Interactive interpreted usage without having installed first
requires adding a search path for the respective environment:

  hugs -98 -P:src:base21
  ghci-7.0.4 -isrc:base40
  ghci-7.4.1 -isrc:base45
  ghci-7.8.1 -isrc:base47

ghc < 7.6 has not been tested recently, reports welcome.