Functions can be used to compute values from other values. CIF has many built-in functions, called standard library functions. An example is the size function, which takes a list and returns the number of elements of the list. Another example is the abs function, which takes an integer or real number and computes the absolute value of that number.

While many built-in functions are available, they may not always suffice. User-defined functions can be added to CIF models, to allow custom computations needed by the model. By putting the calculation in a user-defined function, the calculation can be used in several places, allowing for reuse. Furthermore, functions allow for more complex computations than would otherwise be possible.

CIF features two kinds of user-defined functions: internal ones and external ones. Internal user-defined functions are fully defined within the CIF model. External user-defined functions declare only a header which indicates the existence of the function, while the actual implementation is obtained from an external source. The lessons of this category only explain internal user-defined functions. The language reference documentation gives more information on external user-defined functions.