Re: [Haskell-beginners] Finding the first successful Map.lookup

On Thu, Dec 2, 2010 at 1:04 PM, John Obbele wrote:

{{{ type Key = String type URL = String

lookupDocumentation :: Map Key URL -> Key -> Maybe URL lookupDocumentation myMap key =        let r = map (lookupSymbols myMap) [ key                                          , "a_" ++ key                                          , "b_" ++ key                                          , "g_" ++ key                                          , "gtk_" ++ key                                          , "pango_" ++ key                                          ]        in case catMaybes r of             []    -> Nothing             (x:_) -> Just x }}}

My problem is: will `lookupDocumentation` perform n=6 lookups in my Map, or will Haskell-built-in-lazyness only evaluate till an URL is found ?

You can test this by creating a Map that contains undefined values e.g. {{{ lookupDocumentation (insert "b_somekey" undefined (singleton "a_somekey" "somevalue")) "somekey" }}} Since "a_somekey" is checked before "b_somekey" and "a_somekey" is in the map, this should work if the function is lazy enough. Johan