# safe versions of pred and succ?

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## safe versions of pred and succ?

 Suppose I have a function f :: (Enum a) => a -> ... f x ... = ...pred x ... If a is also an instance of Bounded, then I get a runtime error if pred minBound is evaluated during evaluation of f. If I try to protect the use of pred x with a check using minBound, then I have to add a Bounded constraint on type a, which would mean, for example, that I can no longer use f with type Integer. Do I need two different versions of f, one for Bounded a and one for non-Bounded a? Is there a more elegant way to take care of this problem? I don't know much about all of the type magic available in GHC. For example, from another list message, groupConsecutive :: (Enum a,Eq a) => [a] -> [[a]] groupConsecutive = foldr go []      where go x ls@(hd@(y:_):yss)              | x == y || x == pred y = (x:hd):yss              | otherwise             = [x]:ls            go x [] = [[x]]  > groupConsecutive [1,2,3,7,8,10,11,12] [[1,2,3],[7,8],[10,11,12]]  > groupConsecutive ([1,0,1,2]::[Word]) *** Exception: Enum.pred{Word}: tried to take `pred' of minBound In the first go case, if type a is also Bounded, y == minBound and x /= y, then we know already that x /= pred y, we want the guard to fail and that we pass to the otherwise guard to start a new sublist. But how to implement it without making the function unavailable for un-Bounded types? Graham _______________________________________________ Beginners mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners