Why some functions are not visible in hoogle?

Why some functions are not visible in Hoogle? Example: ppTopElement

ppTopElement :: Element -> String
        -- Defined in `Text.XML.Light.Output'

Is it a bug?

Workaround: go to FPcomplete instead.

haskell.org's Hoogle (http://www.haskell.org/hoogle/) is quite selective about what it searches, whereas FP Complete's hoogle at https://www.fpcomplete.com/hoogle searches more fully:

Search more widely than haskell.org's hoogle

https://www.fpcomplete.com/hoogle?q=ppTopElement&env=ghc-7.4.2-stable-13.09

Searched for: ppTopElement

ppTopElement :: Element -> String
xml Text.XML.Light.Output

Pretty printing renders XML documents faithfully, with the exception that whitespace may be added/removed in non-verbatim character data.

Search more flexibly than hayoo

Being a hoogle rather than a hayoo, you can search by type, partial type, partial name, misordered type etc. For instance, searching for Element -> String gives lots of functions, some showing, some pretty printing, some just doing the top element.

Aside: there's a lovely online IDE

BTW, Their online IDE is superb (click on the homepage https://www.fpcomplete.com/ and scroll down to Start a Project). It's slick, pretty, and you can add dependencies easily without tedious cabal install problems. I've been tempted to even use it at home where I have ghc installed!