If I were to propose this, which I'm not, I would discuss what the [a,b..c] notation is meant to represent. To me, personally and for no good reason, it looks like an iterator that yields numbers starting at a, adding (b-a) at each step and yielding the next number as long as it's less than or equal to c, and continuing as long as c is neither reached nor exceeded.
Yes, the current situation is somewhat odd. $ ghci GHCi, version 7.10.3: http://www.haskell.org/ghc/ :? for help Prelude> [0,5..9] [0,5] Prelude> [0,0.5..0.9] [0.0,0.5,1.0] Prelude> [0.0..0.49999999999999991] [0.0] Prelude> [0.0..0.49999999999999992] [0.0,1.0] Prelude> :m Data.Ratio Prelude Data.Ratio> [0..1%2] [0 % 1,1 % 1] This makes the construct into a serious bug generator for anything numeric. For instance, consider a naive numeric integration of the sort one might wish to exhibit while teaching Haskell this past pi day, Monday the fourteenth of March. Prelude> let h=1e-1 in sum $ map (\x -> 4*h*sqrt(1-x^2)) [0,h..1] 3.304518326248318 Prelude> let h=1e-2 in sum $ map (\x -> 4*h*sqrt(1-x^2)) [0,h..1] NaN Prelude> let h=1e-3 in sum $ map (\x -> 4*h*sqrt(1-x^2)) [0,h..1] NaN Prelude> let h=1e-4 in sum $ map (\x -> 4*h*sqrt(1-x^2)) [0,h..1] 3.141791477784753 Prelude> let h=1e-5 in sum $ map (\x -> 4*h*sqrt(1-x^2)) [0,h..1] 3.1416126164824103 Prelude> let h=1e-6 in sum $ map (\x -> 4*h*sqrt(1-x^2)) [0,h..1] NaN One invariant that people seem to find natural regarding [x,y..z] is that its elements are all between x and z (inclusive), regardless of the value of y. This is, in part, because the mathematical notation [x,z] often denotes the closed interval whose endpoints are x and z. I myself cannot think of any advantage of the behaviour exhibited above. To take a slightly deeper violated invariant, it seems like map f [x,y..z] and [f x,f y..f z] should be the same when f is a linear (in the numeric sense) function. The current behaviour violates this.