Re: [Haskell-cafe] Is it possible to get the information of instances of a type?

No, this lists all the instances of a class.  OP asked for the classes of which a given type is an instace.

Presumably it is possible, since Haddock does it!  In the documentation generated for a type it lists classes of which the type is an instance.  So you might want to look at how Haddock does it.  I suspect the only way is through the GHC API.

ghci :info does it too.