+  (make-parameter "https://repo.hex.pm"))
+(define hexpm-url
+  (string-append (%hexpm-repo-url) "/tarballs/"))

Where did you find this URL?  I took a look at https://hex.pm to download
the source code of an Erlang package, but I'm not finding any download links
for source code, for, say, <https://hex.pm/packages/idna>.  Only diffs and
‘previews’.

The hex.pm api is, well, not quite good documented. This URL I found at <https://github.com/hexpm/specifications/blob/master/endpoints.md>. I'll add a respective comment.

-- 
Regards
Hartmut Goebel

| Hartmut Goebel          | h.goebel@crazy-compilers.com               |
| www.crazy-compilers.com | compilers which you thought are impossible |