# Static testing with JET vs interfaces
**URL:** https://discourse.julialang.org/t/static-testing-with-jet-vs-interfaces/97985
**Category:** Tooling
**Tags:** interface, jet, static-analysis
**Created:** [April 27, 2023, 6:15am UTC](https://discourse.julialang.org/t/static-testing-with-jet-vs-interfaces/97985 "2023-04-27T06:15:13Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@gdalle](https://discourse.julialang.org/u/gdalle)
#### Post date: [April 27, 2023, 6:15am UTC](https://discourse.julialang.org/t/static-testing-with-jet-vs-interfaces/97985/1 "2023-04-27T06:15:13Z")
Hi there!
I’m starting to add [JET testing](https://aviatesk.github.io/JET.jl/dev/jetanalysis/#JET.test_package) to several packages, but I run into problems when I try to define abstract interfaces. As of right now, interfaces in Julia are mostly methodless functions with explicit documentation, so of course the static analysis errors when I make use of these functions downstream. More details here: [JET errors on interface definitions (as it should) - what to do? · Issue #495 · aviatesk/JET.jl · GitHub](https://github.com/aviatesk/JET.jl/issues/495)
Does anyone have a suggestion?
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### Author:  [@Sukera](https://discourse.julialang.org/u/Sukera)
#### Post date: [April 27, 2023, 6:19am UTC](https://discourse.julialang.org/t/static-testing-with-jet-vs-interfaces/97985/2 "2023-04-27T06:19:40Z")
The question is, what do you want to test on the interface definition alone? The example you linked seems to report exactly what it should - there is no fallback implementation for `bar(::AbstractFoo)`, which ought to be interpreted as “implement it yourself”.
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### Author:  [@gdalle](https://discourse.julialang.org/u/gdalle)
#### Post date: [April 27, 2023, 6:22am UTC](https://discourse.julialang.org/t/static-testing-with-jet-vs-interfaces/97985/3 "2023-04-27T06:22:12Z")
The thing is, I’d like to use `JET.test_package` on some of my packages, where I define such an interface but no concrete implementation. It was more of a long shot / general question about design, cause I think the practical solution would be to add an `ignore_function` argument to JET or something
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### Author:  [@Sukera](https://discourse.julialang.org/u/Sukera)
#### Post date: [April 27, 2023, 6:42am UTC](https://discourse.julialang.org/t/static-testing-with-jet-vs-interfaces/97985/4 "2023-04-27T06:42:35Z")
I think using JET for an interface package only makes sense when you have a concrete implementation & signature you want to check. You can’t check whether an interface without any callable signature matches with what you expect of it after all. If you want to check whether some concrete signature of a function was implemented, you could do it like this:
```julia
julia> abstract type Foo end
julia> struct NotImplementedError <: Exception
f
end
julia> func(::Foo) = throw(NotImplementedError(func))
func (generic function with 1 method)
julia> using JET
julia> JET.report_call(func, (Foo,))
═════ 1 possible error found ═════
┌ @ REPL[4]:1 func(::Foo)
│ may throw: throw(NotImplementedError(func)::NotImplementedError)
└─────────────
```
Of course, checking whether a concrete type implements that correctly is also possible:
```julia
julia> struct Bar <: Foo end
julia> JET.report_call(func, (Bar,))
═════ 1 possible error found ═════
┌ @ REPL[4]:1 func(::Bar)
│ may throw: throw(NotImplementedError(func)::NotImplementedError)
└─────────────
julia> func(::Bar) = "Bar!"
func (generic function with 2 methods)
julia> JET.report_call(func, (Bar,))
No errors detected
```
But for forward declarations in the style of `function func end`, there’s just no way to check whether a fallback works correctly, because there are simply no possible methods to call in the first place.
Do you have a concrete example I can take a look at?
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### Author:  [@gdalle](https://discourse.julialang.org/u/gdalle)
#### Post date: [April 27, 2023, 9:46am UTC](https://discourse.julialang.org/t/static-testing-with-jet-vs-interfaces/97985/5 "2023-04-27T09:46:48Z")
I think you’re right, `test_package` is not appropriate in this case and I need to look at calls individually. I thought it neat to be able to include JET tests in the CI, but I guess it takes more finesse.
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### Author:  [@gdalle](https://discourse.julialang.org/u/gdalle)
#### Post date: [April 27, 2023, 9:47am UTC](https://discourse.julialang.org/t/static-testing-with-jet-vs-interfaces/97985/6 "2023-04-27T09:47:51Z")
No particular example, but the issue popped up several times. Defining the signature with a `NotImplementedError` is a good option but it replaces one JET warning with another ^^