[Nix-dev] A Journey into our brand-new Haskell infrastructure: Part II

[Peter Simons]

Hi Thomas,

 > How can I override a package to enable profiling?

commit [1] adds helper functions that allow referring to any package "foo"
as "enableLibraryProfiling foo" to get a variant that supports profiling.

If you want to enable profiling for *all* packages, then you can replace
the 'mkDerivation' function with one that enables the feature by default,
i.e. by adding the snippet

 | haskellPackageOverrides = self: super: {
 |   mkDerivation = expr: super.mkDerivation (expr // { enableLibraryProfiling = true; });
 | };

to your ~/.nixpkgs/config.nix file.


 > Is there an obvious way in general to apply the same override to a
 > package and all its dependencies?

No, we can't do that yet. But we can do something else, that's almost
equally goos: we can modify the package set in which a package (and all
its dependencies) are evaluated. Just add

 | {
 |   packageOverrides = super: let self = super.pkgs; in
 |   {
 |     foo = self.haskellngPackages.foo.overrideScope (self: super: {
 |       mkDerivation = expr: super.mkDerivation (expr // { enableLibraryProfiling = true; });
 |     });
 |   }
 | }

to your ~/.nixpkgs/config.nix to get a variant of "foo" that has profiling
enabled in all its dependencies as well. If you take that stuff to the
extreme, then it looks like [2].

I hope this helps,
Peter

[1] https://github.com/NixOS/nixpkgs/commit/e4a68f0b98c28319913a4f2104ddb0524262f390
[2] https://github.com/NixOS/nixpkgs/blob/9e114ccd2693daab6bca3c05674c549884097925/pkgs/development/haskell-modules/configuration-ghc-7.8.x.nix#L43-87