Add example for ML-like references

[just95]

We want to add an example that shows that Standard ML references can be modelled using the Ref effect. Therefore, the decrement_to_zero example from https://learnxinyminutes.com/docs/standard-ml/ should be implemented.

[jan-christiansen]

I get the following error.

example/Control/Prog/Example/ML.hs:134:6: error: [-Wsimplifiable-class-constraints, -Werror=simplifiable-class-constraints]
    • The constraint ‘Typeable (ref Int)’ matches a built-in instance
      This makes type inference for inner bindings fragile;
        either use MonoLocalBinds, or simplify it using the instance
    • In the type signature:
        decrementToZero :: forall ref sig.
                           (Typeable (ref Int), Ref ref :<: sig, Let ML :<: sig) =>
                           Prog sig (ref Int) -> Prog sig ()
    |
134 |   :: forall ref sig
    |      ^^^^^^^^^^^^^^...

[just95]

Which version of GHC are you using? I don't get the error in 8.6.5. We should setup a CI pipeline to test with multiple versions.