[haskell-cafe] Monad and kinds

[Jake Mcarthur]

On Sep 4, 2008, at 9:52 PM, Tim Chevalier wrote:

> I'm no master, but I've never encountered a situation where strictness

> annotations would be useful as documentation, nor can I imagine one.


I'm no master either, but how about these simple examples?

         data Stream a = Cons !a (Stream a)
         data Vector3 a = Vector3 !a !a !a

The compiler will certainly be able to infer the strictness itself in
most uses, so obviously the purpose for these annotations is not for
optimization, but I still would find these annotations useful. This is
much like explicitly giving the type for a function. It guides the
reader of the program, and just happens to also assist the compiler a
little bit.

> That's because optimization *is* the only reason why programmers
> should care about strictness information. IMO, arguing that
> programmers should care at all amounts to conceding that default
> laziness is treacherous.

If optimization is the only reason to worry about strictness, then
default laziness really _is_ treacherous. Luckily this is not the
case. Laziness is not useful without strictness, otherwise there would
never be any evaluation. Understanding how to apply laziness and
strictness in different situations is critical to writing efficient
but meaningful code in Haskell.

To quote a blog article[1] I wrote in June,

        "It is simple and idiomatic to use strictness where your
         program calls for it. In other words, contrary to many
         complaints, strictness is not a low-level optimization
         technique. Strictness is a strategy for algorithms. As is
         always the case, if you change an algorithm, its time and
         space requirements will change, but it’s a high-level
         change. A strictness annotation or data structure redesign is
         not like dropping to C or assembly."

The article explains laziness and strictness as I understand it. I
could be wrong, but my understanding seems to have served me well so
far.

- Jake McArthur