Changes have been made as of 2020-06-09 to reflect new information about Cabal.
I would like to preface this article by saying that Haskell is great. I have been using it as my go-to for native applications for several years now. Everything that follows is a product of my love and affection for the language, and a desire to see it succeed. And none of this is intended to disparage the efforts of people improving the Haskell ecosystem. It’s hard work and there’s a lot of it, and I thank you all for what has been done so far.
That said, I will not be pulling any punches here.
As of now, there are a number of pain points which frequently occur when using Haskell. These are a product of my own experience and that of some of my good friends at varying levels of Haskell education and practise. If you are an intrepid Haskeller, some or all of these will already be known to you. I would like to lay them out here for a couple of reasons:
- For one, having the pain points made clear will hopefully encourage Haskellers to think about how to overcome them. Attempts have been made (we will come to that!) but given that the problems still exist, there is work yet to be done.
- Secondly, if you are a Haskell learner, I hope that it will help you to see that these things are known problems in the community and you are not just “doing it wrong”. People are aware that these things need to be improved and many people before you have encountered exactly the same problems.
If you just want the skinny, check the very end for a shorthand version of the article.
Contrary to popular belief, there are multiple Haskell compilers.
Even so the vast majority of application code is written with the GHC
compiler firmly in mind. There are good reasons for this. The most
critical is that GHC has the most money and time invested in it, and as
such moves faster to accommodate new concepts than other compilers.
Almost all production code initially written for GHC will not work with
any other compiler, owing to the inclusion of language pragmas such as
OverloadedStrings and so on. This itself is not a pain
point though; Haskell.org simply proposes to download GHC. (Except on
Arch Linux, which is a whole other kettle of
The problems start after downloading GHC. At time of writing, thirty years after Haskell 1.0, there is no bespoke Haskell IDE. There is no featureful Haskell plugin for any major text editor which can be installed without command-line intervention1. Either of these things alone would be somewhat problematic; taken together, it can be enough to put people off the language entirely. Hard to blame them.
There is hope in this regard. The tooling community seem to have
unified on making
haskell-language-server the standard.
Luke Lao wrote a great summary
which I would not be doing justice to summarize here. But it feels
pretty rough to tell people “Hey, just go without tooling until we sort
this whole thing out”, or “stick with this old version of GHC, it is the
one that works”.
For the time being, I suggest people find an extension which
integrates only with
ghci, such as
Simple GHC (Haskell) Integration
for Visual Studio Code. The lack of moving parts is a virtue. I also
strongly recommend the wonderful tool
provides live reloading and testing out of the box, again built only
ghci but with integration with
Cabal is super cool. It is very featureful and powerful, and getting more so with every release.
You know what else is super cool, featureful and helps people write
git. As a sprawling tool with multiple
interfaces for even small tasks, it’s no surprise that
has a million and one books dedicated to working with it in the most
practical way. By contrast, Cabal has the Cabal
User’s Guide. Unfortunately, the guide doesn’t really do much in the
way of guiding. For example, it doesn’t tell you that the
preferred way to use cabal in 2020 is with the
commands which are not even introduced until chapter five of seven,
roughly forty thousand words into the guide2.
Update: Apparently the v2 commands are the default now,
but the documentation is incorrect about this point. Also the tool still
tells you to use “v2-” versions of commands in command-line output
during normal operation.
In my view, the principal reason that many people recommend
cabal is twofold. Firstly, the
defaults are sensible. If you type
stack build, you are
doing it right. And secondly, you do not need to know everything
about it in order to get it to do what you want. As a Haskell
programmer of five years, I am still too intimidated by
cabal to use it as my daily driver, and I will reach for
stack even for small tasks.
Oh, and it doesn’t help that the documentation hosted on
haskell.org, which is the first search result for me, is
three minor versions behind the documentation hosted at
cabal.readthedocs.io, which is itself a minor version
ahead of what comes by default on the usually bleeding-edge
Arch Linux repositories3. Harumph.
If you are new to programming in Haskell,
stack is still
the path of least resistance. I look forward to the day that I can say
cabal is all we need.
The invention of the monad may be the most important piece of programming language theory of the last quarter century. They are so powerful that a virtual cheer goes up any time another language even imitates their implementation in Haskell. Despite many attempts though, I don’t think we have yet found the best way of explaining what they actually are. The Wikipedia article has quite a good example of non-explanation.
At Warwick University we have a competently designed and taught
Functional Programming module for first year students. In the module,
monads are introduced as a mechanism for abstracting sequential
composition. This seems to be quite effective as a motivator, but it is
also hiding a decent amount of complexity. For example, the
List monad does not compose sequentially in the way that
might be expected. Additionally, the effect composition order is not
always defined by the direction of
the reverse state monad, for example).
Functors can be broadly explained in terms of containers, where
fmap modifies the value(s) in the container without
modifying the container itself. The container may be structural (in the
case of data functors) or computational (in the case of control
functors); both of these concepts survive the box explanation. There are
similar analogies that can be made for monads, but they all fall apart
in some critical aspect. “A monad is like a burrito” is famous
for how true it isn’t.4
Hopefully we will find the platonic ideal of a monad tutorial before too long. In the mean time, we will have to stick to teaching by motivated examples.
Monads and Documentation
The introduction of monads to new learners is not the only pain point, however. Friends and colleagues have pointed out that a similar issue arises whenever trying to learn a new library which has a monad at its core.
Very rarely do libraries document the underlying mechanism by which effects are composed. As a result, the user is left guessing at what the core of a library actually does. Even some of the best-documented libraries5 seem to suffer from this affliction.
I implore library writers to define the functionality of their fundamental types upfront. And while we’re at it, please consider documentation-by-documentation as more of a priority than documentation-by-blog-post. The Haskell community is inseparable from the blogosphere (and we have many talented bloggers around, not including myself) but blogs do decay over time. Inline documentation serves as a source of truth, and is found precisely where people first look.
At risk of sounding like a broken, erm… yeah.
Haskell’s record system is outdated. In a universe of abstractions and minimally-constrained types, the monomorphic nature of records sticks out like a sore thumb. For example, there is no built-in way of saying “given a record of this shape, I will produce a record that is the same with this extra field”. Even having two records which share fields is difficult. Such construction is natural in many modern languages. This concretion is frequently used as a stick to beat Haskell programmers over the head with.
A number of libraries have been developed which extend or replace the functionality of built-in records. Each of these libraries is far superior to the default state of affairs, and that makes Haskell look bad.
If you are a learner, I recommend simply believing sensible record management to be impossible in Haskell until further notice. If you desperately need this functionality, see the libraries above.
Fingers crossed that the Haskell community can unify on one approach, at least enough to get built into GHC as a language extension or, ideally, to make its way into the Haskell Report.
I won’t hold my breath.
GHC uses a frankly astonishing amount of time and memory to do its job. It’s a known problem that some programs and libraries literally cannot be compiled on even competent hardware due to a lack of resources. In addition, long compile times are off-putting to people trying out Haskell for the first time.
Part of this is down to another known issue with Haskell libraries
which is that some of them pull in the known universe as dependencies.
is a good example of this. The upshot is using nasty
workarounds or to simply give up and buy a more powerful
But even so, it’s unclear to me why compiling a program which has a hundred or so files uses plural gigabytes of memory to compile. I’m not sure there’s anything we can do about this, because GHC is such a monolith, other than slowly improve its codebase over time.
To library and application developers, please stop depending
on enormous libraries like
wherever possible. I do not think that a dependency on
Cabal is required by almost any program, and
lens can almost always be replaced with
a minimal reimplementation designed with library devs in mind.
No advice for new learners, I’m afraid. Maybe take up swordfighting.
I have left out some pain points that people told me about while I was researching for this article. That’s not because they’re invalid, but because most of them are a product of the environment in which they were learning and might not be as universal as the things listed above.
None of these problems are insurmountable. Indeed, a lot of them are currently in the process of being fixed. But unless we can look each other in the eye and say that they exist, people will keep being turned away.
Below I’ve summarized the content above in a handy flash-card form for your convenience.
|Problem||What do we do about it?||Advice for new learners|
|Tooling is janky||Get a unified toolchain to maturity ASAP!||Use your favourite text editor, and
Haskell tooling which only relies on
|Cabal is impenetrable||Produce a practical guide to
|Advanced topics are hard to teach||Invest a lot of time and effort into pedagogy for advanced topics in Haskell.||Learn by example as far as you can. Ask around for accessible introductions to advanced topics you’re interested in.|
|Documentation is imperfect||Learn from the masters, and document alongside your code and not only in blog posts.||Talk to Haskellers in the community for recommended sources of truth for given libraries.|
|Records are outdated||Unify around a common syntax for polymorphic, extensible records.||Understand that records are imperfect; use alternative abstractions if you need extensibility or polymorphism.|
|Compilation is slow and resource intensive||Minimize your libraries’ and applications’ dependencies aggressively.||Take up swordfighting.|
If you know of regular issues which come up and which I have not mentioned here, I’d love to hear about it. There’s plenty to discuss.
Trust me, I have tried them all. I have been trying for years. Apparently
haskell-modefor emacs may come close; I did not have success with it.↩︎
Copied and pasted chapters 1-4 into wordcounter.net.↩︎
This is without even mentioning the classic examples of “A monad is a monoid over the category of endofunctors” or “A monad is a lax functor over a terminal bicategory”, which both serve as unfair but amusing characterisations of certain members of the Haskell community.↩︎
One example is the fantastic
SBVlibrary. It has a type
Symbolicon which a lot of functionality is based. I have been using this library for a long time, but if you asked me what the
Symbolicdid, I would not be able to tell you. In the docs,
Symbolicis described as “a specialisation of
SymbolicTis defined as “a generalisation of
Symbolic”. I hope the problem is clear.↩︎