In fact with its current compiler (AFAIK) it can only be run as a web server, which means it's not even possible to write a program which just writes Hello World to stdout and exits. This was frustrating enough to me that I soon gave up on the language. :(
As someone else mentioned, Backpack is trying to "fix" the module system in Haskell[2], which should be interesting. That said, I don't have much of a problem with it, but then again I haven't seen the light of the OCaml way yet :)
[0] https://ghc.haskell.org/trac/ghc/wiki/StrictPragma
[1] http://blog.johantibell.com/2015/11/the-design-of-strict-has...
data Front a = Nil | Cons a (Stream a)
type Stream a = Lazy (Front a)
* Idris : modules?, strict by default, effect tracking by default, higher-kinded types, parallelism I think (at least on some platforms)
* Ceylon: modules, strictness, experimental higher-kinded types (possibly only on JS), parallelism (possibly only on Java)
* F#: modules, strict by default, parallelism
* Scala: some modularity, strict by default, higher-kinded types, parallelism
(0) “In principle”, you could encode modules in Idris using dependent sum types, but... Good luck with that! It isn't going to be terribly usable. In general, Haskell and related languages don't consider it worth the effort to give modules proper types. I guess we can be thankful Agda's modules don't suck as badly as Haskell's - at least they can be nested.
(1) Ceylon and Scala's type systems are sophisticated enough to encode some of the use cases for modules with objects, but invariably the result is awkward. And other use cases are just impossible. For example, how should I encode an ML functor that takes as arguments two modules with shared type members? If it's possible at all, I don't even want to imagine how horrifying it will be to manually turn all the sharing by fibration into sharing by parameterization.
(2) F# doesn't allow any encoding of ML-style modules. Nothing. Nichts. Nada. It's the only so-called “ML dialect” where the most important feature from ML is completely missing.
(0) The presence of a value component.
(1) The presence of a type component.
(2) The concrete definition of a type component, while still remembering its presence.
I am not sure you can do this in Idris, other than manually shuffling data between multiple dependent record types. Which is rather inconvenient: If you have a module with 20 components, the last thing you want to do is manually shift 15 of them to another module.
Also, unlike vanilla Haskell (no extensions) type classes, which can only have a single type parameter, ML modules can have more than one type component.
A better module system for Haskell is in the makings (Backpack by EZ Yang), but may or may-not be what you are looking for. To my understanding the module system of Haskell is not very limiting at all.
In a strict language, a lazy thunk can be represented as a single mutable cell. In ML, thanks to type abstraction, the mutation is confined to the module that implements laziness, and, in the rest of the program, there's no way to tell that anything impure is happening.
> To my understanding the module system of Haskell is not very limiting at all.
Even Haskell's own designers admit otherwise. Haskell actually originated as an attempt to standardize a lazy, purely functional language for the research community. Modules and records weren't very high in the priority list, and it really shows. But sometimes behind a black cloud there's a silver lining, and the limitations of records inspired Haskell programmers to invent lenses, which are very, very, very awesome.
But, to this day, Haskell still doesn't have a decent alternative to ML-style modules and functors. As far as I can tell, in vanilla Haskell (no extensions), you can only encode functors on modules with a single type component. With either type families or multiparameter type classes, you can encode modules with more than one type component, but the result is very awkward.
Unfortunately, the general story isn't quite so nice, due to thread-safety.
Immutable data structures are great for thread-safety, as there's no problems with data races or generally invalidating things, because there's no writes... but laziness introduces secret writes, among other problems. This means a type providing laziness either can't be used across multiple threads simultaneously, or is forced to have extra overhead (e.g. atomic operations, blocking & registering for being woken up when trying to force a thunk that another thread is already forcing), and users of a library definitely need to know if a module is using mutation in the former way, and may want to know about the latter.
Of course, if shared-memory concurrency/parallelism is forgone, this isn't a problem, but that omission has its own downsides. And... I would suspect the overhead of just implementing it in a thread-safe way can be considered negligible in many cases, especially if data is guaranteed to be pointer-sized (or less).
(Concurrency/parallelism are sometimes described as "abstraction breaking" for this sort of reason.)
Lack of global type inference is far from the only problem with Scala.