Occured error when follow your instructions.

[ityspace]

let
  a = 1;
in
"the value of a is ${a}" 

THEN output that :
error: cannot coerce an integer to a string

   at «string»:4:2:

        3| in
        4| "the value of a is ${a}"
         |                    ^
        5|

[ityspace]

So the interpolation cannot be an integer？

[ryan4yin]

Yep, I made a mistake here, I found an issue related to it: NixOS/nix#7327

[ryan4yin]

Fixed, I changed it to a = "1";