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

[Johannes Waldmann]

>  - 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.