___         ___ _
  / _ \ /\  /\/ __(_)
 / /_\// /_/ / /  | |      GHC Interactive, version 5.01, for Haskell 98.
/ /_\\/ __  / /___| |      http://www.haskell.org/ghc/
\____/\/ /_/\____/|_|      Type :? for help.

Loading package std ... linking ... done.
Prelude> :unset +s +t
Prelude> -- tests for :def, :undef
Prelude> let void m = m >> return ""
Prelude> let echo s = void (putStrLn s)
Prelude> :def echo echo
Prelude> :echo hello, world!
Prelude> 
Prelude> let echo s = return (":! echo " ++ map Char.toUpper s)
hello, world!
Prelude> :echo hello, world!
Prelude> :def echo echo
hello, world!
command `echo' is already defined
Prelude> :undef echo
Prelude> :def echo echo
Prelude> :echo hello, world!
HELLO, WORLD!
Prelude> Leaving GHCi.