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

[PY]

Hello, Francesco! For me - phantom types. And may be types families.

Also good is "map" function which replace visitor pattern is short way, 
but it exists in most modern languages


11.07.2018 15:31, Francesco Ariis wrote:
> On Wed, Jul 11, 2018 at 12:10:21PM +0000, Simon Peyton Jones via Haskell-Cafe wrote:
>> 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.
> If your most of your audience uses a dynamically typed language,
> I would introduce type inference and how small, painful bugs can
> be tracked down by the compiler without having to write a single
> type signature bar top level.
>
> Another good one is implementing a tricky function with holes
> (in what I have seen described as `hole-driven` development.
>
> Unfortunately, one of Haskell strongest suit (ease of refactoring)
> doesn't really shine in bite-sized examples!
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