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</o:shapelayout></xml><![endif]--></head><body lang=EN-GB link=blue vlink=purple><div class=WordSection1><p class=MsoNormal>So I had a function that would leave me with a Nothing or a Just, and I wanted to use the result of this function to lookup in a Map. So something like this was very easy to do.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>h> let list = Data.Map.fromList [(1,2),(3,4),(5,6)]<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>h> Just 4 >>= flip Data.Map.lookup list<o:p></o:p></p><p class=MsoNormal>Nothing<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>h> Just 3 >>= flip Data.Map.lookup list<o:p></o:p></p><p class=MsoNormal>Just 4<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Now, however, my source function gives me an Either. I ended up writing this function to let me chain Either with Data.Map.lookups:<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>(>>?=) :: (Either a b, a) -> (b -> Maybe c) -> (Either a c,a)<o:p></o:p></p><p class=MsoNormal>(>>?=) (Left x,e) _ = (Left x,e)<o:p></o:p></p><p class=MsoNormal>(>>?=) (Right y,e) f = case f y of <o:p></o:p></p><p class=MsoNormal> Nothing -> (Left e,e)<o:p></o:p></p><p class=MsoNormal> Just z -> (Right z,e)<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>But I can’t help thinking I’m reinventing the wheel here somehow. There must be a better way, right?<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Thanks in advance for any pointers.<o:p></o:p></p><p class=MsoNormal><br>Alan<o:p></o:p></p></div></body></html>