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

Vanessa McHale vanessa.mchale at iohk.io
Fri Jul 13 02:01:25 UTC 2018

Oh, and this may not be appropriate for your particular audience, but I
definitely like the fact that you can write some algorithms extremely
elegantly with recursion schemes, viz.


import Data.Functor.Foldable import Data.Ratio (Ratio, denominator, (%))

isInteger :: (RealFrac a) => a -> Bool isInteger = idem (realToFrac .
floor) where idem = ((==) <*>)

continuedFraction :: (RealFrac a, Integral b) => a -> [b]
continuedFraction = apo coalgebra where coalgebra x | isInteger x = go $
Left [] | otherwise = go $ Right alpha where alpha = 1 / (x - realToFrac
(floor x)) go = Cons (floor x)


I wrote up a whole bunch of examples here:

I think such examples are really great because they are things which are
not possible in Rust (or F#, if I am not mistaken).

On 07/11/2018 07:10 AM, Simon Peyton Jones via Haskell-Cafe 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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