❯  nix shell --impure --expr "(import (builtins.getFlake \"nixpkgs\") {}).pkgsStatic.haskellPackages.shellFor { packages = (p: [ p.aeson p.typed-process p.bytestring p.lens p.lens-aeson]);}"
error:
       … while calling a functor (an attribute set with a '__functor' attribute)
         at /nix/store/s8b6phy15mzjvx56vjdd1rdd8hr07qw5-source/pkgs/development/haskell-modules/make-package-set.nix:613:29:
          612|       # pkgWithCombinedDeps :: HaskellDerivation
          613|       pkgWithCombinedDeps = self.mkDerivation (genericBuilderArgsModifier genericBuilderArgs);
             |                             ^
          614|

       … while evaluating a branch condition
         at /nix/store/s8b6phy15mzjvx56vjdd1rdd8hr07qw5-source/lib/customisation.nix:173:7:
          172|       in
          173|       if isAttrs result then
             |       ^
          174|         result

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: expected a set but found null: null

In reply to @woobilicious:matrix.org

❯  nix shell --impure --expr "(import (builtins.getFlake \"nixpkgs\") {}).pkgsStatic.haskellPackages.shellFor { packages = (p: [ p.aeson p.typed-process p.bytestring p.lens p.lens-aeson]);}"
error:
       … while calling a functor (an attribute set with a '__functor' attribute)
         at /nix/store/s8b6phy15mzjvx56vjdd1rdd8hr07qw5-source/pkgs/development/haskell-modules/make-package-set.nix:613:29:
          612|       # pkgWithCombinedDeps :: HaskellDerivation
          613|       pkgWithCombinedDeps = self.mkDerivation (genericBuilderArgsModifier genericBuilderArgs);
             |                             ^
          614|

       … while evaluating a branch condition
         at /nix/store/s8b6phy15mzjvx56vjdd1rdd8hr07qw5-source/lib/customisation.nix:173:7:
          172|       in
          173|       if isAttrs result then
             |       ^
          174|         result

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: expected a set but found null: null

1. I think you misunderstand the inputs for shellFor. The packages input should be the packages you're developing. shellFor won't install them, only their dependencies.
2. pkgsStatic usually requires cross-compilation. If you run into evaluation or build problems, this is probably why.
3. bytestring is a boot library preinstalled with GHC, so Nixpkgs sets it to null (this is why sterni has suggested you remove it).
4. For smaller outputs, consider also using justStaticExecutables.

λlice: Do you mean generating completion for optparse applicative parameters? There is a helper for that in nixpkgs, but I don’t know how to integrate it with haskell-flake.

There might be other venues, where there is more discussion about haskell-flake, the readme is e.g. linking a zulip, since the maintainer of haskell-flake is banned from nixos community rooms.