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11585 :: how to create a type that accepts a function with any arguments?

• basow53698 (orginal) [2024-05-11T13:23:53+02:00] view original

  How to create a type that accepts a function with any arguments, and later, you can execute this function and get the result?

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• Isofruit (orginal) [2024-05-11T13:40:05+02:00] view original

  Do you just store the function call or do you also store the parameters themselves? If so, why not store a closure instead?

  proc doubleIt(x: int): int = x*2
  
  let someInt = expensiveComputation()
  proc myClosure(): int = myProc(someInt)
  Run

  Now you can execute myClosure wherever you want and it'll execute myProc with the result from expensiveComputation.

  You can also store this closure in an object if that's nicer for you:

  type Store[T] = object
    clos: proc(): T {.closure.}
  
  let store = Store(clos: myClosure)
  store.clos()
  Run

  However, this means that not all stores are interchangeable. You can't store a closure that returns a string in a Store has the type Store[int].

  In those cases you maybe can store the pointer, like so:

  type Store = object
    command: pointer
  
  let s = Store(command: proc(): int = 5)
  Run

  Now you can store multiple Stores in a Seq or whatever you need. This comes with 2 limitations:

  • It is impossible to store closures, so you need to have the parameters inside of Store in one way or another
  • This absolutely requires you to know which type command will return when you want to execute the proc, because that needs to look like so:

  let s = Store(command: proc(): int = 5)
  
  echo cast[proc(): int {.nimcall.}](s.command)()
  Run

  There are other solutions as well depending on your need, but those tend to dive into metaprogramming, so macros and templates.

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