Is your feature request related to a problem? Please describe.
When I'm entering the nix repl, I often like to set up a couple definitions for convenience before I start evaluating expressions, like this:

$ nix repl nixpkgs
Loading installable 'flake:nixpkgs#'...
Added 5 variables.
nix-repl> pkgs = legacyPackages.aarch64-darwin
nix-repl> hsPkgs = pkgs.haskell.packages.ghc96

Unfortunately, there's no way to configure this sort of behavior for nix repl, and even if there was a setting like repl-default-bindings it wouldn't account for the dynamism of the values that might be loaded.

Describe the solution you'd like

I would like a repl-startup-files setting, which could default to a path like ~/.config/nix/repl.nix. The file would contain a function of type AttrSet -> AttrSet. nix repl would call the file with the initial top-level variables as the first argument and replace the top-level variables with the resulting expression.

Then, the bindings I've added manually above could be added automatically like this:

vars: let
  pkgs =
    if vars ? legacyPackages.${builtins.currentSystem}
    then vars.legacyPackages.${builtins.currentSystem}
    else if vars ? packages.${builtins.currentSystem}
    then vars.packages.${builtins.currentSystem}
    else null;

  hsPkgs =
    if pkgs ? haskellPackages
    then pkgs.haskellPackages
    else null;

  optionalAttrs = condition: attrs:
    if condition
    then attrs
    else {};
in
  vars
  // (optionalAttrs (pkgs != null) {inherit pkgs;})
  // (optionalAttrs (hsPkgs != null) {inherit hsPkgs;})

Perhaps we could also provide a global variable like __replSource to indicate which flake or file is being loaded. Then, users could dynamically provide bindings for specific flakes. Alternatively, flakes could set nixConfig.extraReplStartupFiles to check in project-specific bindings.

Describe alternatives you've considered
One alternative would be a repl-bindings setting, which might look like this:

repl-bindings = pkgs=legacyPackages.${builtins.currentSystem} hsPkgs=legacyPackages.${builtins.currentSystem}.haskellPackages

I think this would be much clumsier to use and would introduce another unfortunate whitespace-sensitive DSL. The lack of newlines in configuration files would also make it much harder to keep the definitions organized neatly.

Priorities

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