only way an expression like this could work is if the x on the right comes from a parent scope, such as an argument list, and the assignment operation creates a new x in the current scope. Understanding this concept leads to understanding the beauty of OpenSCAD.

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