Re: Proposal: Add IsString instance for (Maybe a) to base

On Fri, Jul 12, 2013 at 01:29:43PM +0300, Roman Cheplyaka wrote:

* John Lato [2013-07-12 17:03:03+0800]

...

In all seriousness, I'm not that fond of the proposal myself. I just wanted to argue the other side because nobody else was. But so far, I've seen two main arguments against the proposal:

1. IsString is for things that are notionally some sort of string. 2. It makes the types more confusing.

The second seems like a weak argument to me. First off, the proposal seems very similar to the recent generalization of Prelude.map, which had a great deal of support.

The difference is that fromString is called implicitly.

A similar precedent is numeric literals. When I type 1, a method is called, but most of the time I don't need to think about it because those methods agree with my intuitive semantics of numbers.

It's easy to define a more or less proper instance

instance (Applicative t, Num n) => Num (t n)

which would allow to overload numbers even harder, but then I'd need to stop and think every time I see a number.

Roman

I agree. When I see a string literal in the source code of a program, I want to be able to think of it as, well, a string. Not necessarily a String with a capital S, but a string of some sort nonetheless. A Maybe String is no such thing! The idea is only slightly less ridiculous than having a literal denote its singleton list, or singleton set, or tree with one leaf. (so -1 from me)