Nix: String and Path Concatenation

nix_string_and_path_concatenation.md

Nix String and Path Concatenation

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