Is there a way I can pass a variable with a generic type to a method with a concrete type?

Question

Is there a way I can pass a variable with a generic type to a method with a concrete type? Here is some motivating code:

predicate Goo<K, V>(m: map<K, V>)
  requires m.keyType is string // Can I do something like this?
{ Foo(m) }

predicate Foo<V>(m: map<string, V>)

Answer

In short, no.

There are a few problems here. First there is no way to extract the type of the keys of a map as a type value. Dafny does not have the capability to reason about types as first-class entities. In fact the is operator takes a value and a type, not two types.

We could try writing the precondition as requires m.Keys is set<string>, but that is not permitted either as m.Keys is a set<K> and is not comparable to set<string> with is.