It looks like Neil hasn't updated hoogle
to work with the latest wai
package. The short-term solution is to force cabal to select an older version of the offending package (wai):
cabal install hoogle --constraint='wai<2.0'
EDIT: Note wai
2.0 was released just 4 days ago and hackage doesn't proactively inform developers about breakage, so this isn't really surprising.