Re: [ghc-steering-committee] #283: Local modules (again), recommendation: accept

Dear all, Let me raise this proposal again. Very few of us have opined, and while I'd usually be happy to consider silence as assent, this is a rather large proposal which may require a few more pairs of eyes. Please consider giving this one a read and share your thoughts. If you can't do so right now, please let me know when you will be able to, so that we can plan accordingly. This is an important proposal, I'm keen on seeing its design finalised. /Arnaud On Wed, May 26, 2021 at 2:35 PM Richard Eisenberg wrote:

On May 26, 2021, at 3:28 AM, Spiwack, Arnaud wrote:

I'm realising that I inverted additional options 1 and 3 in my reply. To spell things out: I'm in favour of the namespace introduced for every datatype and such; and weakly in favour of anonymous modules, for which I prefer the `_` syntax than simply omitting the name.

Oh, good. I was very confused here, but I decided not to push on it. I'm similarly weakly in favor of (1), but I can't get myself to decide firmly on whether to go with alternative (7). Going with (7) is a little more consistent with other features, but it adds more symbols to the source text that could otherwise be omitted. So I'm pretty ambivalent.

Richard

On Tue, May 25, 2021 at 11:54 PM Richard Eisenberg wrote:

...

On May 25, 2021, at 3:09 PM, Alejandro Serrano Mena wrote:

- I am not sure of the benefit of allowing (1), compared with the possible surprise of users. - I do not fully understand (2). - I think (3) would be great, if we ensure that nothing changes if I don’t use “qualified”, even if -XLocalModules is on.

If in the language, I would use (1) -- anonymous local modules -- regularly, when defining a function or class instance with a bunch of "local" helper functions. Of course, if we can't omit the module name, I will suffer no great harm.

I cannot offer the guarantee you seek in (3), but I don't think you want it. (If nothing changes, then the feature has no effect!) Here is a scenario where (3) could cause trouble:

import Data.Set as Set ( abcde )

data Set = Mk { abcdf :: Int }

blah = Set.abcdf

Previously, GHC would have suggested that you perhaps misspelled abcde. Now, you'll get (presumably) a type error.

Here's another case:

import Data.Set as Set ( Set )

data Set = Mk

x :: Set.Set

Everything is happy today, but with -XLocalModules (and (3)), the type of x is an ambiguous name.

Any example that causes trouble, though, will have something in common: an imported module name (possibly via an alias) that matches a locally defined type name. I would imagine this pattern is rare in practice, and that the benefit of (3) would outweigh the number of times that a problem like this bites.

I, too, could live without (2).

Richard