[Haskell-cafe] What to call Occult Effects

[David Feuer]

First, for clarity, note that

   const id = flip const

Consider a (right-)occlusive functor. We immediately see that

liftA2 (flip const) m (pure x) = pure x

Using the Applicative laws, we can restate this:

x <$ m = pure x

We get the same sort of result for a left-occlusive effect.

So an occlusive effect can't have any *observable* side effects. It must be
"read only".

On Thu, Nov 12, 2020, 3:59 PM Olaf Klinke <olf at aatal-apotheke.de> wrote:

> > First, instead of `const id` I will use `const`, that is, we shall
> > prove
> >
> > const = liftM2 const :: M a -> M b -> M a
> >
> > which I believe should be equivalent to your property.
>
> My belief was wrong, which is evident when using do-notation.
> Kim-Ee stated
>
> do {_ <- b; x <- a; return x} = a
>
> while I examined
>
> do {x <- a; _ <- b; return x} = a
>
> Since do {x <- a; return x} = a holds for any monad, Kim-Ee's property
> can be reduced to
>
> do {_ <- b; a} = a
> or more concisely
> b >> a = a
>
> which I called Kleisli-const in my previous post. As we seemed to have
> settled for "occlusive" I suggest calling
> b >> a = a
> "right-occlusive" or "occlusive from the right" because the right
> action occludes the side-effects of the left action, and
> do {x <- a; _ <- b; return x} = a
> should be called "left-occlusive" or "occlusive from the left" because
> the left action hides the effect of the right action. Under
> commutativity, both are the same but in general these might be
> different properties. I do not have a distinguishing counterexample,
> though, because all monads I come up with are commutative.
>
> David Feuer hinted at the possibility to define occlusiveness more
> generally for Applicative functors. Commutativity might be stated for
> Applicatives as
>
> liftA2 f a b = liftA2 (flip f) b a
>
> So far I can only see two classes of occlusive monads: The reader-like
> (Identity ~ Reader ()) and the set-like monads.
>
> Olaf
>
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