By the way, I have developped  the core of a search engine completely in Haskell.

 

It can index arbitrary objects (including files, of course)

has definitions for filters, with predefined filters for plain text, HTML/XML and a general  filter for Haskell  data types

 

It has basic search capabilities: it returns a list of identifiers (URIs) of the objects that contains all the entered keywords.

 

But it is fairly easy to accept arbitrary AND OR and NOT conditions. Just query parsing is necessary.

 

More info:

 

 

 

The source of Search.hs can be found here