[Nix-dev] Re: Proposal: tryCatch function

[Michael Raskin]

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Andres Loeh wrote:
> Mainly because of the already mentioned trickyness that exceptions in
> lazy languages tend to escape their surrounding try-block, try-catch
> is generally considered not a very useful construct in the face of laziness.
> 
> The escaping should not only be a problem for functions, but also for
> any form of compound lazy data (i.e., attribute sets in the Nix language).
> 
> <religion>Exceptions in a pure language are evil and should be avoided,
> not encouraged.</religion>

We have enabled side effects already. Without caring about evaluation
order. I mean "trace" builtin. It has 2 main properties, though: it
never changes what derivations are constructed and it only gives
end-user a bit more information that can be filtered out if not helpful.

tryCatch lacks the first property, and because of that I consider it
harmful. tryRethrow is just like trace, and I am not sure that IO purity
is worth preserving (if it doesn't affect actual build purity).
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