# Need help with resolving method ambiguities
**URL:** https://discourse.julialang.org/t/need-help-with-resolving-method-ambiguities/47779
**Category:** General Usage
**Created:** [October 5, 2020, 11:25am UTC](https://discourse.julialang.org/t/need-help-with-resolving-method-ambiguities/47779 "2020-10-05T11:25:20Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@tisztamo](https://discourse.julialang.org/u/tisztamo)
#### Post date: [October 5, 2020, 11:25am UTC](https://discourse.julialang.org/t/need-help-with-resolving-method-ambiguities/47779/1 "2020-10-05T11:25:20Z")
Just started to experiment with implementing a proxy type, but I failed very early:
```julia
module Proxies
export AbstractProxy, Proxy
abstract type AbstractProxy end
struct Proxy{TInner} <: AbstractProxy
inner::TInner
Proxy{TInner}(inner) where TInner = new{TInner}(inner)
Proxy(inner) = begin
p = Proxy{typeof(inner)}(inner)
return p
end
end
inner(p) = p.inner
Base.convert(targettype::Type{<:T}, p::Proxy{TInner}) where {T, TInner} = begin
return convert(targettype, inner(p))
end
end # module
```
```julia
using Proxies
using Test
@testset "Proxy creation" begin
p = Proxy(42)
@test p isa Proxy
@test Proxies.inner(p) == 42
end
@testset "transparent proxing" begin
@test convert(Int, Proxy(42)) isa Int
@test convert(Int, Proxy(42.0)) isa Int
@test Proxy(42) == 42
@test_broken Proxy(42) + 42 == 84
end
```
```julia
Test Summary: | Pass Total
Proxy creation | 2 2
Test Summary: | Pass Total
transparent proxing | 2 2
ERROR: LoadError: MethodError: convert(::Type{Any}, ::Proxy{Int64}) is ambiguous. Candidates:
convert(::Type{Any}, x) in Base at essentials.jl:170
convert(targettype::Type{var"#s13"} where var"#s13"<:T, p::Proxy{TInner}) where {T, TInner} in Proxies at /home/krisztian/projects/Proxies.jl/src/Proxies.jl:19
Possible fix, define
convert(::Type{Any}, ::Proxy{TInner}) where TInner
Stacktrace:
[1] Base.RefValue{Any}(::Proxy{Int64}) at ./refvalue.jl:8
[2] Ref{Any}(::Proxy{Int64}) at ./refpointer.jl:96
[3] inferencebarrier(::Any) at ./essentials.jl:718
[4] show_default at ./show.jl:389 [inlined]
[5] show(::IOContext{Base.GenericIOBuffer{Array{UInt8,1}}}, ::Any) at ./show.jl:384
[6] show_delim_array(::IOContext{Base.GenericIOBuffer{Array{UInt8,1}}}, ::Tuple{DataType,Proxy{Int64}}, ::Char, ::Char, ::Char, ::Bool, ::Int64, ::Int64) at ./show.jl:776
[7] show_delim_array at ./show.jl:761 [inlined]
[8] show(::IOContext{Base.GenericIOBuffer{Array{UInt8,1}}}, ::Tuple{DataType,Proxy{Int64}}) at ./show.jl:794
[9] _show_default(::IOContext{Base.GenericIOBuffer{Array{UInt8,1}}}, ::Any) at ./show.jl:406
[10] show_default at ./show.jl:389 [inlined]
[11] show(::IOContext{Base.GenericIOBuffer{Array{UInt8,1}}}, ::Any) at ./show.jl:384
[12] sprint(::Function, ::MethodError; context::Pair{Symbol,Bool}, sizehint::Int64) at ./strings/io.jl:103
[13] Test.Error(::Any, ::Any, ::Any, ::Any, ::Any) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/Test/src/Test.jl:162
[14] top-level scope at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/Test/src/Test.jl:1121
[15] top-level scope at /home/krisztian/projects/Proxies.jl/test/runtests.jl:11
[16] include(::String) at ./client.jl:457
[17] top-level scope at none:6
in expression starting at /home/krisztian/projects/Proxies.jl/test/runtests.jl:10
caused by [exception 1]
MethodError: convert(::Type{Any}, ::Proxy{Int64}) is ambiguous. Candidates:
convert(::Type{Any}, x) in Base at essentials.jl:170
convert(targettype::Type{var"#s13"} where var"#s13"<:T, p::Proxy{TInner}) where {T, TInner} in Proxies at /home/krisztian/projects/Proxies.jl/src/Proxies.jl:19
Possible fix, define
convert(::Type{Any}, ::Proxy{TInner}) where TInner
Stacktrace:
[1] Base.RefValue{Any}(::Proxy{Int64}) at ./refvalue.jl:8
[2] Ref{Any}(::Proxy{Int64}) at ./refpointer.jl:96
[3] inferencebarrier(::Any) at ./essentials.jl:718
[4] show_default at ./show.jl:389 [inlined]
[5] show(::IOContext{Base.GenericIOBuffer{Array{UInt8,1}}}, ::Any) at ./show.jl:384
[6] show_delim_array(::IOContext{Base.GenericIOBuffer{Array{UInt8,1}}}, ::Tuple{DataType,Proxy{Int64}}, ::Char, ::Char, ::Char, ::Bool, ::Int64, ::Int64) at ./show.jl:776
[7] show_delim_array at ./show.jl:761 [inlined]
[8] show(::IOContext{Base.GenericIOBuffer{Array{UInt8,1}}}, ::Tuple{DataType,Proxy{Int64}}) at ./show.jl:794
[9] _show_default(::IOContext{Base.GenericIOBuffer{Array{UInt8,1}}}, ::Any) at ./show.jl:406
[10] show_default at ./show.jl:389 [inlined]
[11] show(::IOContext{Base.GenericIOBuffer{Array{UInt8,1}}}, ::Any) at ./show.jl:384
[12] sprint(::Function, ::MethodError; context::Pair{Symbol,Bool}, sizehint::Int64) at ./strings/io.jl:103
[13] Test.Error(::Any, ::Any, ::Any, ::Any, ::Any) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/Test/src/Test.jl:162
[14] do_test(::Test.ExecutionResult, ::Any) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/Test/src/Test.jl:518
[15] top-level scope at /home/krisztian/projects/Proxies.jl/test/runtests.jl:13
[16] top-level scope at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/Test/src/Test.jl:1115
[17] top-level scope at /home/krisztian/projects/Proxies.jl/test/runtests.jl:11
[18] include(::String) at ./client.jl:457
[19] top-level scope at none:6
ERROR: Package Proxies errored during testing
```
I would really appreciate any help on this!
---
### Author:  [@johnmyleswhite](https://discourse.julialang.org/u/johnmyleswhite)
#### Post date: [October 5, 2020, 11:38am UTC](https://discourse.julialang.org/t/need-help-with-resolving-method-ambiguities/47779/2 "2020-10-05T11:38:42Z")
In your original code, I get the following error:
```julia
julia> p = Proxy(42)
Error showing value of type Proxy{Int64}:
ERROR: MethodError: convert(::Type{Any}, ::Proxy{Int64}) is ambiguous. Candidates:
convert(::Type{Any}, x) in Base at essentials.jl:170
convert(targettype::Type{var"#s2"} where var"#s2"<:T, p::Proxy{TInner}) where {T, TInner} in Main at REPL[4]:1
Possible fix, define
convert(::Type{Any}, ::Proxy{TInner}) where TInner
```
The last line suggests an ambiguity fix:
```julia
function Base.convert(::Type{Any}, ::Proxy{TInner}) where TInner
convert(Any, inner(p))
end
```
That gets rid of the ambiguities, but then you have more methods to implement to get your tests passing.
---
### Author:  [@tisztamo](https://discourse.julialang.org/u/tisztamo)
#### Post date: [October 5, 2020, 12:31pm UTC](https://discourse.julialang.org/t/need-help-with-resolving-method-ambiguities/47779/3 "2020-10-05T12:31:35Z")
Thank you, not clear how I have missed that!
---
### Author:  [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)
#### Post date: [October 5, 2020, 1:32pm UTC](https://discourse.julialang.org/t/need-help-with-resolving-method-ambiguities/47779/4 "2020-10-05T13:32:47Z")
Just as a note, it is quite questionable to have a type that converts to absolutely any other type. Conversions should usually result in something that is very similar to the original thing (`1` → `1.0`) and it is unclear if you can have a type such that this is true for all the types it wraps.
It might be better to have a designated `unwrap` method or something along those lines.
---
### Author:  [@tisztamo](https://discourse.julialang.org/u/tisztamo)
#### Post date: [October 5, 2020, 1:55pm UTC](https://discourse.julialang.org/t/need-help-with-resolving-method-ambiguities/47779/5 "2020-10-05T13:55:53Z")
Thanks for noting this! My understanding of conversion and promotion is not perfect yet, so possibly even the goal of this method could be improved. With
```julia
Base.convert(targettype::Type{<:T}, p::Proxy{TInner}) where {T, TInner} = begin
return convert(targettype, inner(p))
end
```
I have tried to say in Julia that “If a conversion is available from `TInner` to `T`, then here I provide one from `Proxy{TInner}` to `T`”
---
### Author:  [@tomerarnon](https://discourse.julialang.org/u/tomerarnon)
#### Post date: [October 5, 2020, 3:16pm UTC](https://discourse.julialang.org/t/need-help-with-resolving-method-ambiguities/47779/6 "2020-10-05T15:16:35Z")
On the topic of
> [@Finding and fixing invalidations: now, everyone can help reduce time-to-first-plot](https://discourse.julialang.org/t/finding-and-fixing-invalidations-now-everyone-can-help-reduce-time-to-first-plot/45598/1):
>
> I just posted a [new blog post](https://julialang.org/blog/2020/08/invalidations/) on recent work reducing Julia’s latency, a.k.a., “time to first plot” and even “time to second plot (after loading more code)”. The blog post describes recent work on diagnosing and eliminating invalidations, events that cause Julia to have to recompile previously-compiled code. The blog post focuses on (1) explaining the underlying ideas and (2) briefly summarizing the overall progress we’ve made so far. I’m writing this post to emphasize to package developers the…
and
> [@A playful competition: who can define a method that invalidates the most code?](https://discourse.julialang.org/t/a-playful-competition-who-can-define-a-method-that-invalidates-the-most-code/47543/36):
>
> Yikes, realized I forgot to announce the results on Friday. To evaluate the submissions, I tested each using a script that I executed from the linux prompt to ensure there weren’t differences in my typing at the REPL (since hitting certain keys can force the compilation of new methods). In a couple of cases, this led to a pretty different count than provided by the submitter, perhaps mostly because some REPL methods did not compile. (At least the rank order was consistent with the numbers provid…
The method:
```julia
function Base.convert(targettype::Type{<:T}, p::Proxy{TInner}) where {T, TInner}
return convert(targettype, inner(p))
end
```
causes `1934` invalidations…
---
### Author:  [@tisztamo](https://discourse.julialang.org/u/tisztamo)
#### Post date: [October 5, 2020, 3:38pm UTC](https://discourse.julialang.org/t/need-help-with-resolving-method-ambiguities/47779/7 "2020-10-05T15:38:00Z")
Great, better than the winning `convert` in the non-pirating category! 🙂
I have thought that implementing a proxy type (similar to [Proxy - JavaScript | MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy) ) will be relatively simple, so a good way to learn more about how types are working. Never thought that it will be performant when implemented naively, because a lot of evaling seems to be needed. (Not sure if non-naive implementation is possible though)
---
### Author:  [@oxinabox](https://discourse.julialang.org/u/oxinabox)
#### Post date: [October 5, 2020, 5:47pm UTC](https://discourse.julialang.org/t/need-help-with-resolving-method-ambiguities/47779/8 "2020-10-05T17:47:40Z")
My instinct is that you can’t prevent those and you don’t want to.
I am pretty sure it would require changes to how the things you invalidated were written.
Or to the code that calls them.
Invalidations are not, as a rule, a problem for package authors to worry about.
It’s a problem the compiler and standard library folk to worry about.
---
### Author:  [@tomerarnon](https://discourse.julialang.org/u/tomerarnon)
#### Post date: [October 6, 2020, 11:39am UTC](https://discourse.julialang.org/t/need-help-with-resolving-method-ambiguities/47779/9 "2020-10-06T11:39:51Z")
Reading through that link, I think a naive implementation (even a not very naive one…) is not possible in julia. You would need a way to replace arbitrary function calls on the type with different ones (at runtime, without macros and so on presumably, or else you severely limit the usability of Proxy). The way javascript is interpretted is different enough from how julia is compiled that there can’t be an exact equivalent to `Proxy` without call overloading on `::Function` or `::Any` 😬
With a macro you could do:
```julia
@proxy p let getproperty = blah, setproperty! = blah
p.x = p.x + p.y # can be translated by the macro to whatever
end
```
but doesn’t really fill the same niche as `Proxy`. Normally you would just define a new type or a new method, depending on what behavior you need changed…