Hi,
Nicolas Pierron wrote:
> 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.
The `==' operator evaluates both terms to weak head normal form. There is no
realistic way that it could see that "a" and "b" are the same without
evaluating.
Also, `==' doesn't work on lists and attribute sets (not to mention functions)
because the list elements / attributes are not evaluated to normal form. So `{a
= "foo" + "bar";} == {a = "foobar";}' will evaluate to false. In fact, even `{a
= "foobar";} == {a = "foobar";}' evaluates to false because of the position
information in the attributes. The evaluator should really print an error when
trying to compare lists and attribute sets...
See also this issue: http://bugs.strategoxt.org/browse/NIX-62
--
Eelco Dolstra | http://www.st.ewi.tudelft.nl/~dolstra/
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