Re: Drastic Prelude changes imminent

On Tue, Feb 3, 2015 at 5:49 PM, Michael Snoyman wrote:

On Tue Feb 03 2015 at 6:26:58 PM Erik Hesselink wrote:

...

On Tue, Feb 3, 2015 at 4:45 PM, Michael Snoyman wrote:

...

Now that I actually write this all out, I'm beginning to think I *am* opposed to the proposal overall, though not by a wide margin. It seems to fall into the "too little, too late" category: the benefits it brings are not massive due to my three points above, and the disruption introduced to the ecosystem is potential massive.

What is the potential massive disruption?

1. Breakage of existing code due to lack of type inference 2. Breakage of existing tutorials/documentation 3. Confusing error messages

I didn't really want to drill into it because that seems to be the point of discussion of this entire thread.

I was just wondering which of these you were alluding to. Turns out it's all of them :) I'm surprised you mention breakage of code, since in my experience most of the breakage in GHC 7.10 I've seen in my own packages and the Stackage builds is due to other factors: the Applicative/Monad changes, time 1.5, etc. It would be interesting (but time consuming) to gather some statistics on this. The breakage of tutorials is unfortunate, yes. Regarding error messages: we've been using two alternate preludes. One is monomorphic, just the normal Prelude extended with things like Data.List, Control.Monad, etc, and with partial functions replaced by total ones. The other is a polymorphic one, similar to the current FTP with the addition of a generalized (.) and 'id' as well. I've not found the type errors to be confusing. Now I'm not a beginner of course, but the fact is that even beginners quickly have to learn how to treat error messages with type classes in them (which is commonly said to be the confusing part). Many common functions already involve type classes and cause type errors. Things like Show and Monad regularly show up in error messages, so I don't see what makes Foldable/Traversable any different. I'm on the fence about this whole issue like you, but unlike you I'm learning more and more towards the positive side. If something is to be done about the Prelude (which I think is good) this seems like a very well considered step with minimal breakage. Some problems like the weirdness of Data.List can be fixed with deprecations later. Erik