option

programs.vscode.haskell.hie.executablePath

The path to the Haskell IDE Engine executable.

Because hie-nix is not packaged in Nixpkgs, you need to add it as an overlay or set this option. Example overlay configuration:

nixpkgs.overlays = [
  (self: super: { hie-nix = import ~/src/hie-nix {}; })
]
Declarations
Type
path
Default
"${pkgs.hie-nix.hies}/bin/hie-wrapper"
Example
(import ~/src/haskell-ide-engine {}).hies + "/bin/hie-wrapper";