# Please stop using \`error\` and \`ErrorException\` in packages (and Base)

**URL:** <https://discourse.julialang.org/t/please-stop-using-error-and-errorexception-in-packages-and-base/12096>\
**Category:** General Usage\
**Tags:** error\
**Created:** [July 2, 2018, 6:37am UTC](https://discourse.julialang.org/t/please-stop-using-error-and-errorexception-in-packages-and-base/12096 "2018-07-02T06:37:35Z")\
**Posts on this page:** 1\
**Showing post:** 1

<div class="post-metadata">

**Author:** ![oxinabox](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oxinabox/32/206603_2.png) [@oxinabox](https://discourse.julialang.org/u/oxinabox)\
**Post date:** [July 2, 2018, 6:37am UTC](https://discourse.julialang.org/t/please-stop-using-error-and-errorexception-in-packages-and-base/12096/1 "2018-07-02T06:37:35Z")

</div>

In julia we like to distinguish different things using different types (exceptions are one such thing).  
In most languages they also like to distinguish different exceptions with different types.

In particular in julia

```julia
try
    foo()
catch err
    if err isa BarError
            # Handle it
    elseif err isa BlargError
            # Handle that
    else
           rethrow()
    end
end

```

Saying `error("it is borked because the Blarg is flumoxed")` causes an `ErrorException` to be thrown.  
This is an incredibly unhelpful error, typewise.  
You can;t work out what threw it or why.  
So you can’t handle it safely.

`error` exists for when you can’t be bothered declaring an exception type.  
Or working out which existing one to use.  
It belongs in user-scripts, and quick prototypes.  
It doesn’t belong in registered packages, and it particularly doesn’t belong in Standard Libraries / `Base`  
Most gone now. However, the `run` command still throws one ([https://github.com/JuliaLang/julia/pull/27900](https://github.com/JuliaLang/julia/pull/27900)).

I suggest that `ErrorException` should be treated as if it were an uncatchable exception type.

I suspect our over reliance on using `error` might come back to prior experiences we have using other languages where declaring your own exception type is difficult.  
But in julia it is easy

`struct MyError <:Exception end`  
or

```julia
struct MyErrorWithMsg <:Exception
    msg::String
end

```

There is also `@assert` for things that a programmer errors.  
So you can replace

```julia
if val == true
     # do it
elseif val==false
     # do not do it
else
     error("This should never happen") 
end

```

With

```julia
if val == true
     # do it
elseif val==false
     # do not do it
else
     @assert(false, "This should never happen") 
end

```

Which is clearer.

* * *

I am not saying I am perfect, I have packages where I was lazy and used `error`,  
rather than using the right exception type.  
But lets try and clean those out now, while we are getting ready for 1.0.

Save `error` for end-users; keep it out of your packages

---

_[View the full topic](https://discourse.julialang.org/t/please-stop-using-error-and-errorexception-in-packages-and-base/12096)._