Dynamic Resolution

Overriding Functions

With a custom Evaluator object, you can override the built-in implementation of any function. To do so, you would need a value that adopts the FunctionOverrider protocol (declared in DynamicResolution.swift).

This protocol defines a single method:

func overrideFunction(function: String, arguments: Array<Expression>, substitutions: Substitutions, evaluator: Evaluator) throws -> Double?

This takes all of the same arguments as a custom function, as well as the name of the function being evaluated. The return value of this method has the same semantics as the return value of a custom function: provide a Double if you can, throw an ErrorType if something goes wrong, and return nil to continue on with default handling.

There are two important things to note about overriding functions:

1. This overrideFunction(...) method will be called for every function in an Expression tree. If you only want to override a single method, check the function parameter for the appropriate name, and otherwise return nil.
2. The function name passed in as the first parameter is not normalized. It is the function name as it appears in the string, without resolving aliases or standardizing on capitalization. This means, among other things, that you can use this to implement case-sensitive functions (as requested in issue #57).

Function Resolution

Getting a callback for every single function evaluation can be overkill. If you simply want to supply a value for a function when it's evaluated, provide a FunctionResolver to your custom Evaluator to get invoked after built-in and registered functions have failed to supply a value for a function.

Like before, you have the ability to provide a value, throw an error, or return nil.

Variable Resolution

Typically, variable values are supplied up-front as evaluation begins, by passing in a lookup dictionary.

However, you can also provide a VariableResolver value to your custom Evaluator to dynamically supply variable values as variable expressions are evaluated.