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---
synopsis: "Distinguish between explicit throws and errors that happened while evaluating a throw"
cls: 1511
credits: Qyriad
category: Improvements
---
Previously, errors caused by an expression like `throw "invalid argument"` were treated like an error that happened simply while some builtin function was being called:
```
let
throwMsg = p: throw "${p} isn't the right package";
in throwMsg "linuz"
error:
… while calling the 'throw' builtin
at «string»:2:17:
1| let
2| throwMsg = p: throw "${p} isn't the right package";
| ^
3| in throwMsg "linuz"
error: linuz isn't the right package
```
But the error didn't just happen "while" calling the `throw` builtin — it's a throw error!
Now it looks like this:
```
let
throwMsg = p: throw "${p} isn't the right package";
in throwMsg "linuz"
error:
… caused by explicit throw
at «string»:2:17:
1| let
2| throwMsg = p: throw "${p} isn't the right package";
| ^
3| in throwMsg "linuz"
error: linuz isn't the right package
```
This also means that incorrect usage of `throw` or errors evaluating its arguments are easily distinguishable from explicit throws:
```
let
throwMsg = p: throw "${p} isn't the right package";
in throwMsg { attrs = "error when coerced in string interpolation"; }
error:
… while calling the 'throw' builtin
at «string»:2:17:
1| let
2| throwMsg = p: throw "${p} isn't the right package";
| ^
3| in throwMsg { attrs = "error when coerced in string interpolation"; }
… while evaluating a path segment
at «string»:2:24:
1| let
2| throwMsg = p: throw "${p} isn't the right package";
| ^
3| in throwMsg { attrs = "error when coerced in string interpolation"; }
error: cannot coerce a set to a string: { attrs = "error when coerced in string interpolation"; }
```
Here, instead of an actual thrown error, a type error happens first (trying to coerce an attribute set to a string), but that type error happened *while* calling `throw`.
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