request to clarify "illegal name" error

[deliciouslytyped]

Scenario:
While playing in the REPL with the following snippet:

mapDir = f: path:
  let
    files = builtins.readDir path;
  in
    builtins.mapAttrs (n: v: f (path + ("/" + n)) v) files;

I was confused by the following:

l> mapDir (n: v: "${n}") ./.
illegal name: '.git'

I didn't understand why it was complaining about illegal names, and besides, I wasn't renaming any of the attributes!

So the specific problem here was that interpolating a path doesn't just interpolate the string, it imports the file into the store, and interpolates the store path. This all makes sense after having it explained to me, but it would have been nice if the error message clarified that this wasn't an evaluation error or somesuch.

I don't have any specific fixes in mind, but @grahamc has suggested something like the following as a starting point:

Trying to import «path» in to the Nix store with the name «.foo» is invalid: it must begin with a letter or a number
Note: builtins.path can be used to give it an alternative name, if you cannot rename the file no disk.

In my case this would have told me that I need to figure out why its trying to import a path, and I would have eventually figured out the interpolation behavior.

[grahamc]

nix/src/libstore/store-api.cc

Line 86 in 324a5dc

void checkStoreName(const string & name)