- 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.