Overload `/` to mean path append; alternative to `path + "/" + "foo"` footgun

[roberth]

Is your feature request related to a problem? Please describe.

In the Nix language, + associates in such a way that path + "/" + "foo" evaluates to path + "foo" for any path value path.

#6530 pushes people towards this syntax, so perhaps it's worth considering to give them a concise alternative that doesn't lead to almost inexplicable malformed paths every now and then.

Describe the solution you'd like

Instead of

nix-repl> "a" / "b"
error: value is a string while a float was expected

nix-repl> ./. / "b" 
error: value is a string while a float was expected

evaluate to

nix-repl> "a" / "b"
"a/b"

nix-repl> ./. / "b" 
/home/user/src/nixpkgs/b    # assuming such a ./.

Describe alternatives you've considered

Add builtin fetchers to the string context, and add fixed-size holes to strings, so that "${fetchTree foo}/bar" does not immediately fetch the tree.
Holes are a lot to ask though.
Perhaps better phrased as a "rope" of strings and lazy tree thunks. A rope, or lazily concatenated string representation might not be a bad idea?

Additional context

[infinisil]

This is a bit problematic because foo / bar (this is a path concatenation of two variables) would then have a different meaning from foo/bar (this is a path created from two string components)

[ncfavier]

I wonder if something like ${foo}/${bar} could work?

[infinisil]

I should also point out the recently merged #5066, which actually supports almost this:

nix-repl> foo = "foo"

nix-repl> bar = "bar"

nix-repl> /${foo}/${bar}
/foo/bar

I feel like support for ${foo}/${bar} could also be implemented

[roberth]

We could at least do this in C++ actually. SourcePath's + operator freaks me out every now and then. / wouldn't.

@edolstra wdyt?

[roberth]

Discussed in meeting today

• path / "string" is a good idea (@Ericson2314, @edolstra, @roberth)

• "string" / "string" probably good for consistency.

  • We want to use string for "abstract" paths such as those in NixOS configs
  • Slightly trickier "1.0" / "2.0"

• Feels like a gimmick, but solves a real problem

• @Ericson2314: let's talk to the tvix people as well