Module 0439: OpenSCAD programming

• What is this document?
• Syntax
  • Action statement
  • Variables

What is this document?

This document describes how to use OpenSCAD from the perspective of a typical programmer.

Syntax

Action statement

While not the lowest level of language constructs, the action statement is a logical place to start the description of the syntax of OpenSCAD. Generally speaking, an action statement is similar to a C statement where it ends with a semi-colon (;), or it is a compound statement using braces ({}). However, unlike a C statement, an OpenSCAD action statement may have optional “operators.” In certain OpenSCAD documentation, an “operator” is also called a “transformation.” The term “transformation” describes the language construct better.

• The beginning of an action statement can have any number (including 0) of operators (transformations) using at least one space as a separator.
• transformations ::= [ transformation " "... ]
• After the transformations:
  • A 2D or 3D object, terminated by a semi-colon (;) or
  • A control structure

A control structure can be one of the following:

• control-stmt ::=
  • "{" stmt* "}": a compound statement block
  • "for" "(" var "=" "[" start ":" end "]" ")" stmt
  • "for" "(" var "=" "[" start ":" step ":" end "]" ")" stmt
  • "for" "(" var "=" "[" value "," ... "]" ")" stmt
  • "if" "(" cond ")" stmt
  • "let" "(" var-decl "," ...")" stmt
  • atom ";"

An action statement is, therefore:

• stmt ::= transformations control-stmt

Variables

The term “variable” has a different meaning in OpenSCAD. The value of a variable is constant in the same scope, determined by the last update of the variable within said scope. This is because OpenSCAD is a functional programming language, as opposed to an imperative programming language. Specifically, OpenSCAD has referential transparency.

As a result, stepping that involves self-referencing a variable is not allowed in OpenSCAD. This includes increment operators, compound assignments, and any assignment in which the right-hand side explicitly or implicitly references the right-hand side.

For convenience, one can view the term “variable” in OpenSCAD as “constant” in C.

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