# OpenSCAD User Manual/User-Defined Functions and Modules

Users can extend the language by defining their own functions and modules. This allows grouping portions of script for easy reuse with different values. Well chosen names also help document your script.
Functions return values.
Modules perform actions, but do not return values.
OpenSCAD calculates the value of variables at compile-time, not run-time. The last variable assignment within a scope applies everywhere in that scope. It also applies to any inner scopes, or children, thereof. See Scope of variables for more details. It may be helpful to think of them as override-able constants rather than as variables.
For functions and modules OpenSCAD makes copies of pertinent portions of the script for each use. Each copy has its own scope, which contains fixed values for variables and expressions unique to that instance.
The name of functions and modules is case sensitive, therefore test and TEST refer to different functions/modules.

## Scope

Modules and functions can be defined within a module definition, where they are visible only in the scope of that module.
For example
The function y and module plot cannot be called in the global scope.

## Functions

Functions operate on values to calculate and return new values.

### name

    Your name for this function. A meaningful name is helpful later. Currently valid names can only be composed of simple characters and underscores [a-zA-Z0-9_] and do not allow high-ascii or unicode characters.

### parameters

    Zero or more arguments. Parameters can be assigned default values, to use in case they are omitted in the call. Parameter names are local and do not conflict with external variables of the same name.

### value

    an expression that calculates a value. This value can be a vector.

### Function use

When used, functions are treated as values, and do not themselves end with a semicolon ;.
You can also use the let statement to create variables in a function:
It can be used to store values in recursive functions. See the wikipedia page for more information on the general concept.

### Recursive functions

Recursive function calls are supported. Using the Conditional Operator "... ? ... : ... ", it is possible to ensure the recursion is terminated.
There is a built-in recursion limit to prevent an application crash (a few thousands). If the limit is hit, you get an error like: ERROR: Recursion detected calling function ... .
For all tail-recursive functions that calls itself, OpenSCAD is able to eliminate internally the recursion transforming it in an iterative loop. The previous example code is not a tail call as an "add" operation need to be calculated after calling. But the following is entitled to tail-recursion elimination:
Tail-recursion elimination allows much higher recursion limits (up to 1000000).

### Function literals

[Note: Requires version 2021.01]
Function literals are expressions that define functions, other names for this are lambdas or closures.
    function literal
Function literals can be assigned to variables and passed around like any value. Calling the function uses the normal function call syntax with parenthesis.
It's possible to define functions that return functions. Unbound variables are captured by lexical scope.

### Overwriting built-in functions

It is possible to overwrite the built-in functions. Note that definitions are handled first, so the evaluation does indeed return true for both echo calls as those are evaluated in a later processing step.
| Source Code | Console output |
| echo (sin(1)); function sin(x) = true; echo (sin(1)); | Compiling design (CSG Tree generation)... ECHO: true ECHO: true Compiling design (CSG Products generation)... |

## Modules

Modules can be used to define objects or, using children, define operators. Once defined, modules are temporarily added to the language.

### name

    Your name for this module. Try to pick something meaningful. Currently valid names can only be composed of simple characters and underscores [a-zA-Z0-9_] and do not allow high-ascii or unicode characters.

### parameters

    Zero or more arguments. Parameters may be assigned default values, to use in case they are omitted in the call. Parameter names are local and do not conflict with external variables of the same name.

### actions

    Nearly any statement valid outside a module can be included within a module. This includes the definition of functions and other modules. Such functions and modules can be called only from within the enclosing module.
Variables can be assigned, but their scope is limited to within each individual use of the module. There is no mechanism in OpenSCAD for modules to return values to the outside. See Scope of variables for more details.

### Object modules

Object modules use one or more primitives, with associated operators, to define new objects.
In use, object modules are actions ending with a semi-colon ';'.

### Operator modules
#### Children

Use of children allows modules to act as operators applied to any or all of the objects within this module instantiation. In use, operator modules do not end with a semi-colon.
Basicly the children command is used to apply modifications to objects that are focused by a scope:
Objects are indexed via integers from 0 to $children-1. OpenSCAD sets $children to the total number of objects within the scope. Objects grouped into a sub scope are treated as one child. See example of separate children below and Scope of variables. Note that children, echo and empty block statements (including ifs) count as $children objects, even if no geometry is present (as of v2017.12.23).
Deprecated child module
Up to release 2013.06 the now deprecated child module was used instead. This can be translated to the new children according to the table:
| up to 2013.06 | 2014.03 and later |
| child | children(0) |
| child(x) | children(x) |
| for (a = [0:$children-1]) child(a) | children([0:$children-1]) |
Examples

### Further module examples

    Objects
    Operators

### Recursive modules

Like functions, modules may contain recursive calls. However, there is no tail-recursion elimination for recursive modules.
The code below generates a crude model of a tree. Each tree branch is itself a modified version of the tree and produced by recursion. Be careful to keep the recursion depth (branching) n below 7 as the number of primitives and the preview time grow exponentially.
Another example of recursive module may be found in Tips and Tricks

### Overwriting built-in modules

It is possible to overwrite the built-in modules.
A simple, but pointless example would be:
Note that the built-in sphere module can not be called when over written.
A more sensible way to use this language feature is to overwrite the 3D primitives with extruded 2D-primitives. This allows additional to customize the default parameters and to add additional parameters.