Xojo & Closures

General Programming
1

Am I right in thinking that Xojo doesn’t support closures?

I ask because I am currently neck deep in my compiler trying to implement them and it suddenly occurred to me that I might not need to bother. After all, Xojo as a language has done pretty well without them.

2

It does not :slight_smile:

3

Swift does, and I’m still trying to wrap my head around where/why and how to use them.

4

Thought so.

Ditto. Lots of languages do use them but anything you can do with a closure you can do using classes. There are times when it’s more succinct to use them but personally I don’t ever use them.

If I understand correctly, they are essentially functions declared within other functions that capture local variables in the scope of their enclosing function.

5

Java UI used them a lot basically to implement the events
They call them anonymous functions

1 Like
6

Lambdas or Anonymous functions are a kind of function used as parameter for a high order function. A Closure is a function with “bound” variable values that can be passed around. A Closure is a Lambda, a Lambda is not necessarily a Closure (need to have bound values).
So in a pseudo-Xojo, you could do things like:

Function MyFun( f As Function(x As Integer, y As Integer) As Double, s As String)
  Msgbox s + f(x,y).ToString
End

MyFun( (10, 20) (
  Dim n As Integer = x*y
  Dim r As Double = n / 10
  Return r ), "This is x*y/10")

MyFun( (11, 21) (
  Dim n As Integer = x+y
  Dim r As Double = n / 11
  Return r ), "This is x+y/11")
7 
Function GetAClosure(lim As Integer) As Function // lim will be bound to the resulting function
  Return  Function(x As Integer -> (If x<lim Then Msgbox "true")) // Function() is value, Function()() is call
End

Dim LessT10 As Function = GetAClosure(10)
Dim LessT20 As Function = GetAClosure(20)

LessT10(5) // true
LessT10(10) 
LessT10(15) 
LessT10(20)
LessT10(25)
LessT20(5)  // true
LessT20(10)  // true
LessT20(15)  // true
LessT20(20) 
LessT20(25)
8

Cant say that I’ve really missed them (some folks - brock for instance) would have you believe that “OMG you cannot possibly write decent code without them !” :stuck_out_tongue:

Would they be handy - for some things sure

Generics would be something I’d rather see implemented before closures and lambdas

9

Thanks for the further clarification @Rick.A. I’ve decided against implementing them in my language - honestly it mostly because I never use them and I can’t be arsed to rip up the pretty fast stack-based local variable lookup my VM currently uses to shoehorn in heap-managed locals which closures would necessitate.

10

It is very useful for some contexts. You can think as storing a function in a variable and passing around to be used by some other process. Imagine that you have a task like sorting things in some unusual way besides ascending and descending in alphabetic order, so instead of writing many sorting functions, you can write a “generic” function to sort data and one parameter is a function to classify if a<b in some unusual way. You could store one way as Dim sortAsc As Function… Dim sortDesc As Function… Dim sortInsane As Function… and pass those “names” as a parameter to the sort routine. Small expressions can be written just in place like myArray.sort((x,y)->(x<y))

11

I agree that in the list of Xojo desired missing features, Generics takes a bit more precedence.

12

Not holding my breath about either :slight_smile:

13

I see the utility. I’ll think about it. Need to stop adding features get 1.0 of the compiler done though. As a dynamic language, Possum supports generics.