Overlay example

[rsoeldner]

Hey,

this is my first try using overlays, according to #53 this is the recommended way now.

It looks like nix is not able to find my app package,

nix-build release.nix 
error: attribute 'app' missing, at /Users/rsoeldner/work/haskell-nix-overlay/release.nix:11:12
(use '--show-trace' to show detailed location information)

do I still need to override haskell.packages ?
The default.nix was generated by cabal2nix . > default.nix

{ compiler ? "ghc802" }:

let
  a = self: super:
      {   
        app = super.callPackage ./default.nix;
      };  

  overlays = [a];
  pkgs = import <nixpkgs> { inherit overlays; };
in { app = pkgs.haskell.packages.${compiler}.app; }

[PierreR]

@rsoeldner You need to do something alike:

  a = self: super:
    {
        haskellPackages = super.haskellPackages.override {
          overrides = hself: hsuper:  {
             app = super.callPackage ./default.nix;
          };
      };
    }

[rsoeldner]

@PierreR thank you, from what I understood from https://nbp.github.io/slides/NixCon/2017.NixpkgsOverlays/?full#p1-prior-overlays this all overrides will be replaced by overlays.

I struggle understanding what pieces fit together, Is there a documentation about overrides accepting a function with two arguments returning a set ?

In my shell.nix I want to add the same GHC version with hoogle enabled, there should be an easy way adding all packages to the hoogle search ?

[PierreR]

@rsoeldner The overlays is replacing the packageOverrides idioms, so this part:

config = {
    packageOverrides = pkgs: rec {

I doesn't replace the override attribute.

You can see this by yourself on the third slide ;-)

Some explanation about the override attribute can be found here.

[rsoeldner]

@PierreR Thank you, this makes sense.

Currently I'm at:

release.nix

compiler ? "ghc822" }:

 let 
  overlays = [(import ./custom-overlay.nix)];
  pkgs = import <nixpkgs> 
  {
    inherit overlays;
  };  
in
  { app = pkgs.haskellPackages.app;
  }

with the custom-overlay.nix

  self: super:
  {
    haskellPackages = super.haskellPackages.override  {
      overrides = hself: hsuper:
      {   
        app = super.callPackage ./default.nix;
      };  
    };  
  }

and the shell.nix

(import ./release.nix {}).app.env

But entering the nix-shell yields

nix-shell 
error: value is a function while a set was expected, at /Users/rsoeldner/work/haskell-nix-overlay/shell.nix:1:1
(use '--show-trace' to show detailed location information)

Is this the right approach ?
And inside the custom-overlay.nix shouldn't it be a hsuper.callPackage ??? ?

[PierreR]

@rsoeldner yes it should be hsuper.callPackage. I made that mistake why trying to mimic your code sample.

[sboosali]

(wait, doesn't callPackage need self (i.e. `hself`) to pick up on the new packages?)

…

On Wed, Aug 1, 2018, 10:23 AM Pi3r ***@***.***> wrote: @rsoeldner <https://github.com/rsoeldner> yes it should be hsuper.callPackage — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#58 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ACNoMSvIp_fIcoznJIwfjNA-LMnRtm9Zks5uMeQPgaJpZM4Vp3Uq> .

[Gabriella439]

Yeah, custom-overlay.nix should be:

  self: super:
  {
    haskellPackages = super.haskellPackages.override  {
      overrides = hself: hsuper:
      {   
        app = hself.callPackage ./default.nix;
      };  
    };  
  }

If you use hsuper.callPackage then app won't pick up any overrides you make to its dependencies

[rsoeldner]

@Gabriel439 this makes sense too.

nix-shell --show-trace
error: while evaluating the attribute 'app.env' at /Users/rsoeldner/work/haskell-nix-overlay/release.nix:10:5:
value is a function while a set was expected, at /Users/rsoeldner/work/haskell-nix-overlay/shell.nix:1:1

is there a need to pass hself to the app derivation like

app = hself.callPackage ./default.nix (inherit hself);

[sboosali]

assuming `default.nix` has the `cabal2nix` shape: { mkDerivation, lens, <etc> }: ... the inputs are all provided by `callPackage`, and passing `hself` would be an unexpected argument.

…

On Wed, Aug 1, 2018, 9:44 PM rsoeldner ***@***.***> wrote: @Gabriel439 <https://github.com/Gabriel439> this makes sense too. nix-shell --show-trace error: while evaluating the attribute 'app.env' at /Users/rsoeldner/work/haskell-nix-overlay/release.nix:10:5: value is a function while a set was expected, at /Users/rsoeldner/work/haskell-nix-overlay/shell.nix:1:1 is there a need to pass hself to the app derivation like app = hself.callPackage ./default.nix (inherit hself); — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#58 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ACNoMU9hSIDw7OtapnZV-W16fKNYJ1Ksks5uMoPFgaJpZM4Vp3Uq> .

[rsoeldner]

@sboosali right, default.nix was created by cabal2nix.
I don't get the problem entering the shell

[Gabriella439]

@rsoeldner: It's because callPackage needs an extra {} argument, so the correct override section is actually:

  self: super:
  {
    haskellPackages = super.haskellPackages.override  {
      overrides = hself: hsuper:
      {   
        app = hself.callPackage ./default.nix {}; # ← Note the `{}`
      };  
    };  
  }