Merge "repl-overlays: Provide an elaborate example" into main

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rebecca “wiggles” turner 2024-09-10 00:17:01 +00:00 committed by Gerrit Code Review
commit cc183fdbc1

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@ -185,6 +185,54 @@ struct EvalSettings : Config
else
{ }
```
Here's a more elaborate `repl-overlay`, which provides the following
variables:
- The original, unmodified variables are aliased to `original`.
- `legacyPackages.${system}` (if it exists) or `packages.${system}`
(otherwise) is aliased to `pkgs`.
- All attribute set variables with a `${system}` attribute are
abbreviated in the same manner; e.g. `devShells.${system}` is
shortened to `devShells`.
For example, the following attribute set:
```nix
info: final: attrs: let
# Equivalent to nixpkgs `lib.optionalAttrs`.
optionalAttrs = predicate: attrs:
if predicate
then attrs
else {};
# If `attrs.${oldName}.${info.currentSystem}` exists, alias `${newName}` to
# it.
collapseRenamed = oldName: newName:
optionalAttrs (builtins.hasAttr oldName attrs
&& builtins.hasAttr info.currentSystem attrs.${oldName})
{
${newName} = attrs.${oldName}.${info.currentSystem};
};
# Alias `attrs.${oldName}.${info.currentSystem} to `${newName}`.
collapse = name: collapseRenamed name name;
# Alias all `attrs` keys with an `${info.currentSystem}` attribute.
collapseAll =
builtins.foldl'
(prev: name: prev // collapse name)
{}
(builtins.attrNames attrs);
in
# Preserve the original bindings as `original`.
(optionalAttrs (! attrs ? original)
{
original = attrs;
})
// (collapseRenamed "packages" "pkgs")
// (collapseRenamed "legacyPackages" "pkgs")
// collapseAll
```
)"};
};