I'm trying to parse a list of numbers plus four diferent signs (+-*/) in this way: Lets say the list is "32+5/46" result would be "2+5/4" I get: "2+5/4*** Exception: geneticSimple.hs:(55,0)-(65,35): Non-exhaustive patterns in function chromoResult If the list is "32+5**6" result would be "2+5*6" I get: "2+5/*** Exception: geneticSimple.hs:(55,0)-(65,35): Non-exhaustive patterns in function chromoResult If the list is "32+-72" resoult would be "2+7" I get: "2+*** Exception: geneticSimple.hs:(55,0)-(65,35): Non-exhaustive patterns in function chromoResult code: chromoResult [] = [] chromoResult (a:b:c:xs) | elem a "0123456789" && elem b "0123456789" && elem c "0123456789" = chromoResult (c:xs) | elem a "0123456789" && elem b "0123456789" && elem c "+-*/" = b:c: chromoResult xs | elem a "0123456789" && elem b "+-*/" && elem c "0123456789" = a:b:c : chromoResult xs | elem a "0123456789" && elem b "+-*/" && elem c "+-*/" = a:b : chromoResult (c:xs) | elem a "+-*/" && elem b "0123456789" && elem c "0123456789" = chromoResult (b:c:xs) | elem a "+-*/" && elem b "0123456789" && elem c "+-*/" = b:c : chromoResult xs | elem a "+-*/" && elem b "+-*/" && elem c "0123456789" = chromoResult (c:xs) | elem a "+-*/" && elem b "+-*/" && elem c "+-*/" = chromoResult xs | otherwise = chromoResult (b:c:xs) I suspect my approach is flawed but i have exausted my ideas. I need a fresh approach so if anybody would be kind enough and just give me a hint how to approach the problem. Thx in advance.