# Heuristic for checking if a Method always errors
**URL:** https://discourse.julialang.org/t/heuristic-for-checking-if-a-method-always-errors/115125
**Category:** Internals & Design
**Created:** [June 3, 2024, 3:12pm UTC](https://discourse.julialang.org/t/heuristic-for-checking-if-a-method-always-errors/115125 "2024-06-03T15:12:22Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@mrufsvold](https://discourse.julialang.org/u/mrufsvold)
#### Post date: [June 3, 2024, 3:12pm UTC](https://discourse.julialang.org/t/heuristic-for-checking-if-a-method-always-errors/115125/1 "2024-06-03T15:12:22Z")
@oxinabox mused here that it should be possible to use effect analysis to figure out if a method always, sometimes, or never errors.
> [@\[RFC/WIP\] DuckDispatch.jl](https://discourse.julialang.org/t/rfc-wip-duckdispatch-jl/114930/19):
>
> I feel we should really be able to use effects (and a little compile-time code analysisis) to break methods down into Always throws, vs May throw, vs Never throws Probably even to getting a list of all exception types it might throw – should be a reasonable use of abstract interpetetation. Just having “Always throws” or not is enough to eliminate a ton of the false positives – namely the ones that that come from stub definitions that just throw “NotImplementedException”. And that should just …
I don’t know enough about the internals to dig into that at the moment, but it did lead me to wonder if there was a more basic heuristic I could use to at least tell if a method always errors.
I’m wondering if `code_typed` infers that the return type is `Union{}` does that guarantee that the function will error? Like this:
```julia
julia> err_f() = error("")
err_f (generic function with 1 method)
julia> code_typed(err_f, ()) |> only
CodeInfo(
1 ─ invoke Main.error(""::String)::Union{}
└── unreachable
) => Union{}
```
---
### Author:  [@jar1](https://discourse.julialang.org/u/jar1)
#### Post date: [June 3, 2024, 6:22pm UTC](https://discourse.julialang.org/t/heuristic-for-checking-if-a-method-always-errors/115125/2 "2024-06-03T18:22:41Z")
>
>
> \`\`\`jl
> f1(x) = error("hello")
> f2(x) = f1(x)
> @report\_call mode=:sound f2(:a)
> \`…\`\`
>
> says "may throw" but in fact it will throw. If error is the only possible outcome, I should not try to run the code, especially if it will take a long time to execute. The report could distinguish this case when it is sure of the result.
---
### Author:  [@nsajko](https://discourse.julialang.org/u/nsajko)
#### Post date: [June 3, 2024, 9:12pm UTC](https://discourse.julialang.org/t/heuristic-for-checking-if-a-method-always-errors/115125/3 "2024-06-03T21:12:12Z")
> [@mrufsvold](#):
>
> infers that the return type is `Union{}` does that guarantee that the function will error?
An empty union return type means that the function may not ever return a value, so it either runs infinitely or throws.