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

Damian Nadales damian.nadales at gmail.com
Wed Jul 11 13:36:37 UTC 2018

Not sure if this helps, but the following two code snippets (taken from
Rosetta code) for computing the generalized Cartesian product of a list of
lists might exemplify a very important aspect of Haskell: Haskell
programmers must invest a lot of time into learning functional programming,
but once they speak this common language, the code tends to be easier to
understand, maintain, and reason about.

The Java solution requires to think about all sort of lower level details
on how to iterate the lists and construct the result. It even mixes
iterations with more "functional" idioms:

import static java.util.Arrays.asList; import static
java.util.Collections.emptyList; import static java.util.Optional.of; import
static java.util.stream.Collectors.toList; import java.util.List; public
class CartesianProduct { public List<?> product(List<?>... a) { if
(a.length >= 2) { List<?> product = a[0]; for (int i = 1; i < a.length;
i++) { product = product(product, a[i]); } return product; } return
emptyList(); } private <A, B> List<?> product(List<A> a, List<B> b) { return
of(a.stream() .map(e1 -> of(b.stream().map(e2 -> asList(e1,
e2)).collect(toList())).orElse(emptyList())) .flatMap(List::stream)
.collect(toList())).orElse(emptyList()); } }

A programmer that spent a lot of time studying Monads and playing around
with them, and that understands the Monad instances for lists, might come
up with the following solution in Haskell:

cartProdN :: [[a]] -> [[a]] cartProdN = sequence

This also made me realize of two things:
0. Haskell will never be mainstream, because there are not a lot of
programmers out there who are willing to do the investment required for
learning the necessary concepts to understand and write code like the one
shown above.
1. Haskell has rendered me unemployable for almost all jobs that do not
involve Haskell codebases. Imagine having to maintain code like the first
snippet, replicated in a 10K LOC codebase.

And yes, I did use the code above in production :)

On Wed, Jul 11, 2018 at 2:10 PM Simon Peyton Jones via Haskell-Cafe <
haskell-cafe at haskell.org> wrote:

> Friends
> In a few weeks I’m giving a talk to a bunch of genomics folk at the Sanger
> Institute <https://www.sanger.ac.uk/> about Haskell.   They do lots of
> programming, but they aren’t computer scientists.
> I can tell them plenty about Haskell, but I’m ill-equipped to answer the
> main question in their minds: *why should I even care about Haskell*?
> I’m too much of a biased witness.
> So I thought I’d ask you for help.  War stories perhaps – how using
> Haskell worked (or didn’t) for you.  But rather than talk generalities, I’d
> love to illustrate with copious examples of beautiful code.
>    - Can you identify a few lines of Haskell that best characterise what
>    you think makes Haskell distinctively worth caring about?   Something that
>    gave you an “aha” moment, or that feeling of joy when you truly make sense
>    of something for the first time.
> The challenge is, of course, that this audience will know no Haskell, so
> muttering about Cartesian Closed Categories isn’t going to do it for them.
> I need examples that I can present in 5 minutes, without needing a long
> setup.
> To take a very basic example, consider Quicksort using list
> comprehensions, compared with its equivalent in C.  It’s so short, so
> obviously right, whereas doing the right thing with in-place update in C
> notoriously prone to fencepost errors etc.  But it also makes much less
> good use of memory, and is likely to run slower.  I think I can do that in
> 5 minutes.
> Another thing that I think comes over easily is the ability to abstract:
> generalising sum and product to fold by abstracting out a functional
> argument; generalising at the type level by polymorphism, including
> polymorphism over higher-kinded type constructors.   Maybe 8 minutes.
> But you will have more and better ideas, and (crucially) ideas that are
> more credibly grounded in the day to day reality of writing programs that
> get work done.
> Pointers to your favourite blog posts would be another avenue.  (I love
> the Haskell Weekly News.)
> Finally, I know that some of you use Haskell specifically for genomics
> work, and maybe some of your insights would be particularly relevant for
> the Sanger audience.
> Thank you!  Perhaps your responses on this thread (if any) may be helpful
> to more than just me.
> Simon
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