Question

There is no special type for functions in VBA. It is hard for me to see how to add functions as arguments to functions in Excel VBA.

What I am trying to accomplish is something like this:

function f(g as function, x as string) as string
        f = g(x)
end function

Currently, I have a group of little functions all repeating themselves but with one call to a specific function.

Answer 1

From your code, function g takes a string parameter and returns a string. I suggest you create a class module called IStringFunction to act as the definition of an interface that all functions will support, thus:

Class Module: IStringFunction

Public Function Evaluate(ByVal s As String) As String
End Function

Then, create a couple of example functions implementing this interface:

Class Module: HelloStringFunction

Implements IStringFunction

Public Function IStringFunction_Evaluate(ByVal s As String) As String
    IStringFunction_Evaluate = "hello " & s
End Function

Class Module: GoodbyeStringFunction

Implements IStringFunction

Public Function IStringFunction_Evaluate(ByVal s As String) As String
    IStringFunction_Evaluate = "goodbye " & s
End Function

…and finally, some test code to exercise the functions:

(Standard) Module: Test

Sub Test()

    Dim oHello As New HelloStringFunction
    Dim oGoodbye As New GoodbyeStringFunction

    MsgBox Evaluate(oHello, "gary")
    MsgBox Evaluate(oGoodbye, "gary")

End Sub

Private Function Evaluate(ByVal f As IStringFunction, ByVal arg As String) As String
    Evaluate = f.Evaluate(arg)
End Function

Note that the class implementing the interface must have methods named <Interface>_<Method> as in the example above, not just <Method> as you’d expect.

Download the simple demo or intermediate demo here