# Interfaces.jl unable to find implementation
**URL:** https://discourse.julialang.org/t/interfaces-jl-unable-to-find-implementation/124299
**Category:** General Usage
**Tags:** interface
**Created:** [December 31, 2024, 2:49am UTC](https://discourse.julialang.org/t/interfaces-jl-unable-to-find-implementation/124299 "2024-12-31T02:49:59Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@langestefan](https://discourse.julialang.org/u/langestefan)
#### Post date: [December 31, 2024, 2:49am UTC](https://discourse.julialang.org/t/interfaces-jl-unable-to-find-implementation/124299/1 "2024-12-31T02:49:59Z")
Hi.
I am trying to implement Interfaces.jl for my package.
The code can be found here: [SolarPosition.jl/src at main · PVSMC/SolarPosition.jl · GitHub](https://github.com/PVSMC/SolarPosition.jl/tree/main/src)
The interface definition is here: [SolarPosition.jl/src/PositionInterface/PositionInterface.jl at main · PVSMC/SolarPosition.jl · GitHub](https://github.com/PVSMC/SolarPosition.jl/blob/main/src/PositionInterface/PositionInterface.jl)
```julia
module PositionInterface
using Interfaces
using DynamicQuantities: Quantity
export SolarPositionInterface, SolarPositionAlgorithm
abstract type SolarPositionAlgorithm end
function sunpos end
components = (
mandatory = (
sunpos_check = (
"interface implements the sunpos function" => a::Arguments -> sunpos(
a.algorithm, a.timestamp) isa Tuple{Quantity, Quantity}
),
),
optional = ()
)
description = """
Defines an interface to calculate the position of the sun in the sky from a given
observer position and time defined by a location and timestamp.
A solar position is uniquely defined by the elevation and azimuth angles.
"""
@interface SolarPositionInterface SolarPositionAlgorithm components description
end # module PositionInterface
```
The test I implemented is here: [SolarPosition.jl/test/test-interfaces.jl at main · PVSMC/SolarPosition.jl · GitHub](https://github.com/PVSMC/SolarPosition.jl/blob/main/test/test-interfaces.jl)
```julia
using TimeZones: now, TimeZone
using Interfaces
using PVSimBase.GeoLocation: Location
using SolarPosition.Basic
using SolarPosition.PositionInterface
@testset "Basic algorithm" begin
test_location = Location(latitude = 0.0, longitude = 0.0)
alg = BasicAlgorithm(test_location)
@test alg isa BasicAlgorithm
@test alg.location == test_location
pos = sunpos(alg, now(TimeZone("UTC")))
end
@testset "PositionInterfaces" begin
@test Interfaces.test()
end
```
I want every `SolarPositionAlgorithm` to conform to the `SolarPositionInterface` interface (some parts are mandatory, others optional).
`@test Interfaces.test()` throws this error:
```julia
Testing SolarPositionInterface is implemented for BasicAlgorithm
InterfaceError: test for SolarPositionInterface :sunpos_check "interface implements the sunpos function" threw a MethodError
For test object Arguments{(:algorithm, :timestamp), Tuple{BasicAlgorithm, TimeZones.ZonedDateTime}}((algorithm = BasicAlgorithm(Location(0.0 , 0.0 , 0.0 , tz"UTC")), timestamp = TimeZones.ZonedDateTime(2024, 12, 31, 2, 33, 26, 462, tz"UTC"))):
PositionInterfaces: Error During Test at /Projects/2024/PV/SolarPosition.jl/test/test-interfaces.jl:17
Test threw exception
Expression: Interfaces.test()
MethodError: no method matching sunpos(::BasicAlgorithm, ::TimeZones.ZonedDateTime)
The function `sunpos` exists, but no method is defined for this combination of argument types.
Stacktrace:
[1] (::SolarPosition.PositionInterface.var"#1#2")(a::Arguments{(:algorithm, :timestamp), Tuple{BasicAlgorithm, TimeZones.ZonedDateTime}})
```
So for some reason when I run `@test Interfaces.test()` it does not find the correct implementation of the `sunpos` function. It does find the right algorithm implementation.
With a lot of this more advanced stuff I am still a beginner so please let me know if this is completely the wrong direction to take this implementation…
---
### Author:  [@abraemer](https://discourse.julialang.org/u/abraemer)
#### Post date: [December 31, 2024, 7:18am UTC](https://discourse.julialang.org/t/interfaces-jl-unable-to-find-implementation/124299/2 "2024-12-31T07:18:42Z")
I think the issue is just that you don’t attach the methods to right function, i.e. there is a namespacing issue.
See you define the interface for the function `PositionInterface.sunpos` (as posted above) but in [Basic.jl](https://github.com/PVSMC/SolarPosition.jl/blob/main/src/Basic/Basic.jl) you don’t import it and thus don’t attach a method to it. Instead what happens is, that the definition in Basic.jl creates another function `Basic.sunpos` which is distinct from `PositionInterface.sunpos` and you then export that function from Basic.jl. So of course when you test the interface, there is appropriate method of `PositionInterface.sunpos` for your `BasicAlgorithm`.
What you should do instead:
- `export` the function `sunpos` from `PositionInterface`. This is somewhat optional, but I think desired since you want to use the function `sunpos` as part of the public interface and thus `export`ing it makes sense.
- add a `import .PositionInterface` to Basic.jl and change the definition of `sunpos` to `PositionInterface.sunpos` to properly add the method to the correct function. This steps works independently of the previous one, i.e. does not require the `export sunpos` from `PositionInterface`
- I would recommend not exporting `sunpos` from `Basic.jl` again. In principle it works and can also be correct if you take care to not accidently create another `sunpos` but it’s bad style imo. Just export it once from `PositionInterface` where it originates if you want it to be exported.
---
### Author:  [@langestefan](https://discourse.julialang.org/u/langestefan)
#### Post date: [December 31, 2024, 11:52am UTC](https://discourse.julialang.org/t/interfaces-jl-unable-to-find-implementation/124299/3 "2024-12-31T11:52:50Z")
Thank you very much for writing this out. I was able to fix the issue. I had some wrong ideas about how the methods are discovered, but now I know 🙂
One small note:
> [@abraemer](#):
>
> import .PositionInterface
I needed `import ..PositionInterface` here, then it works.