Docs that explain Import From Derivation is missing some context

[itsfarseen]

Problem

Here: https://nix.dev/manual/nix/2.24/language/import-from-derivation
It is mentioned that:

Passing an expression expr that evaluates to a store path,
to any built-in function which reads from the filesystem,
constitutes Import From Derivation (IFD):

import expr
 .. other builtin functions that reads from the file system ..

It also says:

When the store path needs to be accessed, evaluation will be paused,
the corresponding store object realised, and then evaluation resumed.

Counter example:

nix-repl> pkgs = import <nixpkgs> {}
nix-repl> builtins.toString pkgs.zxing-cpp                               
"/nix/store/nvis47j5sxwibkwh0p94iwi4cx8rphgi-zxing-cpp-1.4.0"

# Try to import the path
nix-repl> import "/nix/store/nvis47j5sxwibkwh0p94iwi4cx8rphgi-zxing-cpp-1.4.0"
error:
       … while calling the 'import' builtin
         at «string»:1:1:
            1| import "/nix/store/nvis47j5sxwibkwh0p94iwi4cx8rphgi-zxing-cpp-1.4.0"
             | ^

       error: path '/nix/store/nvis47j5sxwibkwh0p94iwi4cx8rphgi-zxing-cpp-1.4.0' does not exist

# Okay try to import the path passed in as path instead of string
nix-repl> import /nix/store/nvis47j5sxwibkwh0p94iwi4cx8rphgi-zxing-cpp-1.4.0
error:
       … while calling the 'import' builtin
         at «string»:1:1:
            1| import /nix/store/nvis47j5sxwibkwh0p94iwi4cx8rphgi-zxing-cpp-1.4.0
             | ^

       error: path '/nix/store/nvis47j5sxwibkwh0p94iwi4cx8rphgi-zxing-cpp-1.4.0' does not exist

But:

nix-repl> builtins.toString pkgs.zxing-cpp
"/nix/store/nvis47j5sxwibkwh0p94iwi4cx8rphgi-zxing-cpp-1.4.0"

nix-repl> import (builtins.toString pkgs.zxing-cpp)
.. starts downloading ..

How does this work?

Proposal

Checklist

• checked latest Nix manual (source)
• checked open documentation issues and pull requests for possible duplicates

---

Add 👍 to issues you find important.