morning

morning

Morning

morning

Yesterday I saw SPJ in Amsterdam… the dark side

Now back to hot weather coding…

who/what is SPJ?

https://twitter.com/borkdude/status/1018924164089032705

Simon Peyton Jones, the Haskell guy

I find him rather static....

His talk was about dynamic typing in Haskell

shock horror!!!

what happened... is he ill?

it’s a way to get dynamic typing in haskell with static guarantees

sounds rather clever...

it’s kind of like storing something as Object in Java and then casting it back right when you need it to be of some type, but with the guarantee that you can never unsafely cast it to something it isn’t

the checks to safely cast are runtime checks right?

this sounds like something only the Haskell community could come up with

the haskellers very good at inventing complicating stuff to limit themselves, and then inventing even more complicated stuff to overcome these limitations

The first time is saw http://hackage.haskell.org/package/HList i though it was a joke

@borkdude what are the guarantees to the toStatic function in your code?

it looks like a runtime check?

that it will always result in a static type

this can break as soon as you add another option to StringOrInt

only because it's a silly name 🙂

in reality, it will most like be FooType

@chris.blom that’s always the case in Haskell 😛

That's A Feature

@chrisblom that’s always the case in Haskell (hey, I mentioned the wrong chrisblom)

haha, true

@lmergen the point is that you will never have a runtime exception as a result of casting to the wrong type

hmm

can't the runtime exception still occur when toStatic is executed?

No, it will always have something of the type Maybe StringOrInt, never an exception

well, I should have added a case for all other Dynamics that return a Nothing

but, but... then it's just the same like casting an Object to a StringOrInt, and wrapping the exception in a Maybe ?

but maybe this code isn’t the best example, I wrote this only when I discovered that you could do this, not when I properly understood this topic 🙂

hehe

yeah ok but these are not static guarantees

you can still feed it the wrong Dynamic

it's just that it's now a Nothing instead of an exception

there is no wrong Dynamic

a Dynamic that doesn't cast to Int or String, right ?

i think i'm still confused 🙂

fromDynamic returns a Nothing or a Just a of the type a you wanted. You also have unsafeCoerce in Haskell, this won’t get you an exception, but just returns bogus when applied wrongly.

Prelude Unsafe.Coerce> data Foo = A | B
Prelude Unsafe.Coerce> unsafeCoerce A :: Int
283479243147

I’m not an Haskell expert, so sorry for the confusion 🙂

i'm not a haskell expert either

no one is 🙂

but as far as i can see, it's mainly a way to wrap some arbitrary data in a Dynamic, and write your code that uses it with static guarantees

that’s right

One of my takeaways from this talk was how hard it is to do something like this in Haskell 😛

that's also a feature, eh 🙂

I guess so