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 StringEnd Function

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

Class Module: HelloStringFunction

Implements IStringFunctionPublic Function IStringFunction_Evaluate(ByVal s As String) As String    IStringFunction_Evaluate = "hello " & sEnd Function

Class Module: GoodbyeStringFunction

Implements IStringFunctionPublic Function IStringFunction_Evaluate(ByVal s As String) As String    IStringFunction_Evaluate = "goodbye " & sEnd 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 SubPrivate 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

Since VBA has it's roots in an interactive language, it has always had the ability to execute text:

function f(g as string, x as string) as string        f = application.run(g,x)end functionMyStringA = f("functionA",string1)MyStringB = f("functionB",string1)

Edit to Add:I think that in current versions of Excel, the application (Excel) can 'run' only things you can show in a spreadsheet cell. So that means functions, not subroutines. To execute a subroutine, wrap it up in a function wrapper:

function functionA(x as string)    Call MySubA(x)end function

It is possible to pass a function as an argument, and get exactly the behaviour you're after, but it's a bit hacky. This is achieved by exploiting the default property of a class,

Create a class myFunction containing your function

Public Function sayHi(ByVal s As String) As String    sayHi = "Hi " & sEnd Function

Export it, and open the .cls file in a text editor

You should see somewhere in the file, a line that defines your function

Public Function sayHi(ByVal s As String) As String

Beneath it, add the line Attribute Value.VB_UserMemId = 0, so that now it looks like this:

Public Function sayHi(ByVal s As String) As StringAttribute Value.VB_UserMemId = 0    sayHi = "Hi " & sEnd Function

What you've done here is to mark sayHi as the default property of the class. Now you can call the function by the class object alone class(args), rather than class.function(args)

Save the amended file and import it into your VBA project

Test module

Sub test()    Dim parameterFunction As Object         'this is what we'll be passing to our routine    Set parameterFunction = New myFunction  'create instance of the function object    MsgBox f(parameterFunction, "Greedo")End SubFunction f(ByVal g As Object, ByVal x As String) As String        f = g(x)End Function

*NB, this way you can pass any function; but you may instead specify ByVal g As IStringFunction to get only the subset with the correct interface as per Gary McGill's answer