Let’s play a game. Take your favourite languages and tell us what you hate about them. Like I’ve said before – all languages suck. Don’t pretend yours is perfect. If there aren’t things about it which annoy you you’re not trying hard enough.
I’ve already started with Scala. Now I’ll do Haskell. Hopefully someone more experienced with the language will follow up, as I’ll freely admit it’s not my best language.
So, things I hate about Haskell:
Let’s start with the obvious. Monad tutorials. No, not monads. Specifically the tutorials. They’re endless, overblown and dear god are they tedious. Further, I’ve never seen any convincing evidence that they actually help. Read the class definition, write some code, get over the scary name.
Now we’ve over the one sociological issue, let’s get to some actual language ones.
Modularity. Haskell’s module system is about the minimum possible you can get away with and still claim to have a module system. Anything less and you basically have C style header files. Now, this isn’t a huge problem. To a certain degree type classes mitigate the need for anything functorial. But there are examples where it’s an issue. Take ByteStrings for example. Lazy and strict ByteStrings have essentially the same module signatures, but there’s no way to write code which is agnostic as to the type of ByteString it uses. They don’t live in a type class because it was considered that the type class signature would be too long. This is probably fair, but life would be a lot nicer if you could write functors over the ByteString modules.
In general, Haskell has very poor namespacing. If two modules define something of the same name you have to redefine one of them or refer to one of them qualified. It’s not a huge issue, especially as you *can* rename and Haskell modules are anyway not as long as Java packages. It’s not like you have to refer to org.haskell.data.collections.map.mypetcat.lookup. M.lookup isn’t too onerous. This mainly causes problems with symbolic operators, because things like Foo.++ look a lot worse as infix operators than Foo.bar does. I don’t know of a good solution to this problem. Object oriented languages provide a solution to it by basically making the type of the first argument responsible for namespacing. I don’t find this especially satisfactory, but it does work.
On a similar note, Haskell type classes basically live in global scope. There are good reasons for this, and it avoids you having to write a whole lot of painful sharing constraints (The example that is thrown at me every time I complain about this is what do you do if you want to unify two Data.Sets, each of which uses a different instance of the Ord type class)? It can be a real pain though.
There’s another issue related to type classes. Let’s pick an example. Data.List defines both sortBy and sort. The difference is purely that sort uses the standard ordering on the Ord type class while sortBy uses a provided ordering. And you need to define different functions every time you want to be able to use either a type class or something user configurable. Because you can’t redefine type classes locally or pass them as first class instances there’s basically no way around this. You can normally avoid the issue with selective use of newtype and careful choice of your types, but it’s definitely there.
Which brings me to another point – type classes are not first class. The Scala encoding of type classes is clunky in many ways, but a very nice feature is that the things you’re using as type classes are real first class members of the language, and you can bring to bear all the usual tools you have for manipulating stuff.
10,000 compiler extensions. GHC introduces so many compiler extensions, and everyone goes wild about them. This is understandable. Some of them are really useful. Some of them it’s amazing to manage living without (No multiparameter type classes in H98? Yeargh. Getitaway!!). I really like most of these I’ve used, but it sometimes make it feel like you need to employ deep and profound type system voodoo to get anything done. It would be very nice to get these unified into something sane and consistent, but Haskell’ seems to have stalled.
Tuples and records. There’s basically no good generic handling for either of these. Each of the different sizes of tuples is a totally different type (with no common type classes for abstracting over them), records are just a thin wrapper over normal datatypes with no extensibility or namespacing. Your record accessors will clash just as much as any other function name.
Something which has bitten me in the past is sharing code between monadic and non-monadic contexts. You essentially need to write pure and monadic versions of a number of functions. It would be nice if generic monadic code could automagically be specialised to the identity monad in a way that didn’t uglify pure code using it, or pure code lifted to monadic versions (this is harder I think). This doesn’t come up too much, but it means that there’s often functions like foo and fooM for pure and monadic versions.
The prelude and standard type classes are a bit painful sometimes. Things which you’d expect to be overloaded into type classes aren’t (Data.Monoid defines a function mappend for example, which in the List instance is ++. Why isn’t ++ on Data.Monoid?) and sometimes the type classes which are there are poorly thought out (Num shouldn’t extend Eq, and it would be nice if + was factored out in order to provide better support for things like vector spaces). I believe there is some work on alternate numeric preludes.
Plus lots of other little things which don’t spring to mind at the moment.
Also, one final disclaimer. Please please please don’t take this as a “Haskell sucks, don’t use it!!!” post. If you do I’ll… I don’t know, give you a really devastating hurt puppy look or something.
I’d also appreciate it if you don’t use this post to start a language war. Remember – you’re only allowed to say bad things about languages you actually like. Otherwise you’re cheating. :-)