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Diffstat (limited to 'doc/manual/rl-next/distinguish-throw-errors.md')
-rw-r--r-- | doc/manual/rl-next/distinguish-throw-errors.md | 70 |
1 files changed, 70 insertions, 0 deletions
diff --git a/doc/manual/rl-next/distinguish-throw-errors.md b/doc/manual/rl-next/distinguish-throw-errors.md new file mode 100644 index 000000000..243e33d61 --- /dev/null +++ b/doc/manual/rl-next/distinguish-throw-errors.md @@ -0,0 +1,70 @@ +--- +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`. |