It sounds from https://softwareengineering.stackexchange.com/questions/2632... like they are reading the existence of an explicit patent clause in the Apache license (regardless of what that clause is!) as an "additional restriction". I want to know whether they believe the MIT license has a patent grant, and if not, what they think "Permission to use" means.

I think Elm makes a better introduction to FP concepts because there's much less you have to absorb before you reach the point where you can start practicing by doing useful work. Obviously part of that is the fact that Elm removes or hides certain things Haskell has, but an even bigger reason is that you can just say "...and then 'main' returns the HTML element or Html.program that actually gets displayed" and not have to go down the road of IO actions, functors, etc. You can stop at that point and start making working useful applications while getting comfortable with immutability, purity, the type system, and control flow and iteration under those constraints.

Learning Haskell first, you don't have that opportunity to stop and start practicing. You need to move on an understand at least IO actions, functors, applicatives, typeclasses, and other higher-level concepts before you can construct even a simple practice project. Dreaming up a coherent program structure/flow in this weird new immutable and pure world seems hard enough to a beginner without also having to understand how applicative functors fit into the equation. Having that opportunity to stop and stretch your legs by actually doing a project is a major help to a lot of people, that's what makes all the difference. And then 95% of what you've learned transfers directly into Haskell.

Also, though it has been posted here before (and I don't think it has been worked on for a while), I recommend playing with the Monad Challenges[0]. Well, specifically, just do set one (random numbers). You can easily write your own rand function that returns the seed value as a "random" number and then increments the seed. This will generate successive integers (1,2,3,4...). It makes it very easy to test. Then once you've done set one, go back and write map, apply, etc for Gen. One other nice thing you can do is to make a union type/ADT for your random number (i.e., (Int, Seed) ) and then try to see if it is a functor and applicative (also try to understand why or why not). Finally, you can figure out why Gen is structured the way it is.

I've played with that kata over and over and over again. It is simply beautiful.

[0] - http://mightybyte.github.io/monad-challenges/

Of course, I guess I don't have any real data on it, so this is just my intuition based on observing various languages. So, you know, just my 2 cents.

What needs to happen is both, immutable code, and versioned structs with pure functions that can upgrade and possibly downgrade structs as needed. The larger the distributed system, the versions of a struct (message) will be in-flight at a time. Services need to contain no state, so that they can be micro-rebooted and brought up with the new version.

Joe Armstrong had a comment on globally accessible but immutable code, which I think would go a long way towards the ability to statically type the inputs to a function in a distributed system. Interposition and routing would be the only way to upgrade or deprecate old code paths.

WSDL based APIs on the other hand have clearly defined contracts at both ends but there's more overhead involved.

But this does not mean I am happy with the current mainstream status quo :)

Every node becomes an "edge" in it's own right, and doesn't necessarily have global coherence with the rest of the system.

I don't use Erlang, but I have developed an Actor system for C# [1] which is based on its (and Akka's) concepts. Clearly without a static type-checker for the whole distributed system we have to manually get involved and patch the old and new so that we can hot swap processes. Versioning I've found is best done by maintaining the old process that accepts the old message format, maps it to the new one, and then forwards it on to the new process that accepts the new message format. Any other node that is lagging behind will continue to work, and any new one will send to the new address for the process.

This isn't really rocket science, and if you stick to a few basic rules it tends to work out just fine. That doesn't mean that type safety goes out of the window, it just means that in creating a distributed process you must accept that you can't retire the old contract without it causing potential problems.

Apologies if I'm missing your point about OTP, but ultimately it seems that at some point (as the GP says) you are marshalling a message into a text or binary format, and then unmarshalling. At that point if the unmarshalled static type doesn't match the type that the process expects, then it will be off to the dead-letter queue. I don't really see how that's any different to giving the wrong type to a function in a dynamic language, or using an incorrectly typed variable that is picked up by a compiler in a statically typed language. In each case it's type checking at the earliest possible opportunity.

[1] https://github.com/louthy/echo-process

No, that's not how you do it. You marshal things directly into the desired types. Check out either aeson for Haskell or how Go does things via either the json modules or the generic Text/Binary Marshaler/Unmarshaler.

"but also the myriad deployment & upgrade & versioning scenarios."

The answer to all of those things is mostly that even a lot of Erlang shops don't use live upgrading. You really have to have a very particular use case for that to be the best solution vs. a rolling upgrade and server restarts. Even if the language is capable of it, it still requires you to write services that can handle being upgraded, and it's much easier to write services that can handle being restarted, especially since you 100% have to write that anyhow because services get restarted anyhow. Most people don't have that use case. Web services certainly don't have that use case.

Once you drop that, it's a lot simpler.

"Even thinking about something like a gen_server"

gen_server is partially as complicated as it is as a side-effect of other decisions in the language. While the concept of a gen_server is a strength in Erlang, the specific implementation of gen_server as this "behavior" thing is mind-blowingly complicated for what you actually get. (It reminds me of Python's "metaclasses". I spent many hours wrapping my head around what that was, but in the end, all that it amounts to is what is now called a class decorator, which is way more sensible. A metaclass isn't a class decorator in theory, but in practice, class decorators are way easier to understand and cover 99.9% of the use cases, if not 100%.) When I implemented supervisor trees in Go, my solution for gen_server/gen_fsm/gen_* was just to... not. Behaviors are just a very, very weird half-object-ish system with a lot of limitations. They are easily replaced by simply having some sort of "interface" system, be it via conventional classes or interfaces. It's why you don't see "behaviors" as Erlang defines them anywhere else. Erlang has a lot to learn from and copy from, but that part isn't it.

Which is exactly why we want to employ static types: in order to catch the difficulties in implementing it correctly. We describe the complications in the type system, through a model that captures them, to allow compiler errors -- rather than runtime errors -- to guide us in implementing it correctly.

Types only hinder getting an invalid program to compile -- which is exactly what we want.

On a suitably complex/large system this is a recipe for disaster. Things start to slowly rot. It is far better to maintain the old function, accepting the old struct, map it to the new struct and forward it on to the new function that accepts the new struct. Let the old one consume anything that's already queued, or being sent from other nodes that haven't yet been upgraded whilst the new one takes the new format.

Some of it's parallelism and concurrency features should look familiar to you (it has an M:N threading model), complete with channels, and some stuff you've probably never heard of like STM. It compiles to a binary, has a type system much more powerful and expressive than Go's, and the community is very helpful.

I will say the compile times aren't very speedy, I assume you want fast compile times in order to type check your code, and for that there is ghc-mod.

It could be done without a big lock by splitting into three steps:

1) Push an upgrade that changes the types and adds the conversion functions. The valid type is the union of the old type and the new type. Wait until all nodes complete the upgrade.

2) Push an upgrade that instructs the nodes to convert their data and start using the new types by default. Wait until all nodes complete the upgrade.

3) Push an upgrade that removes the old types and conversion functions.

Yep, I develop one myself. And have gone to the extent of not allowing senders to even post a message if it's of the wrong type (processes in nodes publish the types they accept to a central store). I initially went along with the 'accept anything' approach (which Akka really majors on too), but found that for the large systems I was developing that it became a real headache to deal with.

> but my point is that I believe OTP is more flexible for better or worse. Whether that flexibility is worth it to you for what you get is another matter.

Yep, fair enough, if it works for you, who am I to complain? It's not worth it for me, because I feel quite strongly that the code I write should understand the types it's working with. It feels like this super-late binding can give false positives, appear to work, when in fact it's not. That scares the shit out of me when systems get large.

In Go, I agree. But that's not necessarily the case for all simple languages. Take ML, for example. It's a very small, easy-to-learn language with excellent abstraction features that make it easy to avoid duplicate code, as well as excellent static checking and error handling.

Unfortunately, ML lacks a comprehensive, modern standard library like Go has.

More generally, I think RPC interfaces need to be forward-compatible by design. If you have two binaries that are released on different schedules, and the API between them is fully rigid, how do you ever change it? Version the whole API, for a change that adds one boolean feature flag to one struct somewhere? Write a converter for fifty existing fields every time you add a new field, leading to O(n^2) programmer work over time? Come on.

Even more generally, I think static types are a great idea, but they work best locally. Communication over longer distances (in space and time) requires a different set of tradeoffs. There's a reason why people design network protocols and file formats with open-ended forward compatibility in mind. RPCs are kind of a middle ground, and I've found that the tradeoffs in Protocol Buffers work pretty well. YMMV.