[Nix-dev] rec {} // {} ?

[Eelco Dolstra]

Marc Weber wrote:

> let a = rec {  x=y;.. }; in
> let b = rce {  y=x;.. }; in
> a // b;
> 
> Should a // b be rec as well ?

"rec" is basically desugared to a normal attribute set by substituting the rec
inside itself.  So

a = rec { x = y; y = 123; ... }
  = { x = (rec { x = y; y = 123; ...}).y; y = 123; ... }

So you can update the y attribute, but other attributes will still get the old
value of y.

Note that if this weren't the case, you would get very weird scoping.  In your
example

  let a = rec {  x=y;.. }; in

"y" would presumably refer to some "y" in the surrounding scope, but if you then do

  let b = rec {  y=x;.. }; in
  a // b;

it would suddenly refer to the "y" in "b".  This would break equational reasoning.

BTW, Martin and I have thought about this in the past, since it *would* be very
nice to be able to update a rec in this way.  For instance, right now you can't
easily update something in all-packages.nix, say, update the "gtk" attribute
with the // operator and have all other packages build against the new gtk.  But
we couldn't really come up with a semantically clean way to do this.

-- 
Eelco Dolstra | http://www.cs.uu.nl/~eelco