On Sat, Oct 18, 2014 at 1:02 PM, htebalaka
On 10/17/14 12:32, Alexander Berntsen 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
On 17/10/14 00:40, Austin Seipp wrote: 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/