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

[Doaitse Swierstra]

> Op 12 jul. 2018, om 15:01  heeft Brett Gilio <brettg at posteo.net> het volgende geschreven:
> 
> Alexey, could you expand on what you mean in your first point? I am quite intrigued. I do not use Haskell often, but that could be something of interest to me in-and-out of Haskell.

In the old days, when I wrote Pascal programs, and I wanted to swap values I wrote code like:

function swap (var x, y: integer); begin x := x+y; y := x - y; x := x - y end;

Only to find out that if I wanted a swap for a different type this does not work. Hence a lot of coding, and hardly re-use and very error prone. How many functions can’t you write that have this type.

So the question is how many functions can you write with the type (a, a) -> (a, a). If you randomly generate functions of this type there is a chance of 25% you get the right one. 

But things become even better:

swap (a, b) = (b, a)

Once you ask for the type you get (a, b) -> (b, a), hence the type completely specifies what swap computes, and the function is even more general than the version of the type above.

 Doaitse



> 
> Brett Gilio
> brettg at posteo.net | bmg at member.fsf.org
> Free Software -- Free Society!
> 
> On 07/12/2018 07:46 AM, Alexey Raga wrote:
>> Not sure if it counts as "aha moments", but when I started with Haskell I had two major reasons (not in any importance order):
>> 1. The ability to define the specification (types) and then "just" follow them in implementation. Sometimes even without having a clear understanding of the things I was using, I felt (and still feel) guided towards the right solution.
>> 2. The ability to refactor fearlessly is a _massive_ productivity boost. Hard to underestimate.
>> Regards,
>> Alexey.
>> On Wed, Jul 11, 2018 at 10:10 PM Simon Peyton Jones via Haskell-Cafe <haskell-cafe at haskell.org <mailto: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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