[Nix-dev] Re: [Nix-commits] SVN commit: nix - 18113 - eelco - nixpkgs/trunk/pkgs/top-level

[Michael Raskin]

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Eelco Dolstra wrote:
> Log:
> * Before selecting an attribute from `x', check whether it's an
>   attribute set.  `tryEval' no longer catches this.

By the way, that looks like a symptom that your watering tryEval down
breaks the purity. Imagine our pattern of passing derivation or null.
Let us have
C = pkg: { a = A pkg; b = B pkg; };
Now let A assert that the pkg is a package, and B just uses its outPath.
"tryEval (C null)" now depends on execution order. It seems to be
unfortunate.. Maybe catch all errors not damaging execution environment?

Also, if A asserts and B aborts we again have execution order problem.

As you periodically state that some "aggregation" tasks could be done
with less centralized lists and more expression attributes, tryEval
seems to fit such tasks. But the only way it can not break purity is to
restore its power.
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