This is why we have Data.Dynamic which does safe dynamic types.
It almost never comes up because it turns out to be virtually useless, but that's a story for another time.
Re:Clojure's core.typed vs Haskell
by brandonbloom on Monday September 30, 2013 @04:10PM
(#6472038)
> it turns out to be virtually useless
That's debatable, however Data.Dynamic is built on top of Data.Typeable, which provides a lower level runtime type safety facility. I think we can both agree Typable has lots of interesting uses.
Typeable is interesting in theory and generic traversals are a godsend, but usually I find that when I'm reaching for that particular hammer I should check twice.
Re:Clojure's core.typed vs Haskell
by brandonbloom on Monday September 30, 2013 @05:28PM
(#6472495)
I'll agree it's best to avoid fully open unions when you can, but some (super useful) things truly don't work that way. Check out
http://okmij.org/ftp/Computation/monads.html#ExtensibleDS for a cool example.
I always think of Control.Exception as the poster child for open unions.
Re:Clojure's core.typed vs Haskell
by brandonbloom on Monday September 30, 2013 @08:08PM
(#6473172)
Perfect, since exceptions are a subset of effects! Check out http://math.andrej.com/eff/ and its literature.
As the word "unsafe" implies, these Haskell primitives forego type safety in addition to type correctness. That means you can get segfaults and other undefined behavior at runtime. Such a type error on the JVM will simply produce an exception at runtime.