From Bas van Dijk:
To understand these things I would recommend using nix-repl:
$ nix-repl
Welcome to Nix version 1.11.2. Type :? for help.
nix-repl> name = "bar"
nix-repl> ./foo + "${name}"
/home/bas.van.dijk/foobar
Note that the previous is equivalent to the simpler:
nix-repl> ./foo + name
/home/bas.van.dijk/foobar
The reason that you get "foobar" is that we didn't include a "/". So lets try to do that:
nix-repl> ./foo/ + name
error: syntax error, unexpected '+', at (string):1:8
Apparently the Nix path parser doesn't like a slash at the end of a path literal. So lets try adding the slash dynamically:
nix-repl> ./foo + "/" + name
/home/bas.van.dijk/foobar
What happened here? Well, + is left associative so it is interpreted as:
(./foo + "/") + name
Lets try evaluating that left expression alone:
nix-repl> ./foo + "/"
/home/bas.van.dijk/foo
Apparently Nix performs normalization on paths since the final slash is not included. So lets put the parenthesis differently:
nix-repl> ./foo + ("/" + name)
/home/bas.van.dijk/foo/bar
That's better! Now we can shorten this using some antiquotation:
nix-repl> ./foo + "/${name}"
/home/bas.van.dijk/foo/bar
Note that if you use import pkgs.path the path type of pkgs.path means that the contents will not be stored in /nix/store. However if you use import "${pkgs.path}", the pkgs.path gets interpolated as a string, and the pkgs.path then imported into the /nix/store. The former is more efficient than the latter. Remember you can always convert a path to a string with builtins.toString, but to do it other way you can use instead ./. + "string" or /. + "string".
You saved me some time today. Thank you!