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

[Joachim Durchholz]

Am 11.07.2018 um 14:10 schrieb Simon Peyton Jones via Haskell-Cafe:
> 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;

You can expound more on that: People don't have to write loops anymore 
(with their fencepost issues etc.). It's like a "for" loop, but written 
using in-language facilities instead of having to live with what the 
language designer does.

This means you can roll your own loop constructs.

Consider iterating over the elements of a data structure such as List, 
Tree, etc. Most languages offer an Iterable interface, which tends to be 
somewhat messy to implement.
With a higher-order function, you can just write down the loop, *once*, 
and the loop body will be provided by a function parameter, and voilà! 
you have your loop construct.

HTH
Jo