why does `nix flake show` builds ghc? - haskell

Question

If I look at the outputs provided by the haskell.nix flake from a M1 computer, it starts building ghc-8.8.4 etc..
❯ nix flake show github:input-output-hk/haskell.nix
github:input-output-hk/haskell.nix/1b54ea01568299a0eda578ae9395e20d5c699ee1
├───checks
│ ├───aarch64-darwin
trace: haskell-nix.haskellLib.cleanGit: /nix/store/jmx2m0ldgrjq7p3gb4yyca47nvbvspfl-source does not seem to be a git repository,
assuming it is a clean checkout.
trace: No index state specified for haskell-project, using the latest index state that we know about (2022-02-07T00:00:00Z)!
trace: No index state specified for haskell-project, using the latest index state that we know about (2022-02-07T00:00:00Z)!
trace: No index state specified for haskell-project, using the latest index state that we know about (2022-02-07T00:00:00Z)!
trace: WARNING: No materialized dummy-ghc-data for ghc-8.8.4-aarch64-darwin.
trace: To make this a fixed-output derivation but not materialized, set `sha256` to the output of the 'calculateMaterializedSha' script in 'passthru'.
trace: To materialize this entirely, pass a writable path as the `materialized` argument and run the 'updateMaterialized' script in 'passthru'.
[1/0/579 built, 0.1 MiB DL] building ghc-8.8.4 (buildPhase):.....
From an Intel Mac, I get
❯ nix flake show github:input-output-hk/haskell.nix
github:input-output-hk/haskell.nix/1b54ea01568299a0eda578ae9395e20d5c699ee1
├───checks
│ ├───aarch64-darwin
trace: haskell-nix.haskellLib.cleanGit: /nix/store/jmx2m0ldgrjq7p3gb4yyca47nvbvspfl-source does not seem to be a git repository,
assuming it is a clean checkout.
trace: No index state specified for haskell-project, using the latest index state that we know about (2022-02-07T00:00:00Z)!
trace: No index state specified for haskell-project, using the latest index state that we know about (2022-02-07T00:00:00Z)!
trace: No index state specified for haskell-project, using the latest index state that we know about (2022-02-07T00:00:00Z)!
error: --- Error ----------------------------------------------------------------------------------------------------------------------------------------------------------------------- nix
a 'aarch64-darwin' with features {} is required to build '/nix/store/sc11rh4l348yw3z4q4fy4byw324nm5yz-nix-tools-plan-to-nix-pkgs.drv', but I am a 'x86_64-darwin' with features {benchmark, big-parallel, nixos-test, recursive-nix}
(use '--show-trace' to show detailed location information)
Why do those flakes need to build anything for showing their outputs ?
If I run this command on some other flake I have, after a few fetches :
❯ nix flake show github:edolstra/dwarffs
github:edolstra/dwarffs/69d73417d83ebeb7711912e33515d87049b39de0
├───checks
│ ├───aarch64-linux
│ │ ├───build: derivation 'dwarffs-0.1.20220128.69d7341'
│ │ └───test: derivation 'vm-test-run-unnamed'
│ ├───i686-linux
│ │ ├───build: derivation 'dwarffs-0.1.20220128.69d7341'
│ │ └───test: derivation 'vm-test-run-unnamed'
│ └───x86_64-linux
│ ├───build: derivation 'dwarffs-0.1.20220128.69d7341'
│ └───test: derivation 'vm-test-run-unnamed'
├───defaultPackage
│ ├───aarch64-linux: package 'dwarffs-0.1.20220128.69d7341'
│ ├───i686-linux: package 'dwarffs-0.1.20220128.69d7341'
│ └───x86_64-linux: package 'dwarffs-0.1.20220128.69d7341'
├───nixosModules
│ └───dwarffs: NixOS module
└───overlay: Nixpkgs overlay

Answer 1

haskell.nix depends heavily on what is commonly called "import from derivation" or IFD. These are expressions such as
foo = import "${mkDerivation bar}/expr.nix";
or
qux = builtins.readFile (somePackage + "/data.json");
These can not be evaluated without building bar and somePackage.
haskell.nix does have a feature that lets you avoid such expressions altogether. They've called it materialization.