>But you make an interesting point in a way. One can ask, ok here are two non-strictly functional languages that seem to be fairly practical. Used for large distributed mission critical systems.
I think there're at least these problems with Haskell:
* The language is badly designed. See the number of extensions which aren't standardized but widely used. See also obvious problems such as requirement to use renames when importing code, and problem with record names conflicts which isn't solved for years.
* The focus of the language isn't correct. Lazy evaluations creates a lot of problems both in program performance and complexity of the compiler. The most important parts are pureness and side effect control.
* The language is already obsolete. Many its constructions and related complexities are redundant if we unify type and expression level, and get as a result dependently typed system.
I hope some language in the future will implement these ideas, and we will have a widely used pure functional dependently typed language with explicit control of effects.
I see two candidates for such a language:
* Pure script - it's Haskell done right, compiledd to JS. Unfortunately, it doesn't support dependent types.
* Idris - it's the language which I describe, but I fear that it's impossible to create a widely used commercial language without major industry players' support.
> the number of extensions which aren't standardized but widely used.
Actually, I think this is something that Haskell did right. It allows experimentation with new language ideas in a way that allows them to be tested safely with existing codebases. Edward Kmett recently had this to say about it:
> I appreciate Haskell's willingness to take a very very long view, and explore the design options, before committing to a path. In many ways, it is this feature of our community and our language that keeps other programming languages looking to us for features and designs to steal.
> The language is already obsolete.
By that argument so are C, C++, Java, etc. But it's about the existence of a mature ecosystem, not about what is theoretically known.
> Pure script
PureScript is great, but its compile to JS basis makes it inappropriate as a general purpose language.
> Idris
Idris looks promising, but it doesn't have a large enough ecosystem yet. I would certainly be willing to switch from Haskell to Idris in the future, but Haskell already is at least a little bit dependently typed [1] and as hinted at above I wouldn't be surprised if Haskell is able to continue improving its dependent typing capability as research advances.
> PureScript is great, but its compile to JS basis makes it inappropriate as a general purpose language.
Except that Javascript is rapidly making inroads on everything except systems programming. If Java is a general purpose language (can't be used for systems without translation due to the JVM), then so is anything-which-compiles-to-JS.
I think the really important thing is that, while I've been enthusiastic and interested in Haskell for a long time now (~8 years?), the vast majority of people will never understand even a comparatively simple concept like monads, let alone anything higher level than that.
Until you can 'get work done' without the equivalent of graduate-level math classes, there's simply no way to develop that 'mature ecosystem' that everyone is hoping for. People have been using 'better if you understand the complication' languages for a long time now, and there's a good reason they're still niche, unfortunately.
Erlang is a great language, but it's very definitely not of the same scale of difficulty to understand as Haskell. You can noodle around with Erlang and end up with something that works reasonably well - in my experience with Haskell, if you can't mathematically formalize what you're talking about, you're going to have a bad time, unless you do go the 'embedded scheme interpreter' route, which is a fun project, but why not just use Racket to begin with?
> If Java is a general purpose language (can't be used for systems without translation due to the JVM), then so is anything-which-compiles-to-JS.
Fair point. I guess my thinking here is that PureScript is unlikely to replace Haskell. It is possible, but I think there are a few reasons that probably won't happen. The first is that it's mission statement "PureScript is a small strongly typed programming language that compiles to JavaScript" points it in the wrong direction. Maybe Javascript is the counter to this, but that feels like an anomaly. Second is that it is way behind Haskell in terms of infrastructure, ecosystem, and a lot of core language/compiler features that enable general purpose programming. Could swarms of people come in and change this? Sure. But if they did, it seems to me that PureScript is similar enough to Haskell that people could just switch to Haskell and avoid that work.
> Until you can 'get work done' without the equivalent of graduate-level math classes
This is a big misconception that is demonstrably untrue. I know many people who have developed a solid working grasp of monads without doing graduate-level math.
> Second is that it is way behind Haskell in terms of infrastructure, ecosystem, and a lot of core language/compiler features that enable general purpose programming.
No, it's way ahead because it's based on Javascript, and javascript is having umpty-billion libraries developed for it right now. Of course that's not actually a good thing at the moment but they'll settle down at some peoint.
> This is a big misconception that is demonstrably untrue. I know many people who have developed a solid working grasp of monads without doing graduate-level math.
Ref my reply to Mr. Church, but it's not just monads - you never stop running into category theory if you're using Haskell, it's baked in.
If someone without any knowledge of category theory rewrote the language, that statement might become true (though I'd be skeptical they'd get it right), but as of right now, you need to know category theory to understand the language and it's base libraries.
I know, I've struggled with it for a long time - every time I have a problem, I have to go back and reteach myself category theory to understand it.
Until you can 'get work done' without the equivalent of graduate-level math classes
The problem is the name "monad", not the concept itself. You don't actually need to know anything about category theory to do I/O. You just need enough exposure to the concepts to know why the Monad type class is so repeatedly relevant as to be given a named abstraction.
People have a hard time getting past the name. You don't need to be a category theorist to use monads in Haskell.
> You don't need to be a category theorist to use monads in Haskell.
I find that demonstrably false. Every time I run into problems and ask for help, the answer is "there is this other mathematical concept that solves that problem".
It's not just monads, it's comonads, monad transformers, GADTs, Functors, phantom types, arrows, arrow transformers, the list keeps going. In addition, they're only described in terms of other mathematical terms - the same problem that plagues monads: nomenclature.
And these aren't just 'things you can use', if you want to understand the libraries you're using, you need to be able to keep up with all of those and more.
It's not like lisp, where you can mostly rearrange cons cells to make nearly any data type you like - there's no way to bootstrap your brain into understanding arrows from a standing start, at least in my experience as an autodidact trying to learn.
GADTs, phantom types, and arrows (not to mention monad transformers) aren't "mathematical terms" in that the names aren't borrowed from mathematical terminology.
They're just _terms_. But to you they are new terms. So you call them "mathematical terms" because obviously if you haven't heard of it, then it must be complicated math.
It isn't.
If you stop thinking that every new concept you meet in the world (or at least in Haskell) is a "mathematical concept" and start thinking it is "just a thing that is new and I can learn" then maybe you will stop feeling this way.
For that matter, none of the things you list except for Functors and Monad Transformers (which are just "ways to build monads") are idiomatic for "basic" haskell programming, and the next step up, GADTs is a straightforward feature to learn when you need.
If you read something like RWH or LYAH, you'll notice they don't cover arrows or GADTs that much. That's because they're not considered core concepts. If you set out to learn "all the things" and then discover that there are a lot of things, perhaps the problem is just that you set out to learn "all the things" under the false impression that you need to in some immediate way.
I should amend this. I have never noticed something to be an arrow transformer in the wild, neither because it was called out in documentation or naming, nor independently. I have much less confidence nothing I have encountered has ever been an arrow transformer...
Sometimes you have to find the tutorials and blog posts first. The lens type signatures aren't the easiest to understand, and sometimes its easier to just use things "empirically" without fully understanding them, and then get a sense of the type signatures when you really want to know what the compiler's actually doing. Being able to use lenses is not as hard as understanding why its intimidating rank-2 type signature is the right one.
Finally, sometimes it helps to de-generalize. To replace (Monad m) with IO, for example, and think about what that special case might mean.
For example, the type signature for Control.Arrow.first is:
first :: Arrow a => a b c -> a (b, d) (c, d)
If you're a beginning Haskeller, your reaction is going to be "WTF"? Well "Arrow" is a type class more general than function. (It could be the "effectful function" type or Kleisli arrow, a -> IO b.) If you specialize with a = (->), you get the more concrete type signature:
first :: (b -> c) -> (b, d) -> (c, d)
... and it's much more clear what it does.
As for nomenclature, I don't disagree. I had this discussion with Brian Hurt (and owe him for the insights): we'd be more marketable if we called Monoid "Appendable" and Functor "Mappable". Monad is one where it's hard to come up with a more accessible name for it: it abstracts over computational context (i.e. what do we actually mean when we talk about computational effects) but that would be a mouthful of words and not any more clear.
I know all these things now - my point is that it took me years to get them to a level where I understand what they mean well enough to implement them anew. You can't say that about other languages.
Nobody is going to 'build a community' around a language that is so effectively obtuse, and when called on it, says 'here are tutorials' - tutorials are not the point. RTFM isn't a decent response to 'your language is difficult to learn and use'.
Given that not even basic functional programming has become popular yet, I can't see dependent types being widely adopted for decades. The learning curve for using dependent types is incredibly steep and capturing interesting properties in types inevitably requires you to understand and write tricky maths proofs.
I wouldn't say that was the case until there is wide support for algebraic data types, type inference, pattern matching and immutable data. I see more languages allowing functions as parameters, basic type inference and encouraging immutable data, but they're still missing out on a lot of benefits offered by functional languages like Haskell and OCaml by ignoring these features.
I do see functional programming features making their way in to imperative languages but most people are still coding in an imperative way. Mutable data is still the default rather than immutable data for example.
This is a terrible and also hilarious argument. Instead of saying Haskell has too many newfangled ideas, you're instead arguing that it has an insufficient amount of them.
So Haskell has newfangled innovations compared to the big players, and it _also_ has a mature ecosystem in widespread use.
This puts it ahead of Purescript and Idris for adoption in large scale projects (although I would readily agree that both those languages are pretty great too).
I maintain that lazy evaluation works great, it just requires optimizing along different paths than devs are used to -- but it doesn't incur more "overhead" to optimize code than is the case in typical strict languages, which _also_ have to pay that cost, just in different sorts of optimization and reasoning.
I think there're at least these problems with Haskell:
* The language is badly designed. See the number of extensions which aren't standardized but widely used. See also obvious problems such as requirement to use renames when importing code, and problem with record names conflicts which isn't solved for years.
* The focus of the language isn't correct. Lazy evaluations creates a lot of problems both in program performance and complexity of the compiler. The most important parts are pureness and side effect control.
* The language is already obsolete. Many its constructions and related complexities are redundant if we unify type and expression level, and get as a result dependently typed system.
I hope some language in the future will implement these ideas, and we will have a widely used pure functional dependently typed language with explicit control of effects.
I see two candidates for such a language:
* Pure script - it's Haskell done right, compiledd to JS. Unfortunately, it doesn't support dependent types.
* Idris - it's the language which I describe, but I fear that it's impossible to create a widely used commercial language without major industry players' support.