Extract all elements of a set which have a certain attribute (are in relation with a certain value)

Question

I am trying to write an Alloy function to retrieve all elements of a certain type that are in relation with the parameter of the function (let me say, that have that value for one of their "fields/attributes"). I have tried in various ways, none of them worked.

It's something like

fun get[a:A] : set X{
    (x.name :> a)
}

but this return a set of A while I want a set of X

Answer 1

You can do this more simply:

name.a

returns the set of X's that map under name to an element in a.

Checking equivalence to your version:

sig A { }
sig X {
  name: set A
  }

fun get [a:A] : set X{
    ((X <: name) :> a).A
}

fun get' [a:A] : set X{
    name.a
}

check {
  all a: A | get[a] = get'[a]
}