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

[Simon Peyton Jones]

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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