Aspell dictionaries missing when installed into nix-shell - nix

Question

When I install the Aspell package with the Aspell dictionary for English into my global env using Nix on Mac OSX:
$ nix-env -iA nixpkgs.aspell nixpkgs.aspellDicts.en
Then Aspell behaves as I would expect:
$ aspell dump dicts
en
en-variant_0
...
However if I install the packages into a nix-shell then the Dictionary does not appear to be correctly installed:
$ nix-shell -p aspell aspellDicts.en --pure
$ aspell dump dicts
# nothing printed
$ echo 'word lister to check' | aspell --list
Error: No word lists can be found for the language "en_US".
Each of the following variations produce the same problem behaviour:
Installing into a nix-shell that is not pure,
Putting the nix expression into a default.nix file and just running nix-shell.
Installing other Aspell dictionaries
Can anyone advise how to get this working?

Answer 1

The aspell binary is wrapped by NixPkgs, to provide the installation paths through ASPELL_CONF environment variable, if not already specified. (For details, cat -v result/bin/aspell)
You can either specify ASPELL_CONF manually, or use the NIX_PROFILES environment variable. For example:
$ nix-build -E 'with import <nixpkgs> {};
buildEnv { name = "aspell-env"; paths = [aspell aspellDicts.en]; }'
$ NIX_PROFILES=./result ./result/bin/aspell dump dicts
en
en-variant_0
[...]
en_US-wo_accents
However, if you want to build a self-contained aspell install with dictionaries, you can use aspellWithDicts:
nix-build -E 'with import <nixpkgs> {}; aspellWithDicts (d: [d.en])'
This will hardcode the ASPELL_CONF, so you don't need to think of those environment variables again.