Hiding import behaviour

Sat Oct 18 18:42:17 UTC 2014

And also, the ultimate confusing case: if two modules exported
identifiers with the same name *and* type. At least under the current
scheme, you'd be required to clearly disambiguate them in all cases.
Under the proposed scheme, there's no telling what behavior your
program might have, based solely on the shadowing rules/ordering of
the imports of your module and nothing else.

I don't think this would be a common occurrence. But it seems deeply
upsetting that in such a case, rather than the compiler complaining
loudly about ambiguity in a very obvious case (a compiler which
catches many *other* very obvious static code errors), it would
instead silently accept accept your program under a very implicit
"DWIM-ish" import rule.

On Sat, Oct 18, 2014 at 1:33 PM, Austin Seipp <austin at well-typed.com> wrote:
> On Sat, Oct 18, 2014 at 1:02 PM, htebalaka <goodingm at gmail.com> wrote:
>> On 10/17/14 12:32, Alexander Berntsen wrote:
>>> On 17/10/14 00:40, Austin Seipp wrote:
>>> > Maybe there are some cases today where something like this could
>>> > happen, but this seems awfully, awfully implicit and hard-to-follow
>>> > as a language feature.
>>> >
>>> > In general I think a program that has imports like this that may
>>> > clash can be automated to make it easier to manage - but ultimately
>>> > such imports tend to represent a complex relationship between a
>>> > module and its dependencies - I'd prefer it if these were as clear
>>> > as possible.
>>> Very strong +1 from me. It seems awfully implicit and obscure for very
>>> little benefit, and it may mean quite a bit of work for tool developers.
>>
>> I guess my central point is I don't see how anyone can benefit from the
>> current behaviour. For instance, a simple real world example:
>>
>> import Prelude
>> import Data.Text.Lazy.IO (putStrLn)
>>
>> Regardless of the ordering of the imports, is there any way for me to use
>> putStrLn in any context without hiding it from the Prelude (and any other
>> modules that I might be unintentionally importing it from)?
>
> I suppose my point isn't that the current behavior is more useful, but
> the *proposed behavior seems more confusing for humans*. I would
> rather have GHC inform me of an ambiguous import as opposed to
> silently accepting or rejecting my program based on the import list,
> and whether it shadows something prior to it. I don't even always know
> what identifiers may get imported in the first place, due to
> transitive module reexports. It just seems like pretty confusing
> behavior - shadowing of identifiers is rarely a 'feature' for humans,
> IMO.
>
> In the example you have, what happens if I change the import list of
> Data.Text by removing it, for example, or what happens if I *remove*
> the Prelude import, and stick it after the Text import? Rather than
> getting an out of scope identifier error, or something ambiguous, I'd
> get a confusing type error based on Prelude's use of putStrLn in the
> context of needing Texts', because the shadowing would fail to apply
> since it didn't occur before the Text import. Shadowing of previously
> imported identifiers only works one-way, so to speak, where with
> 'hiding', order no longer matters in the import list.
>
> Of course you might say, "Well, of course Prelude exports putStrLn, so
> you wouldn't move the import, and it wouldn't be a problem". The
> problem is I don't know what exports an arbitrary module has; it
> doesn't seem to scale mentally for humans at all. In this case, I know
> Prelude exports that, but in the general case of:
>
> import Frob
> import Knob (xyz)
>
> Today, this means I only import 'xyz' from Knob, and there are no
> other ambiguous names. But under your proposal, I have zero clue if
> 'xyz' is actually shadowing a prior import. So unless I check *all*
> the transitive exports of 'Frob', I have no clue if it's actually safe
> to move the import of 'Knob' higher up - an identifier may not be
> shadowed if I do that. OTOH, I know *for a fact* when I see this:
>
> import Frob hiding (xyz)
> import Knob (xyz)
>
> which 'xyz' I'm referring to later, without ambiguity. Also, what
> happens if I do this:
>
> import Knob (xyz)
> import Frob
>
> legitimately, without shadowing, and 'Frob' later ends up exporting
> its own 'xyz'? Do I just get an ambiguous identifier error, like I
> would today? Again, shadowing in this sense only works 'one-way': top
> to bottom, and it fails any other case they might be rearranged.
>
> This all just seems like a relatively large amount of hoops to jump
> through, just to avoid writing 'hiding' in a on a few things, so to
> me, the cure looks worse than the disease. But I may just be missing
> something completely.
>
>> Any unqualified use will be ambiguous, unless you hide it from every other module that might
>> export a function with the same name. I would think the fact that it
>> shouldn't be implicitly imported from other modules would directly follow
>> from the fact you imported it explicitly (otherwise, why did you import
>> it?). I'm having trouble coming up with a single example where the current
>> behaviour is useful.
>>
>> I can't speak to tooling, though I suppose if this doesn't get implemented
>> I'll write my own. Just to be very clear, supposing you have some Import
>> datatype which stores a list of any identifiers that are being explicitly
>> imported unqualified (or conversely, a list of any identifiers that are
>> being hidden), then the behaviour I'm suggesting is a pragma to enable
>> something like this:
>>
>> hide :: [Import] -> [Import]
>> hide = flip (fmap fmap appendHiddenImports) <*> collectOnly where
>>     collectOnly :: [Import] -> [Identifier]
>>     collectOnly = concat . mapMaybe getExplicitImports
>>     appendHiddenImports :: [Identifier] -> Import -> Import
>>     getExplicitImports :: Import -> Maybe [Identifier]
>>
>> where appendHiddenImports would only change import statements that import an
>> unspecified number of unqualified identifiers, like "import X hiding (x, y)"
>> or "import Y".
>>
>>
>>
>> --
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>
> --
> Regards,
>
> Austin Seipp, Haskell Consultant
> Well-Typed LLP, http://www.well-typed.com/

-- 
Regards,

Austin Seipp, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com/