`builtins.storePath` allows you to use "external" knowledge of present
store paths in your store to be used in your expressions.
This is sometimes used to avoid double wrapping store paths, for example
when dealing with nixpkgs sources (`builtins.storePath pkgs.path` is a
frequent pattern).
Unfortunately, using your external knowledge of present store paths is
technically impure because Flakes is about not depending on the *STATE*
of your store, no matter what you know about it or what you do to ensure
that your knowledge is accurate.
That being said, `builtins.storePath` is already possible in the
fragment of "pure Nixlang" via a clever use of `builtins.appendContext`.
Precisely:
```
storePath = path:
let path' = builtins.unsafeDiscardStringContext path;
in
# NOTE: merging the context set in all generality is impossible
because getContext on a pure path doesn't work.
builtins.appendContext path' { ${path'} = { path = true; }; };
```
works in pure contexts.
Because of this, we deem that storePath should be allowed in pure
contexts and users do not have to come up with a `storePath` polyfill.
Fixes#402.
Change-Id: I5fff750a4feed082745431835db451f945249cfb
Signed-off-by: Raito Bezarius <raito@lix.systems>
