Haskell Review

How Haskell and I met

I started my programming journey with Python and got quite fond of it before I learned Go and, after some reluctance, learned to see the upsides of both languages. Python and Go are my two strongest languages to this day, but some time after I got comfortable with Go I realized that I wasn't a huge fan of either language (albeit I still have a soft spot for Python and probably always will). I wanted the best of both worlds. I thought, "There has to be a better language out there, and it's worth my time to find it and learn it".

Python review

Go review

I tried out Perl, Ruby, Lisp, and probably one other I don't remember, but I didn't end up sticking with any of them. Ruby I lost interest in because I wasn't coming across any substantial difference from Python (and I didn't hear about any by web search either) so I figured it wouldn't be worth the investment. Ruby is dynamically typed, name errors at runtime, object-oriented... basically all the same flaws Python has. I knew that even if I had ended up preferring it to Python, it wouldn't be a big enough improvement to conclude my search.

Empty comparisons

I gave Perl the longest chance. I was referred to Perl by someone else and told that it was "way less verbose" than Python, not as readable but particularly good for scripts and batch editing - which I always used Python for at the time. I did get far enough into Perl to write an actually useful text processing script which I used to convert all the pose codes to a new paradigm in my VN project Return To The Portrait (I ended up rolling back the redesign after I realized it wouldn't work, but that wasn't Perl's fault, it was Renpy's). Still, I never liked Perl and couldn't understand what the hype was about. It has C-like syntax and just generally looked like an uglier version of Python with *nameless function parameters*, *no nested arrays*... seriously. I also didn't find it to be any more concise.


Lisp I don't remember why I quit; I only gave it a couple of hours. I do know I had some issues installing it and running something trivial. Probably I just tried it at a bad time in my life where I ended up not having the time to put into long-term skill investments for a week after that, and just never went back to it.

I came across Haskell. I knew very little of functional programming when I started. But when I read in Learn You A Haskell For Great Good that you can't change the value of a variable, I was too skeptical to not go on.

Learn You A Haskell For Great Good

I had some bad experiences early on and thought about quitting, but someone told me to keep going and I did; eventually I got past the early blocks and started to think I actually really liked Haskell. But that wasn't the end of the road. My enthusiasm dropped over a long time as I watched the cost of learning all this stack higher and higher. It was months before I started to think I understood monads, and more months before I actually did understand them and the rest of the stuff like Writer/Tuple, Reader/Function, State, monad transformers, and the rest of that black magic.

As of this writing, I have no major accomplishments in Haskell, but I've used it with GTK using haskell-gi (as well as helped out with the missing GObject Introspection annotation problem there), briefly collaborated on a Snake game AI (not linked because the repository got unpublished), dabbled in AST parsing, and I think I understand enough to use Haskell for something serious if I wanted to.


So while I'm nowhere near Haskell mastery (and probably never will be), I'm done holding off on giving my opinion of Haskell, even though it's not as informed as my other language opinions.

Compiled AND interactive

When I found out that there was a language that both compiles to native code and has an interactive mode, I seriously started to think I'd stumbled onto the perfect language. It sounded too cool to be true, y'know? In this regard at least, Haskell really is the best of both worlds between Go and Python.

The type system

Haskell's type system has so many great ideas. Without any "cheat the type system and give up safety" features, you can have parameterized types, sum types, interfaces, enums, and you don't suffer from the "losing type information when going through a general function" problem that some other statically typed languages have. What I mean is: in Go, for example, if you have a function that takes an interface type, does something to it, and returns it, it has to return it as the interface type because it doesn't know what its concrete type is. Meaning if you pass it a value of a known concrete type, you lose the information of which type it is when it comes back out. In Haskell, the lost type information "reappears" on the other side. This lets you reuse a ton of stuff you wouldn't be able to reuse in Go.

Not only is the type system extremely flexible and powerful, but you usually don't have to write the types. GHC will infer them as generally as possible: if you have a function that adds two variables, it'll know they have to be some type that implements the Num class, but it can be any such type.

It's still a good idea to put type signatures on top-level bindings as it improves readability and can help puzzle through type errors, but things are helped a lot by not needing type signatures on local variables and lambdas. (You also don't need to declare local variables.)

Against variable declarations

Null is dead. Long live types!

Sum types replace null! The Maybe and Either monads offer ways of dealing with failure in a safe, compile-time-checkable way. And you can also use exceptions when they're convenient.

No struct namespacing

This is the big problem with Haskell's type system: there's no syntax for accessing a struct field; struct fields are actually functions that take the struct type and return the right part of it... and that of course means they're in the global namespace, so structs can never share field names.

Ever worked on an application with data models that share field names like customer_id, creation_date? I have, and I would pay a fuckton to not have to call them job_customer_id, quote_customer_id, customer_creation_date, job_creation_date...

Oh, and as a result, there obviously can't be struct inheritance.

It's like the Haskell designers never worked in that kind of application. It wouldn't surprise me if few of them did - Haskell's following seems to be mostly academic mathematicians who publish papers on increasingly mind-blowing abstractions and few people in the industry actually using them, so its development is kind of in an ivory tower.

There's a compiler extension (DuplicateRecordFields) to sort of fix the namespacing, but not really.

Usefulness of DuplicateRecordFields

You can also get around it with classes (ie. interfaces), by defining an instance of the class for each type, but that's insanely clunky as you have to write all the shared field names three times.

Oh, and God help you if you have to deal with *nested* structs. Since you can't mutate anything, to update a field of a nested struct you have to also update the nested struct itself as a field in the parent struct. There's another elaborate abstraction to "simplify" that: lenses, which I've tried to learn multiple times to learn after understanding monad transfomers and failed.


The big unusal thing about Haskell's syntax is that it's *shell*-like: `func arg1 arg2` instead of `func(arg1, arg2)`. It makes more sense for a language with automatic currying, since there's no difference between evaluating a function's name and calling it with no arguments.

Haskell's generally pretty concise. The above syntax combined with functional purity, the powerful type system and type interference, automatic currying and super-terse lambdas let you express code more succinctly than most other static languages, competitive with Python, in fact.

Branching is a mess

There are four different syntaxes for branching:

And the types of things to branch based on:

Pattern matching in function definitions is really just syntactic sugar for case expressions; cases support structural comparisons and exact value comparisons, but not arbitrary value comparisons. For example, if you case on a number, you can't have a branch for `x > 5`.

`if` supports anything value-level, but nothing structural, and its indentation doesn't work the way it does in other languages; `then` is supposed to be indented under `if`, meaning nested `if`...`else if` expressions flow to the right (and multiline `let` bindings have weird quirks that still confuse me, making it even more difficult to use this workaround).

Pains of multiline let

Guards are an if-elif-else tree-like syntax that can accept an arbitrary value comparison for each guard, but they can only be used in the context of pattern matching. For all the different syntaxes Haskell has for branching, there's just no good way to do a good old if-elif-else tree.

There are compiler extensions to fix this, like MultiWayIf, but they're *compiler extensions*, and they come with quirky syntax of their own (the `if` and first guard both have to be on the same line as the preceding `=` which can be unreasonable).


Oh, and since `let` can only be used inside an expression (so it can't span over multiple guards), the `where` keyword exists which does exactly the same thing but applies to a set of guards (and goes at the end). If only guards could just be used in an expression...

Some more information on Haskell's myriad of should-be-but-aren't-redundant branching syntaxes:

A Gentle Introduction to Haskell

Someone else on Stack Overflow struggling with the mysteries of layout

Bad debugging

Some exceptions don't give stack traces, including some particularly common ones like `Prelude.!!: index too large` and `Prelude.head: empty list` (index out of range errors).

As for logging: pure functions can't do IO so you can't log from within them. And of course, a pure function can't call an impure one, so it takes a pretty onerous refactor to get a log statement deep inside the main logic; one that usually isn't worth it for debugging. (No, the Writer monad is not a solution because it's a comparably onerous refactor for each function it has to touch and still can't log without IO.)

In theory, you shouldn't need to log in a pure function because you can compose them from small pieces that you can prove are individually correct. At least, that's the talking point I heard from some Haskell evangelists when I was new. But that's not how things go in practice. One way or another, complex logic gets complicated, and there are bugs you won't find from testing components in isolation, even ignoring that some (eg. GTK stuff) can be difficult to test in isolation because of the nature of what they do.

The learning curve is absurd

Haskell might be worth it, but *damn* is it hard to understand. To really unleash the power of Haskell you have to understand things like Functors, Applicatives, Monads, Monoids, Monad transformers, and tons of other stuff I haven't even touched yet. This is a very real downside because time spent learning these concepts is time not spent using the language at its fullest.


Features are costs

In another language I could be on my first hour and look up the name of the standard library function to generate random integers and then import the module and get it done. In Haskell, to even start to do a lot of basic things you have to first understand these ridiculously abstract concepts. I remember spending about a day learning how to generate random numbers (and that was *not* my first day with the language).

Haskell feels like a never-ending rabbit hole of ludicrously elaborate abstractions to learn, and sometimes it seems like it's just to win back the functionality that was trivial in other languages.

This might sound like an "it's bad because I don't understand it" criticism, but it's not. A steep learning curve does directly undermines the point of a tool (making work more efficient), even if it can be compensated for.

Recursion is looping, but harder

Since loops involve state, Haskell doesn't have loops. When mapping over a sequence doesn't cut it, you resort to recursion.

But that doesn't actually eliminate state as far as the benefits of doing so are concerned. Recursion is still effectively state. It's a bit harder to think about because it's a call stack instead of a loop, meaning the first iteration isn't removed from the picture when the second one starts, but you're still going over the same code with different values in the same names. For all intents and purposes, recursion is looping, but harder.

Package management and build system hell

It's a bloody nightmare.

Build systems are a scourage

There are several different pieces of software involved: `ghc-pkg`, Cabal, cabal-install (which is a separate package from Cabal), Stack, and hpack (the Haskell Platform seems to just bundle a few of those things). All, besides hpack which I haven't used, are poorly documented and seem to freak out with inexplicable behavior at random times (by which I mean, *every* time).





the Haskell Platform

The most obvious solution is just to use cabal-install with GHC directly. Unfortunately, cabal-install is a capricious demon that hates coders and exists to confound them and inflict depression. Sometimes I get errors saying an import is ambiguous because there are two packages it could refer to... and they're *the same version of the same package*. And that happens without doing anything weird, just right after running a single install command on an otherwise pristine system. Sometimes I can import something in the REPL but not build with it. Sometimes I finally get a package to work and then the next day I can't import it because "the package is hidden", and I have no idea what I changed. `ghc-pkg expose` isn't the solution either. All the time I run into situations where it seems like the only way to fix a problem is to delete and reinstall everything related to Haskell.

`ghc-pkg expose` isn't the solution

Cabal also apparently doesn't install libraries by default, but the initial output you get if you forget the flag *says* it's building a library:

Followed by a successful-looking build log and then:

And sure enough the install failed.

Oh, and the ability to *remove* packages is in a separate package:


Cabal is actually mainly a build tool, so I probably haven't even seen the worst of it.

Stack was created on top of Cabal to supposedly fix the problems with it. Unfortunately, I don't think it does anything of the sort. it seems to install things in a way that's inaccessible to Haskell tools outside of Stack, meaning I can only use it as a package manager if I'm also using it as a build tool, and as a build tool it adds at least one new necessary config file *in addition* to the ones Cabal needs, is poorly documented, and that's just not worth it for me.

Why is Stack not Cabal

Also, it's not bad but install time is non-negligible, unlike with `pip`, `npm`, and `go get`, because Cabal seems to actually build everything it installs into a lib rather than just downloading source.

The one good thing about Haskell tooling-wise is that GHC can actually be pretty helpful. The type error messages are absurdly arcane, but that's mostly because the concepts are arcane, not because GHC is unfriendly. GHC can (with warning flags) point out unused variables and imports, and if you misspell or forget to import something, it sometimes knows what you mean or what module it's in. Even with the arcane errors, sometimes it suggests the solution directly (like "probable fix: use a type annotation to specify what a0 should be"), or recommends a compiler extension. It was GHC, not the Haskell community, that introduced me to the wonderful `ScopedTypeVariables` (which really should be part of the language standard).

Stdlib and ecosystem

The ecosystem reminds me of Javascript in that a single package on top of a base install often pulls in two dozen dependencies. And the standard library is tiny compared to other languages.

That statement is actually a bit unclear because there's some contention about what counts as the standard library. I define the standard library to be whatever comes with the compiler install, which is almost nothing. Here's a map of a superset of the standard library - if I filter out stuff that doesn't come with GHC, what all is left? Besides type system stuff, basic operations on lists and strings, and stuff related to the internals of GHC, pretty much *just* an FFI, OS interfaces, and Windows-specific graphics stuff.

What is the Haskell standard library

Map of a superset of the standard library

Here are a few examples of important stuff that's left out: regex, JSON, HTTP, and *randomness*. There are multiple packages for the first three of those, but being not built-in means you have to do research to decide which one to use, they don't tend to be well documented, and worst of all: you have to go through Cabal hell to get them.


Documentation tends to be subpar. I heard someone remark that "the Haskell community doesn't understand that type signatures are not a substitute for documentation", and I find that pretty insightful.

There also isn't an easy way to view it from the command-line as far as I know. But to compensate, there's Hoogle, which is something that doesn't seem to have an equivalent in other languages. Hoogle is a CLI tool that lets you search a database of packages for function names or type signatures. I don't know much about it, but I think it's really powerful.

The conceptual tutorials tend not to be very good - they're hard concepts to teach, with the curse of knowledge applying far stronger than usual. There are some good ones, but they're drowned by bad ones due to the monad tutorial fallacy.

the curse of knowledge

the monad tutorial fallacy

Strings are a mess

5 different string types. With overloaded function names for conversion. The OverloadedStrings compiler extension helps, but the string types are still confusing and inconvenient.

Untangling Haskell strings

List operations

Haskell is pretty par on concise list operations. Some are easy: index, search, reverse, sort, and of course map, filter, and comprehensions (though comprehensions are way less readable than Python's). But not others: negative index, slice, insert, remove (although `take` and `drop` fill the majority of your slice and remove needs), and update at position.

There's the `Seq` type in `Data.Sequence` which is more aimed at being a replacement for mutable lists so it supports insert/update/remove, but of course since it's a different type (and doesn't have the same interface), you have to convert to use it with anything that's designed to work on the normal list type. `Seq` also doesn't have syntactic support. You have to use it with `fromList` or something similar. And you index it with `lookup i items`. And to make matters worse, their function names overlap so you have to use a qualified import.


I haven't done much with Haskell's concurrency, but it seems pretty solid. `forkIO` starts a green thread and the thread communication mechanism, `MVar`, sounds a lot like Go's channels. Parallel evauation in pure code involves a function called `par` from the the `parallel` package, which takes two arguments and itself evaluates to the second one, but internally, makes GHC start doing the work of evaluating the other so it can already be done when a later expression evaluates to it. It's a counterintuitive approach, but it is incredibly concise - you could parallelize an expensive operation *by editing only one line.*

Haskell definitely taught me a lot about type system theory and language design. But there are a few reasons I won't be going farther with it.

The biggest one is that I don't believe in pure functional programming anymore. I was still unsettled on it when I last touched Haskell; it was mostly Rust that convinced me, by showing that most of the benefits of functional programming can be had without outlawing side effects.

Why I don't believe in pure functional programming anymore


The other reason is that I found out about Idris. Idris seems like basically a remake of Haskell with design mistakes and historical artifacts fixed. If I ever do want to use a pure functional language, I'd go for Idris. It seems like the effort I put into learning Haskell would carry over with about 90% efficiency (I played with Idris a bit).



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