Mac & Linux support of Flake template

[peterbecich]

Is your feature request related to a problem? Please describe.
My understanding is that a haskell.nix Flake does not work in exactly the same way on Mac and Linux. Based on this comment #1401 (comment) and some testing, it looks like a haskell.nix Flake can be used on both Mac and Linux by:

• modifying evalSystem
• the --impure flag
  or
• having a system with both Mac and Linux builders

Describe the solution you'd like
The haskell.nix Flake in this project https://github.com/input-output-hk/cicero-pipe appears to use a fourth solution. nix build and nix develop run on Mac and Linux, without using any of the three solutions above. I have tested this on an amd64-linux and amd64-mac.

The key may be the materialization: https://github.com/input-output-hk/cicero-pipe/tree/master/nix/materialized

Can the approach taken in the cicero-pipe Flake be copied into the haskell.nix Flake template, to improve the ease-of-use of the Flake template?

Additional context

Discussion about builtins.currentSystem: #1401

Thank you for this project, haskell.nix has been very useful to me!

[peterbecich]

I think the materialization in cicero-pipe prevents Mac/Linux errors in nix develop, nix build and nix run.

These issues concern nix flake show and nix flake check:

nix flake show should be filterable by the current system #6985
NixOS/nix#6985

nix flake check is running checks for all systems in checks #6806
NixOS/nix#6806

nix flake check breaks on IFD in multi-platform flake #4265
NixOS/nix#4265

PR:

nix flake show: don't evaluate derivations for foreign systems by default #6988
NixOS/nix#6988

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[hamishmack]

Would it make sense to add ciEvalSystem (like evalSystem, but only for flake hydraJobs)?