Version 1.3.2 of Haskell for Mac introduced documentation links in the info popovers for identifiers (names of functions, types, classes, etc).

More specifically, if you use the shortcut ⌘I when on an identifier (or three-finger tap on an identifier), Haskell for Mac provides general information about the identifier's meaning, such as its type (or the kind, in case of a type name) and its definition. The latter includes the name of the module that contains this definition.

This name is now a link to the online documentation for the defined identifier. If you click that link, it opens the relevant part of the module's Haddock documentation in your browser. This is much faster than searching for it and indispensable when you are using libraries that you are not very familiar with. (The main limitation is that it only works for libraries that come bundled with Haskell for Mac and not for any additional packages that you might have installed with cabal install. We are planning to lift this limitation in the future, though.)