[Nix-dev] maximal sharing and comparisons.

[Michael Raskin]

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Nicolas Pierron wrote:
> Hi,
> 
> I have a problem with the NixOS implementation.  I have remarked the
> following problem:
> 
> let
>   v = {foo=true;bar=false;};
>   a = builtins.trace v v;
>   b = a;
> in
>   assert a == b; true
> 
> where "v" is printed.  "v" should not be printed because unprocessed
> values of "a" and "b" are identical.  Therefore the assertion should
> succeed without evaluating "a" and "b".

Well, exactly this case (where you could get something for nothing) is
rare. Evaluation uses normal form cache. The only side-effects in the
language are output and creating derivations (neither of them actually
creates any problems for the paths intended to be built). So, I do not
see why it is a problem..
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