[codeflo]

I've written small stuff in Haskell a decade ago. I have a soft spot for the language -- it has clearly influenced many notable languages that came after it. But I also admire the patience of anyone who actually manages to use it in practice, there are so many little papercuts that don't get resolved, basically for a decade or more. If I'm cynical, I'd say that's because little practical stuff is often not worth publishing papers about.

Error handling was, for me, a big one. For a functional language, Haskell seems very obsessed with exceptions. Even supposedly pure stuff, like "head" (first element of a list) throws an exception if the list is empty. You'd think Haskell would be the first language to have it return a Maybe value, but no. (Rust, BTW, gets functions like this right; they all return an Option.)

This reliance on exceptions clashes hard with the functional paradigm. Exceptions are "magic": They are special additional values that any type can have (so an Int can either be an actual integer or an exception value), but you can't test for them or handle them in any way pure code, you need IO for that. Which the language makes intentionally hard to use, that's Haskell's entire thing.

[Tainnor]

Partial functions and exceptions are a compromise solution for the fact that you sometimes do know more than the compiler does. I think it's fine to throw an exception in the case of "programmer error". It's the equivalent of assertions in other languages. Yes, it can blow up, but at least the error is a bit more localised.

Having head return a Maybe means that you'll have to awkwardly handly a Nothing case even in situations where there is no sane behaviour to be added because it just simply would make no sense for a particular list to be empty unless you've introduced a bug somewhere else. It's hard to "recover" from such an error.

The same goes for e.g. division, which is partial too (can't divide by 0), but having it return Maybe would make arithmetic incredibly awkward. You could instead define e.g. x/0=0 or any other value—some languages like Coq or Pony do that, but I think that has the drawback that this makes it rather easy to mask some ugly errors.

In many such cases, the Haskell type system (without advanced extensions) is not expressive enough to encode everything you know about your values. In a language with dependent types, such as Idris, you can specify the length of the list in your type; then you can have a type-safe, total head function that doesn't return Maybe. You can also write a division function that requires a proof (possibly implicit) that the denominator is not zero. But dependently typed languages are much more niche than Haskell.

[bspammer]

Haskell has had non-empty lists as a type for a long time: https://hackage.haskell.org/package/base-4.16.0.0/docs/Data-...

Having partial functions in the Prelude is, as far as I know, widely regarded as a mistake and they are only kept around for backwards compatibility. Anyone writing code nowadays should be using safeHead or non-empty lists.

[oats]

Or pattern matching that takes account of the empty list case. Orrrrr using a fold! I usually find when I start matching on list values that the function could be better expressed with a fold instead.

[Tainnor]

Sure, but non-empty lists don't generalise easily to the case where you need the nth element, unless you nest them awkwardly.

[youerbt]

> But I also admire the patience of anyone who actually manages to use it in practice

Nothing you point out gets even close, in my mind, to stuff like null pointers or untyped code. So I wonder what languages you have in mind that require less patience.

> you need IO for that. Which the language makes intentionally hard to use

Well, that is simply not true.

[caente]

>Nothing you point out gets even close, in my mind, to stuff like null pointers or untyped code. So I wonder what languages you have in mind that require less patience.

You two are talking about two different things: - The parent is talking about the ecosystem, how menial tasks have tooling in "less interesting" languages - You are talking about the language itself

I would venture to guess that the parent would agree with you, if talking about the language in a vacuum.

An interesting competition would be to develop a complex product, without external dependencies.

My sad guess is that languages that are filled with escape hatches, like Java, Javascript, or python, would defeat more strict languages.

It's a sad guess, because I actually do prefer the Haskell way.

[smegma2]

> Well, that is simply not true.

I think it's true, for example catch [1] requires IO. Do you mean with unsafePerformIO, or something else?

1: https://hackage.haskell.org/package/base-4.16.0.0/docs/Contr...

[ParetoOptimal]

> But I also admire the patience of anyone who actually manages to use it in practice, If I'm cynical, I'd say that's because little practical stuff is often not worth publishing papers about.

I feel my impatience pushes me towards Haskell if anything... local-reasoning for instance rather than "understand this entire call chain" requires less patience and is easier to get right.

> there are so many little papercuts that don't get resolved, basically for a decade or more.

I've been using Haskell a decade in practice, can you tell me what papercuts you had in mind? I'm assuming I and other real world Haskellers might just see them as much less of a priority all things considered, but I'd like to be sure I'm not missing something.

[exdsq]

In Haskell you’d use pattern matching guards for the empty list which works better for recursion & doesn’t require you to handle the Maybe monad in primitive data structures.

Even though Monads were introduced to programming after Haskell had been written (to deal with IO, SPJ and Wadler have a good paper on this) I don’t know if this would have been worth changing. After all, you can always wrap a custom Maybe<List> if you need it!

[dundarious]

OP is saying the existence of head means someone will use it and get the paper cut. It's true you just shouldn't use it (even when you know it's non-empty, write the throw yourself). But that's why it's an annoyance. Arguably it's even more of a problem, it's a "foot gun". It would be nice if the Prelude was just replaced, but that obviously presents a host of annoying challenges. Several alternative Preludes exist, but none appear to be becoming the new center of mass.

[codeflo]

True, but I think enabling more cases where you can use function composition instead of pattern matching and explicit recursion would be a win.

[odyssey7]

This is why I see PureScript as a better starting point. It was modeled after Haskell, but since it was created in 2013, many of the design choices were to avoid these sorts of things.

[PragmaticPulp]

> But I also admire the patience of anyone who actually manages to use it in practice, there are so many little papercuts that don't get resolved, basically for a decade or more. I

Great summary of what it’s like to use any niche language. You don’t realize the value of a mature and highly used ecosystem until you have to chase issues in an ecosystem where maybe 5 other people total are doing the same thing you’re doing and nobody has updated some library you need for 3 years.

Fun for hobbies, terrible for real work.

[toomanydoubts]

There are some alternative Preludes that attempt to fix this, bringing a safer std lib to the table.

[madsbuch]

What you are running into is the pureness of Haskell. The `head` function in Haskell is only partially defined. What you see as an exception is a case where a function is not defined. This is all by intent.

defining a `head :: [a] -> Maybe a` is a very simple matter and definitely something a developer should be encouraged instead of using the prelude.

exceptions in Haskell are not meant to be used as a first class thing, but is the way to ensure a full Turing complete language where it is possible to define non-terminating behavior. Hence it really is by design.

[dundarious]

The issue as I see it, is that one of the main selling points of a pure language like Haskell, is that you have to explicitly state where a certain class of surprises/failures (from IO) lie, and therefore, you can account for them better, handle them cleanly, prevent them from arising accidentally or in some ways maliciously, etc. Partial functions are another kind of surprise/failure, but they are not at all explicit.

This is a bit strange. It's like caring deeply about whether printf fails, but not so much whether array indexing is out of bounds. Haskell has a great story for both kinds of issue, and even its exceptions are better than panics IMO, even if they are about as tricky to use as POSIX signals, but it is relatively obscure and stigmatized to do a gross thing like use unsafePerformIO, but actually quite common/natural and accepted to use head. Lots and lots of people know to do the right thing for the latter, and there is something of a community push to avoid them, but it's just interesting to note how easy it is to make one mistake versus the other, when both matter a lot. One is treated as fundamental, and the other is not, but day to day, both kinds of issue lead to a similar magnitude of headaches, so the disparity is noteworthy.

I'd love it if even just the type signature recorded that exceptions are possible, even if there is no practical effect on how or where it is used.

[Rusky]

IO is not (primarily) about where failures lie, but about where side effects lie- side effects are where you start caring about the order of execution.

Array indexing failures, on the other hand, are not something you typically care about at quite that granularity- they're usually just bugs, not something to recover from except perhaps at a much higher level.

The parent comment lumps these kinds of failures in with non-termination, which in pure functions is also typically just a bug rather than a recoverable failure. And this one isn't something you can generally check for, either- with lazy evaluation, every type in a Haskell program by default includes a "bottom" value.

I think both choices were made for a similar reason- actually handling array bounds check failures everywhere is pointless tedium (and often better folded into the iteration itself), and actually handling possible non-termination by using a total language can also get pretty tedious. There are languages that do both, and they have their uses, but Haskell went a different direction.

[dundarious]

You make a good point about IO, I forgot how it's also not great about errors (but isn't there are an IO monad with better error treatment? -- it has been several years...). I also agree about granularity and tedium, but that's orthogonal to whether exceptions are the best way to approach such errors, and I don't think they are. Even Go's approach of explicit if-return is not tedium to me, but there are even less tedious approaches, that still let you handle the handle-able errors and do some last-ditch cleanup or just panic on the unhandle-able ones like indexing errors.

The interesting thing about Haskell exceptions are the async ones and the ability to `throwTo`, but I never really had a use for that, so on the whole, that was a bit of an encumbrance too. It's like trying to write exception safe C++ -- tedious and easy to get wrong. I remember a fair few sections of Parallel and Concurrent Programming in Haskell that temporarily didn't handle exceptions correctly, and it often wasn't for pure pedagogical reasons. Great book though.

[Tainnor]

Idris does that. If you add "%default total" to a file (or the equivalent compiler flag), it will make sure every function terminates unless it's annotated with "partial". In the best case, only your main function and a couple others need to be partial.

[DarylZero]

> quite common/natural and accepted to use head

No it isn't, not at all.

What absurd slander.