This module implements variables which may contain values from types different from the native types (Proc_* types).

To make a type available to custom variables, it needs to be instantiated in the CustomValue class, which is subclass of the VarLength class. These instances are derivables using the DeriveGeneric extension. Things you need are: enable the DeriveGeneric language extension, import GHC.Generics, derive a Generic instance of your type and then write the following instances (where Foo is any type of interest):

 instance VarLength Foo
 instance CustomValue Foo

Note that Foo must be made from other types that are instances of CustomValue. Also, note that instances of VarLength or CustomValue can not be recursive or sum types. An example:

 {-# LANGUAGE DeriveGeneric #-}

 import Graphics.Web.Processing.Mid
 import Graphics.Web.Processing.Mid.CustomVar
 import GHC.Generics

 data Point = Point Proc_Float Proc_Float
                deriving Generic

 instance VarLength Point
 instance CustomValue Point

Types instance of the CustomValue class can be contained by a special type of variables, called CustomVar (Custom Variable). Functions for custom variables are equal to the function for regular variables, except that they all end in C. For example, newVar is called newVarC for custom variables.

The dependency of this module in several language extensions was the reason to make it separate from the rest of the mid interface where it belongs to. Somehow, it forces the user to use DeriveGeneric and import GHC.Generics to do something useful with it (more than use custom variables for tuples).