# Proposal for SharedFunctions.jl package for optional dependency management

**URL:** <https://discourse.julialang.org/t/proposal-for-sharedfunctions-jl-package-for-optional-dependency-management/23526>\
**Category:** Internals & Design\
**Created:** [April 25, 2019, 5:18pm UTC](https://discourse.julialang.org/t/proposal-for-sharedfunctions-jl-package-for-optional-dependency-management/23526 "2019-04-25T17:18:42Z")\
**Posts on this page:** 15\
**Page:** 4

<div class="post-metadata">

**Author:** ![tkf](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkf/32/17635_2.png) [@tkf](https://discourse.julialang.org/u/tkf)\
**Post date:** [April 30, 2019, 9:18am UTC](https://discourse.julialang.org/t/proposal-for-sharedfunctions-jl-package-for-optional-dependency-management/23526/61 "2019-04-30T09:18:41Z")

</div>

Honestly I found Context Dispatch an interesting solution first. But I think there are some difficulties (or maybe I’m missing some details):

1. For example, `Plots` and `PyPlot` exports the function named `plot` and I believe

```julia
julia> using Plots

julia> using PyPlot

```

shouldn’t silently merge them.

1. More in general, Context Dispatch makes it impossible to define generic interface like `f(::Any)` since you don’t own `f` anymore if you want to export `f`.

2. You need to export function to merge the methods. However, some methods are primary for overloading and does not make sense to export it (e.g., `Broadcast.broadcasted`).

3. In some situations it could be very confusing. Consider that modules `A` and `B` are imported in `Main` and they have the dependency graph

```nohighlight
A -> B .-> C
       `-> D

E -> F .-> D
       `-> G

```

where `->` means `using`; i.e., module `A` has `using B`. Suppose the modules `C`, `D`, and `G` (i.e., the “third level” module) define and export the function `f` specialized for its internal type. Also suppose that `B` and `E` do not export `f`. If I understand the definition of Context Dispatch, `B.f` and `E.f` do not share the method table. However, does module `D` “smuggles” method `f(::TypeInG)` to `B.f` even if `G` is not in the context of `B`?

---

<div class="post-metadata">

**Author:** ![tkf](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkf/32/17635_2.png) [@tkf](https://discourse.julialang.org/u/tkf)\
**Post date:** [April 30, 2019, 9:20am UTC](https://discourse.julialang.org/t/proposal-for-sharedfunctions-jl-package-for-optional-dependency-management/23526/62 "2019-04-30T09:20:28Z")

</div>

Well, I myself don’t have a large set of packages that needs this solution. So it was just an exercise for me and I’m not planning to do anything with it. But if somebody wants to use it I’ll register it.

> [@Raf](#):
>
> also using the macro for every method definition?

Yes, for all interface functions… At least that’s the plan. But, actually, there is a “bug” [Extending a constructor is possible without explicit `import` or module prefix · Issue #25744 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/25744) in Julia that makes it “work” without the macro at the moment.

---

<div class="post-metadata">

**Author:** ![TsurHerman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tsurherman/32/1234_2.png) [@TsurHerman](https://discourse.julialang.org/u/TsurHerman)\
**Post date:** [April 30, 2019, 12:14pm UTC](https://discourse.julialang.org/t/proposal-for-sharedfunctions-jl-package-for-optional-dependency-management/23526/63 "2019-04-30T12:14:10Z")

</div>

> [@tkf](#):
>
> shouldn’t silently merge them.

You are right it should issue a warning if you try to use them unqualified.  
please also note that I make a distinction between functions and methods.

A function is a name in a module with several methods.

Fusing is supposed to happen either semantically(implicitly), where two modules export the same name  
and a third module is `Using` them, or explicitly similar to the mechanism that exists today or the mechanism you suggested.

You could look up previous posts regarding Context Dispatch, maybe it will clear things up.

> [@tkf](#):
>
> In some situations, it could be very confusing. Consider that modules `A` and `B` are imported in `Main` and they have the dependency graph

I promise you it is less confusing than it looks, please post some code it will be easier for me to follow what you are asking.

And another thing… I don’t own this idea, I merely researched it, and the more I did so it became clear to me that it is a generalization of the current scheme of things, and that there is room for further understanding.

---

<div class="post-metadata">

**Author:** ![StefanKarpinski](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stefankarpinski/32/24_2.png) [@StefanKarpinski](https://discourse.julialang.org/u/StefanKarpinski)\
**Post date:** [April 30, 2019, 3:29pm UTC](https://discourse.julialang.org/t/proposal-for-sharedfunctions-jl-package-for-optional-dependency-management/23526/64 "2019-04-30T15:29:29Z")

</div>

This seems to have become another thread on context dispatch, which was clearly not what Jacob intended, but as I still don’t have a clear picture of what you mean by the term, I guess I’ll use it as an opportunity to ask more about this:

> [@TsurHerman](#):
>
> I don’t own this idea, I merely researched it, and the more I did so it became clear to me that it is a generalization of the current scheme of things, and that there is room for further understanding.

If there are other references, in PL literature or elsewhere, on the “context dispatch” concept, it might be helpful if you could share them. The closest thing I can figure out to what you’re proposing is [Ruby refinements](https://docs.ruby-lang.org/en/2.4.0/syntax/refinements_rdoc.html). Note, however, that Ruby refinements were introduced to mitigate rampant monkey patching issues in the Ruby ecosystem, which would suggest that a similar feature in Julia would be primarily to avoid type piracy, which would be more limited than what it sounds like you’re proposing (although, again, I’m still unclear on precisely what that is).

There are very basic apparent problems with “context dispatch” that were pointed out in previous threads but never addressed as far as I could tell. The most fundamental one is something like this scenario:

- Base defines `sort!` which calls `isless` by default
- Package `A` defines type `T` and `isless` methods for it
- Package `B` defines function `f` which calls `sort!` on arguments of unknown type
- Package `C` uses `A` and `B`: it constructs objects of type `A.T` and passes them to `B.f`

Since `A` and `B` know nothing about each other and `Base` knows about neither of them, and they all—from the way you’ve described it—have different versions of `isless` and `sort!` that don’t know about types defined elsewhere, how does this work? Can `C` pass objects of type `A.T` to `B.f` and have them be sorted according to the `isless` ordering?

---

<div class="post-metadata">

**Author:** ![chakravala](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chakravala/32/6832_2.png) [@chakravala](https://discourse.julialang.org/u/chakravala)\
**Post date:** [April 30, 2019, 3:50pm UTC](https://discourse.julialang.org/t/proposal-for-sharedfunctions-jl-package-for-optional-dependency-management/23526/65 "2019-04-30T15:50:29Z")

</div>

> [@StefanKarpinski](#):
>
> If there are other references

Another topic which we [discussed previously](https://discourse.julialang.org/t/possibility-of-local-import-statements-in-future/10611) is the notion of a `local import` statement

> [@Possibility of \`local import\` statements in future?](https://discourse.julialang.org/t/possibility-of-local-import-statements-in-future/10611/1):
>
> The problem is, any new module that is defined with `import` statements extend on a global scope before `using` it.
> 
> Therefore, it would have language applicability if the `import` statement could be handled locally, and then optionally extended to the main scope with `using` . I’ve been told it isn’t possible with Julia, but I am optimistic. It would require some kind of a table that keeps track of which method extensions are available in what scope. Saying that it is entirely impossible seems rather improbable to me.
> 
> Perhaps, having this could speed up the method-look-up in some cases, since there would be fewer extensions to search through in a given scope.
> 
> Before you try to shove this issue under the rug, consider that multiple packages that all define arithmetic operations on symbols and expressions cannot be loaded at once because of this issue. However, if `local import` where possibe, then different packages that depend on different extensions of the same methods can be used together without interference (because then scope determines the method extension selection).

This is what resulted in making the `@force import` macro in [ForceImport.jl](https://github.com/chakravala/ForceImport.jl) … however, the real source of the problem is as I had mentioned, the context-based dispatch problem:

> [@Possibility of \`local import\` statements in future?](https://discourse.julialang.org/t/possibility-of-local-import-statements-in-future/10611/4):
>
> Essentially, what I am noticing is a greater extent of context sensitive ambiguity in the Julia language. What prevents the Julia language from truly being able to have a 1-1 correspondence with mathematical vernacular is its ability to discern context sensitive ambiguities. Mathematicians are able to do this, but most programming languages, including Julia, cannot. One example of contex sensitive ambiguity is the fact that Julia cannot discern between the overloaded definitions of + by scope. …

One of my major other ideas was to have a context-sensitive dispatch with `local import` ability.

The `@force import` macro only simulates this feature by completely separating the method tables for the conflicting methods, and then using forwarded dispatching to handle `Base` dispatch.

What is being talked about here is the full solution to this problem.

---

<div class="post-metadata">

**Author:** ![TsurHerman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tsurherman/32/1234_2.png) [@TsurHerman](https://discourse.julialang.org/u/TsurHerman)\
**Post date:** [April 30, 2019, 9:11pm UTC](https://discourse.julialang.org/t/proposal-for-sharedfunctions-jl-package-for-optional-dependency-management/23526/66 "2019-04-30T21:11:56Z")

</div>

> [@StefanKarpinski](#):
>
> This seems to have become another thread on context dispatch

There are ~~three~~ several topics here as I see it and I’ll do my best to be brief and concise, with as little errors as possible

1. The discussion started on having the need to define some abstract base module with names, so different modules can coordinate upon(extend common interface)

2. It evolved to:

> [@cstjean](#):
>
> There should be a way to define a `LightGraphs.cartesian_product` method without importing `LightGraphs` .

1. @tkf made the observation that we can extend a function in a module before it is loaded, in a fashion that is roughly equivalent to the following:  
let  
`Function{UID,NAME} where UID where NAME`  
be the abstract base type of all functions in julia, where UID stands for the module or package UUID  
Then any module can extend a function in another module without having to specifically load it.

Here is an example of extending function plot in module Plots using module name for UID

```julia
(plot::Function{:Plots,:plot}(x::MyVecType) = plot(toBaseVec(x))

```

1. Jeff suggested that there will be a way to tell the system to merge functions in different modules(hope I understood correctly)

2. @chakravala wants to do the same but make the merge visible only locally as not to affect the integrity of other modules

3. Context Dispatch is a generalization of the way multiple-dispatch is handled today as it enables you to do the same things, using the same code, however, an implicit merging of exported function names as in 5 feels very natural in this way and can often replace explicit coordination as in 3

> [@Binary Caching and Pre-compilation .. again](https://discourse.julialang.org/t/binary-caching-and-pre-compilation-again/19589/1):
>
> **The building blocks:**
> 
> - A Context is a set of modules.
> - The Context of module M is the set of all modules that module M depends upon.
> - Extension: The method table for a Context consists only from methods belonging to modules in the Context
> - Propagation: the context is propagated recursively down the AST, and is narrowed if proven that it will resolve the call just as specific as the wider context

> [@StefanKarpinski](#):
>
> - Base defines `sort!` which calls `isless` by default
> - Package `A` defines type `T` and `isless` methods for it
> - Package `B` defined function `f` which calls `sort!` on arguments of unknown type
> - Package `C` uses `A` and `B` : it constructs objects of type `A.T` and passes them to `B.f`

so for your toy example, calling function B.f from the context of module C uses modules Base, A and B for its method table so there is no problem.

Now let’s say module D who uses C, and calls ` C.g()` which in turn calls B.f with the constructed types.  
Since it can be proved that the context of C will suffice, the context can be narrowed.

And as long as you don’t change the state of module C or its dependent modules you can re-use the  
binary resulting from compiling the code for ` call C.g() from the context of C`

Another aspect is that the automatic merging of exported names suddenly looks very appealing.

Your example with Context Dispatch and automatic merging of exported names would look like:

```julia
module Base
    sort!(x,y) = isless(x,y)
    isless(x,y) = ErrorException("unimplemented")
    export isless,sort!
end

module A
    struct T end
    isless(x::T,y::T) = true
    export isless
end

module B
    using Base
    f(x,y) = sort!(x,y)
end

module C
    using Base, A, B
    
    g() = B.f(A.T(),A.T())
end

```

In the context of module C, the functions Base.isless and A.isless got merged.

> [@StefanKarpinski](#):
>
> If there are other references, in PL literature or elsewhere?

I don’t think so, like many others here, I think, I learned about compilers and LLVM and multiple-dispatch through the feat of engineering we call “julia”.

It came up initially to solve what @chakravala was writing about, it was a little bit obscure in the beginning but it starts to feel more and more like the right way to go.

O.k it took me more than 3 hours to write this, and I am losing focus, what I meant by the saying  
“I don’t own the idea” is that it feels like a larger subject than just a small eureka moment.

---

<div class="post-metadata">

**Author:** ![tkf](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkf/32/17635_2.png) [@tkf](https://discourse.julialang.org/u/tkf)\
**Post date:** [May 1, 2019, 3:37am UTC](https://discourse.julialang.org/t/proposal-for-sharedfunctions-jl-package-for-optional-dependency-management/23526/67 "2019-05-01T03:37:46Z")

</div>

Thanks a lot for summarising the whole thread! I wish every long thread has something this!

> [@TsurHerman](#):
>
> please post some code it will be easier for me to follow what you are asking

Here is the code for what I was asking. I realized I don’t need modules `A` and `E` so it’s not here anymore:

```julia
module C
    struct TC end
    f(::TC) = TC
    export f
end

module D
    struct TD end
    f(::TD) = TD
    export f
end

module G
    struct TG end
    f(::TG) = TG
    export f
end

module B
    using ..C, ..D
end

module F
    using ..D, ..G
end

using .B, .F

```

Note that I’m not evaluating `using .C` etc. in the top-level namespace and `B` and `F` do not have `export`. So, `f` is not defined in the top-level namespace. I suppose this means `f` in `B` and `f` in `F` have different Contexts.

In the post above, I was asking what would happen if I do `B.f(F.G.TG())`. But I realized that more important question is what happens if I evaluate the following expressions:

```julia
F.f(F.G.TG()) # TG?
D.f(F.G.TG()) # throws MethodError?
B.D.f(F.G.TG()) # throws MethodError?
F.D.f(F.G.TG()) # throws MethodError?

```

If my guess were right, it would mean

```julia
D.f === B.D.f # true?
D.f === F.D.f # true?
F.f === B.f # false?
F.f === D.f # false?

```

So, `export` and `using` for context dispatch does not just construct a local method table but also create a new function object? Otherwise, it’d be impossible to understand

```julia
F.G.TG() |> F.f
F.G.TG() |> D.f
F.G.TG() |> B.D.f
F.G.TG() |> F.D.f

```

(i.e., `x |> f` does what `f(x)` does).

---

<div class="post-metadata">

**Author:** ![TsurHerman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tsurherman/32/1234_2.png) [@TsurHerman](https://discourse.julialang.org/u/TsurHerman)\
**Post date:** [May 1, 2019, 8:43am UTC](https://discourse.julialang.org/t/proposal-for-sharedfunctions-jl-package-for-optional-dependency-management/23526/68 "2019-05-01T08:43:43Z")

</div>

replying @tkf

> [@tkf](#):
>
> Here is the code for what I was asking.

First I want to make a distinction between context-dispatch(considering only modules in context for the purpose of dispatch) and automatic merging of exported names,  
although they fit great together they are two different things.

Your question is about the order of things when both are enabled, so I will introduce a notation similar to  
@eval

```julia
@context M f(args...)

```

which means execute f(args…) in the context of module M

and for simplicity of writing, let’s assume that all names are exported.

```julia
@context F f(TG())
# since f is comprised of method-tables from modules G and D
# imported by module F and it can be proven
# in compile time that 
# using the context of G
# will resolve the call just as if the greater context was used.
# The context is narrowed and it becomes
@context G f(TG()) #=> TG
# note that context narrowing is just an optimization,
# but without it context dispatch does not offer a solution to binary caching 

```

```julia
#D.f(F.G.TG())
@context D f(TG()) # yes, Method error

#B.D.f(F.G.TG()) # throws MethodError?
@context B @context D f(TG()) # yes,Method error

```

> [@tkf](#):
>
> If my guess were right, it would mean
> 
> ```julia
> D.f === B.D.f # true?
> D.f === F.D.f # true?
> F.f === B.f # false?
> F.f === D.f # false?
> 
> ```

you are right

> [@tkf](#):
>
> So, `export` and `using` for context dispatch does not just construct a local method table but also create a new function object? Otherwise, it’d be impossible to understand

you are right again, in my first POC I used the following type to represent  
the method table

> struct Func{Context,Mod,Name} end

and some sets to keep track of instantiated types because

```julia
struct Func{Context,Mod,Name} end

a = Func{:B,:A,:f}()

typeof(a) <: Func #true

subtypes(Func) # empty vector :-( why?

```

Edit:  
another point for thought  
The most general way to define a type for a function might be

```julia
struct Func{Context,Mod,Name,Args} end

```

---

<div class="post-metadata">

**Author:** ![Zach\_Christensen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/zach_christensen/32/7220_2.png) [@Zach\_Christensen](https://discourse.julialang.org/u/Zach_Christensen)\
**Post date:** [May 1, 2019, 11:28am UTC](https://discourse.julialang.org/t/proposal-for-sharedfunctions-jl-package-for-optional-dependency-management/23526/69 "2019-05-01T11:28:06Z")

</div>

I’m a big fan of simplicity and I think the easiest way to do this is something like StatsSharedFunctions. It has the same basic premise as the OP where the functions wouldn’t have anything other than `function food end` but would allow a community standard to be defined for the intended use. If we aim for a shared definition in all of Julia I just see a lot of nit picky arguments arising across fields.

This is what R is starting to do with some things like `fit`. A package has some functions that act as a S3 generics (kind of like multiple dispatch if don’t know R) and only have documentation.

---

<div class="post-metadata">

**Author:** ![tkf](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkf/32/17635_2.png) [@tkf](https://discourse.julialang.org/u/tkf)\
**Post date:** [May 1, 2019, 10:59pm UTC](https://discourse.julialang.org/t/proposal-for-sharedfunctions-jl-package-for-optional-dependency-management/23526/70 "2019-05-01T22:59:09Z")

</div>

> [@TsurHerman](#):
>
> I will introduce a notation similar to  
> @eval
> 
> ```julia
> @context M f(args...)
> 
> ```
> 
> which means execute f(args…) in the context of module M

I still think it’s more reasonable to understand Context Dispatch as “import-site function object auto-creation” rather than thinking it as some call-site modulation. For example, how do you interpret `(B.f ∘ F.f)(F.G.TG())`? The object returned from `∘(B.f, F.f)` needs to know that the context of the first (second) argument is in `B` (`F`).

> [@TsurHerman](#):
>
> ```julia
> # note that context narrowing is just an optimization,
> # but without it context dispatch does not offer a solution to binary caching
> 
> ```

I wonder if it is enough. Consider:

```julia
module X
    module Y
        abstract type TY end
    end

    using .Y: TY
    f(::TY) = TY
end

module Z
    using ..X.Y: TY
    struct TZ <: TY end
end

z = Z.TZ()
@context X X.f(z)

```

Can caching be useful for the “methods” like `@context X X.f(::Z.TZ)` which is unknown at the time the module `X` is precompiled? Here, I’m pretending `X` and `Z` are in different packages.

---

<div class="post-metadata">

**Author:** ![TsurHerman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tsurherman/32/1234_2.png) [@TsurHerman](https://discourse.julialang.org/u/TsurHerman)\
**Post date:** [May 2, 2019, 6:22am UTC](https://discourse.julialang.org/t/proposal-for-sharedfunctions-jl-package-for-optional-dependency-management/23526/71 "2019-05-02T06:22:33Z")

</div>

> [@tkf](#):
>
> @context X X.f(z)

From the context of X the type of z (its concrete type) is unknown so it is a MethodError

But for a context that has both X and Z, once this method is compiled it can be cached for later use as long as the context did not change (no new names or types).

I still don’t have a clear solution on where to store compiled code, I was thinking of introducing “virtual modules” something like:

```julia
module Virtual_X_Z
using X 
using Z
end

```

and storing compiled code here. so any module that uses both X and Z automatically loads the so/dylib/dll associated with the Virtual\_X\_Z module

@tkf let’s continue in another thread

---

<div class="post-metadata">

**Author:** ![tkf](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkf/32/17635_2.png) [@tkf](https://discourse.julialang.org/u/tkf)\
**Post date:** [May 2, 2019, 9:10pm UTC](https://discourse.julialang.org/t/proposal-for-sharedfunctions-jl-package-for-optional-dependency-management/23526/72 "2019-05-02T21:10:27Z")

</div>

> [@TsurHerman](#):
>
> @tkf let’s continue in another thread

Sounds good 👍

---

<div class="post-metadata">

**Author:** ![quinnj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/quinnj/32/11_2.png) [@quinnj](https://discourse.julialang.org/u/quinnj)\
**Post date:** [May 4, 2019, 4:16am UTC](https://discourse.julialang.org/t/proposal-for-sharedfunctions-jl-package-for-optional-dependency-management/23526/73 "2019-05-04T04:16:09Z")

</div>

Thanks to everyone for the lively discussions and thoughtful responses. In line with general recommendations and consensus with this thread, I’ve created the [DataAPI.jl](https://github.com/JuliaData/DataAPI.jl) package as a domain-specific shared namespace for data-related packages to help decouple some key packages. Thanks again for all the responses.

---

<div class="post-metadata">

**Author:** ![tkf](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkf/32/17635_2.png) [@tkf](https://discourse.julialang.org/u/tkf)\
**Post date:** [May 7, 2019, 12:28am UTC](https://discourse.julialang.org/t/proposal-for-sharedfunctions-jl-package-for-optional-dependency-management/23526/74 "2019-05-07T00:28:52Z")

</div>

@quinnj Did you see [my suggestion](https://discourse.julialang.org/t/proposal-for-sharedfunctions-jl-package-for-optional-dependency-management/23526/38) which is implemented here: [https://github.com/tkf/IndirectImports.jl](https://github.com/tkf/IndirectImports.jl) ? I thought you might have missed it because there are many messages in this thread (which is kind of my fault). Compared to DataAPI.jl’s approach, I think it is better because the owner (upstream) packages don’t have to update DataAPI.jl and hence can be more flexible in defining functions that can be extended and “used” without importing the owner packages (i.e., the functions that would be registered in DataAPI.jl).

---

<div class="post-metadata">

**Author:** ![jpsamaroo](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jpsamaroo/32/46804_2.png) [@jpsamaroo](https://discourse.julialang.org/u/jpsamaroo)\
**Post date:** [May 7, 2019, 12:00pm UTC](https://discourse.julialang.org/t/proposal-for-sharedfunctions-jl-package-for-optional-dependency-management/23526/75 "2019-05-07T12:00:38Z")

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Maybe the best option is a mixture between the two approaches; DataAPI.jl will be used for anything where common agreement on a (reasonably formal) API between multiple packages is desired (in this case, the Julia data ecosystem), and then IndirectImports can be used for more informal or ad-hoc interfaces are needed (maybe on the GPU compute side of the house). Especially when you only want to share one or a few functions across packages, IndirectImports is probably a good initial approach, with the option to “upgrade” to a shared XBase or XAPI package once there’s consenseus on what the shared API looks like.

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