[VBA] Lambda Syntax - No script control or cheats! Happy for ports to VB6

[sancarn]

Lambda Expressions

I've already posted this library elsewhere but figured that people on VBForums would find it useful too! This is currently only written to work in VBA but I believe a port to VB6 would only require the alteration of a few declarations. Or perhaps in quite a few within evaluateFunc... Happy to have pull requests if anyone wants to make it usable in VB6!

What is a lambda expression?

A lambda expression/anonymous function is a function definition that is not bound to a name. Lambda expressions are usually "1st class citezens" which means they can be passed to other functions for evaluation.

I personally believe this is best described with an example. Imagine we wanted to sort an array of sheets by their name. In VBA this would be relatively complex and require an understanding of how to sort data in the first place, as well as which algorithms to use. Lambda allows us to define 1 sorting function and then provide our lambda function to provide the ID to sort on:

Sub Main
    myArray = Array(Sheets(1),Sheets(2))
    newArray = sort(myArray, stdLambda.Create("$1.name"))
End Sub
 
Function sort(array as variant, accessor as stdICallable)
    '... sorting code ...
       elementID = accessor(element)
    '... sorting code ...
End Function

Download

The file can be found on github here:
stdLambda.cls.

stdICallable will also be required: stdICallable.cls

How to use stdLambda

The Create() constructor is the main way to create an instance of the stdLambda object.

Sub test()
    Dim cb as stdLambda
    set cb = stdLambda.Create("1+1")
End Sub

To define a function which takes multiple arguments $# should be used where # is the index of the argument. E.G. $1 is the first argument, $2 is the 2nd argument and $n is the nth argument.

Sub test()
    Dim average as stdLambda
    set average = stdLambda.Create("($1+$2)/2")
End Sub

You can also define functions which call members of objects. Use xxx#xxx() to call functions and xxx.xxx() to call properties.

Sub test()
    Debug.Print stdLambda.Create("$1.Name")(someObject)  'returns ThisWorkbook.Name
    Call stdLambda.Create("$1#Save")(someObject)         'calls ThisWorkbook.Save
End Sub

The lambda syntax comes with many VBA functions which you are already used to...

Sub test()
    Debug.Print stdLambda.Create("Mid($1,1,5)")("hello world")        'returns "hello"
    Debug.Print stdLambda.Create("$1 like ""hello*""")("hello world") 'returns true
End Sub

As well as an inline if statement:

Sub test()
    Debug.Print stdLambda.Create("if $1 then 1 else 2")(true)        'returns 1
    Debug.Print stdLambda.Create("if $1 then 1 else 2")(false)       'returns 2
 
    'Note: this will only call someObj.CallMethod() and will not call someObj.CallMethod2() (unless 1st arg is supplied as false of course)
    Debug.Print stdLambda.Create("if $1 then $2#CallMethod() else $2#CallMethod2()")(true,someObj)
End Sub

With stdLambda you are not limited to a single lines, you can also use multiple lines. Note the result of the last line in the lambda is returned:

Call stdLambda.Create("2+2: 5*2").Run()
 
'... or ...
 
Call stdLambda.CreateMultiline(array( _ 
  "2+2", _ 
  "5*2", _ 
)).Run()

You can also use variables, much like in VB6:

'the last assignment is redundant, just used to show that assignments result in their value
Debug.Print stdLambda.CreateMultiline(array( _
  "count = $1", _
  "footPrint = count * 2 ^ count" _
)).Run(2) ' -> 8

Finally you can use Function definitions if you want to use recursion:

stdLambda.CreateMultiline(Array( _
  "fun fib(v)", _
  "  if v<=1 then", _
  "    v", _
  "  else ", _
  "    fib(v-2) + fib(v-1)", _
  "  end", _
  "end", _
  "fib($1)" _
)).Run(20) '->6765

Evaluating lambdas

Use default member execution:

Sub test()
    Dim average as stdLambda
    set average = stdLambda.Create("($1+$2)/2")
    Debug.Print average(1,2)   '1.5
End Sub

Use Run method:

Sub test()
    Dim average as stdLambda
    set average = stdLambda.Create("($1+$2)/2")
    Debug.Print average.Run(1,2)   '1.5
End Sub

Use RunEx method, supplying an array of arguments:

Sub test()
    Dim average as stdLambda
    set average = stdLambda.Create("($1+$2)/2")
    Debug.Print average.RunEx(Array(1,2))   '1.5
End Sub

Sometimes it's useful to use an interface. In this case use stdICallable interface:

Sub test(ByVal func as stdICallable)
    func.Run(ThisWorkbook, 1, "hello world")
End Sub

An update as of 16/09/2020 added the Bind() method to stdLambda as well. The Bind() method creates a new ICallable that, when called, supplies the given sequence of arguments preceding any provided when the new function is called. This ultimately saves on expression compilation time.

'Expression created, argument bound.
Dim cb as stdLambda: set cb = stdLambda.Create("$1 + $2").Bind(5)
Debug.Print cb(1) '6 
Debug.Print cb(2) '7
Debug.Print cb(3) '8
 
'No compilation required, cached lambda is used with new bound argument
set cb = stdLambda.Create("$1 + $2").Bind(6)
Debug.Print cb(1) '7 
Debug.Print cb(2) '8
Debug.Print cb(3) '9

How it works

Finally, how does the class work internally?

Create first looks to see if a lambda already exists, if it does it is returned, else it calls Init which:

• Tokenises the string using Regex
• Calls parseBlock() which uses a top-down parsing algorithm to parse the entire block to an array/stack containing operations (i.e. compiles to byte code)

Then when an expression is executed, Run calls evaluate which:

• Loops over all operations, detects the type and subtype of the operation
• Performs the operations function
• After all operations have executed the 1st item in the stack is returned.

Integration with the STD-VBA Library

Thought i'd give a taste of one of the core reasons I built this library!

'Create an array
Dim arr as stdArray
set arr = stdArray.Create(1,2,3,4,5,6,7,8,9,10) 'Can also call CreateFromArray
 
'More advanced behaviour when including callbacks! And VBA Lamdas!!
Debug.Print arr.Map(stdLambda.Create("$1+1")).join          '2,3,4,5,6,7,8,9,10,11
Debug.Print arr.Reduce(stdLambda.Create("$1+$2"))           '55 ' I.E. Calculate the sum
Debug.Print arr.Reduce(stdLambda.Create("Max($1,$2)"))      '10 ' I.E. Calculate the maximum
Debug.Print arr.Filter(stdLambda.Create("$1>=5")).join      '5,6,7,8,9,10
 
'Execute property accessors with Lambda syntax
Debug.Print arr.Map(stdLambda.Create("ThisWorkbook.Sheets($1)")) _ 
               .Map(stdLambda.Create("$1.Name")).join(",")            'Sheet1,Sheet2,Sheet3,...,Sheet10
 
'Execute methods with lambda:
Call stdArray.Create(Workbooks(1),Workbooks(2)).forEach(stdLambda.Create("$1#Save")
 
'Sort objects by date, and then print names concatenated with comma
Debug.Print stdArray.Create(ObjA,ObjB,ObjC,ObjD,ObjE).sort(stdLambda.Create("$1.Date")).map(stdLambda.Create("$1.Name")).join(",")
 
'We even have if statement!
With stdLambda.Create("if $1 then ""lisa"" else ""bart""")
  Debug.Print .Run(true)                                              'lisa
  Debug.Print .Run(false)                                             'bart
End With

Long term goals

The intermediate representation is good, but it would be even better if we could compile to machine code... I'm pretty sure this is even more difficult, but in the pursuit of speed that's maybe where we'll have to go!

Happy Coding!
~Sancarn

[wqweto]

Wow, where did this come from :-)) Probably from the wonderful world of VBA 7 where cross-pollination with modern languages and new ideas is still strong!

Btw, in the ancient VB6 world we do have x86 machine code generated lambdas (kind of) but unfortunately there is no x64 implementation currently.

I might try to implement C emitting backend (OTCC subset) for your lambda syntax based on your current frontend.

cheers,
</wqw>

[sancarn]

Wow, where did this come from :-)) Probably from the wonderful world of VBA 7 where cross-pollination with modern languages and new ideas is still strong!

Over the past year I've been simultaneously working on a "VBA-Like" to "VBA" transpiler with the intention of making something akin to BabelJS but for VBA. Was also hoping that might be able to compile to VB6 too and thus simultaneously "elevate" the version of VB6. Anyhow that experience and some help from a friend, allowed me to write a first version (which evaluated the tree at runtime...). It really all developed from there, with 2nd and 3rd versions implementing more operations. In an attempt to try to improve performance TarVK took it and added the compiler and evaluator here

Btw, in the ancient VB6 world we do have x86 machine code generated lambdas (kind of) but unfortunately there is no x64 implementation currently.

Wow! Wish I had known that before I made this library lol... I had initially hoped to do more in machine code, but at the time didn't have the knowledge or compiler tooling to make that work...

I might try to implement C emitting backend (OTCC subset) for your lambda syntax based on your current frontend

This would be cool, an alternative approach would be to re-implement the evaluator in compiled C for both x86 and x64. At least I believe this should provide the fastest runtime. Although it may be less fun for you! :P Anyhow any help you can provide would be much appreciated of course