[Haskell-cafe] What is your favourite Haskell "aha" moment?

David Feuer david.feuer at gmail.com
Wed Jul 11 17:09:55 UTC 2018

Haskell is the only language I know of with typeclass coherence.

On Wed, Jul 11, 2018, 12:25 PM Simon Peyton Jones via Haskell-Cafe <
haskell-cafe at haskell.org> wrote:

> Also, languages are borrowing from each other at a rapid rate these days
> (eg Rust traits are equivalent to type classes) so it's hard to find a
> "killer feature" in Haskell any more
> That’s true, and to be celebrated!
> One thing that stands our for me is the ability to abstract over type
> *constructors*:
>             f :: forall (m :: * -> *) (a :: *). Monad m => a -> m a
> That ability is what has given rise to a stunningly huge collection of
> abstractions: not just Monad, but Functor, Applicative, Traversable,
> Foldable, etc etc etc.   Really a lot.  It opens up a new way to think
> about the world.  But only made possible by that one feature.   (Plus type
> classes of course.)
> Do any statically typed languages other than Haskell and Scala do this?
> Simon
> *From:* Haskell-Cafe <haskell-cafe-bounces at haskell.org> *On Behalf Of *Neil
> Mayhew
> *Sent:* 11 July 2018 17:12
> *To:* Haskell Cafe <haskell-cafe at haskell.org>
> *Subject:* Re: [Haskell-cafe] What is your favourite Haskell "aha" moment?
> I came to Haskell from C++, and I was used to the idea of parametric types
> and functions from C++ templates.
> However, what I really liked about Haskell's way of doing these was that,
> due to type inference, not only is there a lot less ceremony involved, but
> code is generic (parametric) *by default*, and thus much more reusable,
> although you still have the option of tightening it up by adding a type
> annotation (eg for performance reasons). For a while, I was writing all my
> C++ code as templates, but this ended up being a pain.
> Also, traditionally C++ has not allowed you to place any constraints on
> template arguments (type parameters) whereas Haskell's type classes are a
> very elegant way of doing it. (C++ now has concepts, but I haven't taken
> the time yet to see how they compare with type classes.)
> I was also used to function overloading, which is somewhat similar to type
> inference, in that the compiler will pick an implementation based on the
> types of a function's arguments. This is similar to Haskell picking an
> implementation from among type class instances. However, what blew me away
> is that Haskell can overload based on *return type* as well as argument
> types. I haven't seen any other production-ready language that can do this.
> A great example of how this is useful is the regex library, where you can
> select from among widely varying styles of regex matching result simply by
> type inference, ie without needing any type annotation.
> There were a *lot* of other things I found amazing, but others have
> covered many of these already. Also, languages are borrowing from each
> other at a rapid rate these days (eg Rust traits are equivalent to type
> classes) so it's hard to find a "killer feature" in Haskell any more
> (except laziness, perhaps). I think it's more the well-balanced combination
> of all the features that makes Haskell so pleasant to work in, and it's
> hard to demonstrate all of these in a single example.
> My own favourite "gem" is this code for computing all primes, based on
> code in a paper[1] by Doug McIlroy:
> primes = sieve [2..] where sieve (p : ns) = p : sieve [n | n <- ns, n
> `mod` p /= 0]
> I think care must be exercised, when using examples like this one, to
> avoid giving the impression that Haskell is a "toy" language. However, what
> I find interesting about this example is that all other sieve
> implementations I've seen work on a fixed size of sieve up front, and if
> you later change your mind about how many primes you want, eg because
> you're expanding a hash table and want a bigger prime for the size, you
> typically have to start the sieve from scratch again.
> [1]: http://www.cs.dartmouth.edu/~doug/sieve/sieve.pdf
> <https://na01.safelinks.protection.outlook.com/?url=http:%2F%2Fwww.cs.dartmouth.edu%2F~doug%2Fsieve%2Fsieve.pdf&data=02%7C01%7Csimonpj%40microsoft.com%7Cce59cb95fe304d442b8d08d5e7490739%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636669223196857658&sdata=1UGE8hoL8B9yMjXqJbe5HV%2FsAXlpoB653kePOrgpCNY%3D&reserved=0>
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