nix-shell as #! interpreter for runghc

I'm trying to use nix-shell as a #! wrapper for runghc, as suggested in the manpage. But it cannot find the libraries. Given the following example, cut-down from the manpage

#! /usr/bin/env nix-shell
#! nix-shell -i runghc -p haskellPackages.HTTP

import Network.HTTP

main = return ()

I get the following result:

[~:0]$ ./nixshelltest 

nixshelltest:4:1: error:
    Failed to load interface for ‘Network.HTTP’
    Use -v to see a list of the files searched for.
[~:1]$ 

to my mind, that's exactly what nix-shell -p is to avoid.

Am I doing something wrong, missing the point, or is there a bug? This is both on a nixOS 17.03 host, and also a host running nix 17.09 on top of Ubuntu.

Thanks,