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

[Alexey Raga]

Some code examples that I usually show/explain to someone who is interested
in Haskell but have no previous exposure:

ADTs. Even for something as simple as

data ShoppingCartCommand
  = CreateCart UserId
  | AddToCart CartId ProductId Quantity
  | ClearCart CartId
  deriving (Eq, Show)

This would be many-many lines of non-trivial C# or Java code (you would
need to think about abstract classes or interfaces, correctly override
toString, equality and getHashCode, write tests for all this, etc).

If the audience is familiar with C#, then explaining the ability to
abstract over type constructors may work well. In C# there is no way to
generalise over, say, IEnumerable<T> and IObservable<T>. If you want to
accept both then you'd have to write the same LINQ statements twice (or
convert one into another, which is not always possible).

Regards,
Alexey.


On Wed, Jul 11, 2018 at 10:10 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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