[andolanra]

There are very much ways of expressing that computation in SML—or in Haskell or other languages—but not in the language of modules. Haskell typeclasses would have a similar problem. Consider the following pseudocode:

    class Map m where {- some implementation -}
    
    instance Map HashMap where {- ... -}
    instance Map TreeMap where {- ... -}

    mkMap :: Int -> {- ??? -}
    mkMap size | size > threshold = newHashMap
               | otherwise        = newTreeMap

What is the type of mkMap? It won't actually type-check in Haskell. It can't be (Map m) => m, because it needs to have a concrete type. We could do some kind of type-level work, relect the size variable into the type level, and have a type family that decides whether the resulting type is a HashMap or a TreeMap—but that has the same problem as the OCaml example, where we're using a different language (the language of type-level computation) to talk about something that'd be simple with value-level programming.

One place where you could write this easily is in a dynamically typed language:

    def mkMap(size):
        if size > threshold:
            return HashMap()
        else:
            return TreeMap()

That is exactly why this kind of research is interesting: we'd like to capture the flexibility of idioms that dynamic languages can sometimes afford us, with the extra safety and security guarantees of a static type system!

[mercurial]

You don't need a dynamic language for that. In Java:

    Map<K,V> mkMap(int size) {
        return size > threshold ? new HashMap<K,V>() : new TreeMap<K,V>();
    }

[rtpg]

Thanks for the explanation, this is illuminating

I'm still kind of unsure of why (Map m) => Int -> m isn't a valid type signature for this though..

[andolanra]

That type means something slightly different: (Map m) => Int -> m doesn't mean that it returns some Map type, it means it returns any Map type. We could write a function of that signature, but that would imply that the caller/context would choose what type of Map it returns (and so it would not depend on the integer argument.)

Think about what it means if a type variable has no typeclass constraint: the type t isn't just a specific unknown type, it's literally any type. Similarly, C t => t isn't a specific unknown instance of a typeclass, it's any instance of that typeclass. If we tell Haskell that we're returning an instance C t => t and then try to give some concrete type, Haskell won't allow it, because it wants that type to encompass any possible type that implements C, not just the one we happened to use.

One way of making the above code work in Haskell is to use an existential type, which expresses that we're talking about some (not any) map type, but we'd have to wrap it inside a different constructor:

    -- this expresses that a SomeMap constructor contains some kind
    -- if Map, but we can't tell which one:
    data SomeMap = SomeMap (forall m. Map m => m)

    -- We can now express this by hiding the choice of Map inside the
    -- SomeMap type:
    mkMap :: Int -> SomeMap
    mkMap size | size > threshold = SomeMap newHashMap
               | otheriwse        = SomeMap newTreeMap

which is, again, a solution with some advantages and some disadvantages.

[octatoan]

If I remember correctly, isn't this one of the problems Idris and Agda are good at solving?

[andolanra]

We could in fact write mkMap in Idris with very little change, and give it a proper type:

    -- This might not be the best/cleanest way of writing it, as my
    -- Idris is a little rusty, but this will work:
    mkMap : (size: Int) -> (if size > threshold then HashMap else TreeMap)
    mkMap size with (size > threshold)
      | True  = newHashMap
      | False = newTreeMap

So yes, although again, other tradeoffs are made to allow this kind of radical expressiveness.