nixpkgs/pkgs/development/haskell-modules/replacements-by-name/README.md

haskell-modules/replacements-by-name

This directory is scanned, and all .nix files are called in order to replace their respective packages in the haskellPackages set. They're loaded after hackage-packages.nix but before any overrides are applied. See non-hackage-packages.nix for the implementation.

This is used for selective backports of updates, as the hackage package set won't be updated in its entirety.