import/using docs: examples & explanation

[StefanKarpinski]

Add some form of https://discourse.julialang.org/t/julep-taking-multiple-dispatch-export-import-binary-compilation-seriously/10882/100 and https://discourse.julialang.org/t/julep-taking-multiple-dispatch-export-import-binary-compilation-seriously/10882/109 to the documentation of import and using.

[mauro3]

For reference, the linked example above is:

julia> module A
           export a, b, c
           a = b = c = 1
       end
Main.A

julia> module B
           export c, d, e
           c = d = e = 2
       end
Main.B

julia> module C
           using ..A
           using ..B
           import ..B: c
           @show a, b, c, d, e
       end
(a, b, c, d, e) = (1, 1, 2, 2, 2)
Main.C

and the text:

A little further explanation. If a name is already ambiguous and you try to use it, then it is simply undefined. If you use a name when it’s unambiguous, that resolves its meaning. We can’t predict that you’re going to later do something that would cause it to become ambiguous, so the best we can do at that point is warn you.

The current design has the property that if you have code that works without warnings or errors and if you reorder the imports and it still works without warnings or errors then it does the exact same thing. That seems like a fairly important property to have. Combined with the requirement that Julia code be evaluated one top-level expression at a time, I don’t think there’s any other design we could have with this property. In older Julia versions the order of usings was significant which is not great because people tend to reorder them.

[mauro3]

I cannot quite reconcile what Stefan said with below examples, in Julia 0.6.2:

   _       _ _(_)_     |  A fresh approach to technical computing                                                                                                         
  (_)     | (_) (_)    |  Documentation: https://docs.julialang.org                                                                                                       
   _ _   _| |_  __ _   |  Type "?help" for help.                                                                                               
  | | | | | | |/ _` |  |                                                                                                                                                  
  | | |_| | | | (_| |  |  Version 0.6.2 (2017-12-13 18:08 UTC)                                                                                                            
 _/ |\__'_|_|_|\__'_|  |                                                                                                                                                  
|__/                   |  x86_64-pc-linux-gnu                                                                                                                             
                                                                                                                                                                          
julia> using Distributions                                                                                                                                                
                                                                                                                                                                          
julia> logpdf(Uniform(1,2), 1)                                    
-0.0                                                                                                                                                                      
                                                                                                                                                                          
julia> loglikelihood(x) = 1                                                                                                                                               
ERROR: error in method definition: function StatsBase.loglikelihood must be explicitly imported to be extended

but this works:

julia> using Distributions                                                                                                                                               
                                                                                                                                                                         
julia> loglikelihood(x) = 1                                                                                                                                              
loglikelihood (generic function with 1 method)                                                                                                                           

julia> logpdf(Uniform(1,2), 1)                                                                                                                                           
-0.0                                                                                                                                                                     

Why is loglikelihood being "reserved" in the first case (without me ever using it) and not in the second?

[GunnarFarneback]

I can't reproduce.

   _       _ _(_)_     |  A fresh approach to technical computing
  (_)     | (_) (_)    |  Documentation: https://docs.julialang.org
   _ _   _| |_  __ _   |  Type "?help" for help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 0.6.2 (2017-12-13 18:08 UTC)
 _/ |\__'_|_|_|\__'_|  |  
|__/                   |  x86_64-linux-gnu

julia> using Distributions

julia> logpdf(Uniform(1,2), 1)
-0.0

julia> loglikelihood(x) = 1
loglikelihood (generic function with 1 method)

[mauro3]

I can't reproduce it either anymore, although scrolling up in my terminal shows the edited (now including banner), above output?! Sorry about the noise...