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

[Vaibhav Sagar]

I'm late to this discussion, but one aspect of Haskell that really 
impressed me was the idea of *typed holes*, which I think revolutionise 
the act of programming. I tried to communicate my love for them to 
people who mostly don't know Haskell here: 
https://www.youtube.com/watch?v=0oo8wIi2qBE


On 17/07/18 03:28, Johannes Waldmann wrote:
>>   - Large-scale refactoring with types:
>> this is a huge selling point for Haskell in general
> I fully agree with the general idea, but the specific example
>
>> data Shape
>>    = Circle Double
>>    | Square Double
>>    | Rectangle Double Double   -- ^ added
> is not convincing (well, to programmers
> that know about static typing) Think of a Java program
>
> interface Shape { .. }
> class Circle implement Shape { ... }
>
> When you add   class Rectangle {  } ,
> then  Shape foo = new Rectangle ()  would be an error,
>
> until you put   class Rectangle implements Shape {  }
> then the compiler tells you what methods are missing.
>
>
> I think the extra value of types in Haskell
> (for everything, including refactoring)
> is that they tend to express more of the program's properties
> (w.r.t. statically typed imperative programs).
>
> Examples:
>
> the distinction between  a  and  IO a,
> between  IO a  and   STM a.
>
> In  Data.Set, function  "fromList"  has an  Ord  constraint,
> but  "toList"  does not. Does  "singleton"  need the constraint?
> I used this as an exam question just last week.
>
>
> - J.W.
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