dafnyformal-verification

Dafny: Generic method on a base type and a subset type?


Can I define a generic method on a base type and subset type?

For example, we might later use the generic function by having

I image something like:

method MergeAndTouch<T0, T1>(Touch: (T1,T1)->bool, Plus: (T0,T0)->T0, unit: T0, xs: seq<T1>, a: T1)
  requires T0 >= T1 // NOT ALLOWED
  requires Reflexive(Touch)
  requires Symmetric(Touch)
  requires IsAssociative(Plus)
  requires IsUnital(Plus, unit)
  requires forall i:T1, j:T1 :: Touch(Plus(i, j), i) && Touch(Plus(i, j), j)
  requires forall i:nat, j:nat :: i < j < |xs| ==> !Touch(xs[i], xs[j])
{
// code goes here
}

I need this to exclude things like the empty set from the generic Touch while still having things like the empty set as the unit for operations such as FoldLeft.


Solution

  • In this case subset language feature can be used. This is how it will look like

    ghost function NonEmptySetExample<T(00)>() : (r: set<T>)
    {
        var t : T :| true ; {t}
    }
    
    type NonEmptySet<T(00)> = s : set<T> |  s != {} 
         ghost witness NonEmptySetExample<T>()
    

    You can use set methods (in here) on NonEmptySet.

    ghost function Test<T(00)>(t: NonEmptySet<T>) : T 
    {
        var x : T :| x in t; x
    }  
    

    Additionally you can define method taking set as argument and call with NonEmptySet

    method TypeMethod<T>(s: set<T>) 
    {
    }
    
    method CallTypeMethodOnSubSet<T(00)>(t: NonEmptySet<T>)
    {
       TypeMethod(t);
    }
    

    Note: T(00) means type parameter itself is non empty.