A user boldly types:

> import Data.Foo

<no location info>: error:
    Could not find module ‘Data.Foo’
    It is a member of the hidden package ‘package-foo-0.1.0.0@package-foo-0.1.0.0-5Itxx5SAgKEAspB2MHVKqi’.

The above error message is rather useless for a newcomer to ghci. After lots of googling, the user figures that :set -v -package package-foo seems to solve it.

The original error should be:

> import Data.Foo

<no location info>: error:
    Could not find module ‘Data.Foo’
    It is a member of the hidden package ‘package-foo-0.1.0.0@package-foo-0.1.0.0-5Itxx5SAgKEAspB2MHVKqi’.
Try :set -package package-foo to make it visible.

Or something like that.