non-negative-0.1.1: Non-negative numbers

PortabilityHaskell 98
Safe HaskellSafe-Inferred



A type class for non-negative numbers. Prominent instances are T, peano numbers and types from Data.Word. However, there is Monoid class for Word types. Maybe Word is not a candidate, since it has wrap-around semantics. This class cannot do any checks, but it let you show to the user what arguments your function expects. Thus you must define class instances with care. In fact many standard functions (take, '(!!)', ...) should have this type class constraint.



class (Ord a, Monoid a) => C a whereSource

Instances of this class must ensure non-negative values. We cannot enforce this by types, but the type class constraint NonNegative.C avoids accidental usage of types which allow for negative numbers.

The Monoid superclass contributes a zero and an addition.


split :: a -> a -> (a, (Bool, a))Source

split x y == (m,(b,d)) means that b == (x<=y), m == min x y, d == max x y - min x y, that is d == abs(x-y).

We have chosen this function as base function, since it provides comparison and subtraction in one go, which is important for replacing common structures like

 if x<=y
   then f(x-y)
   else g(y-x)

that lead to a memory leak for peano numbers. We have choosen the simple check x<=y instead of a full-blown compare, since we want Zero <= undefined for peano numbers. Because of undefined values split is in general not commutative in the sense

 let (m0,(b0,d0)) = split x y
     (m1,(b1,d1)) = split y x
 in  m0==m1 && d0==d1

The result values are in the order in which they are generated for Peano numbers. We have chosen the nested pair instead of a triple in order to prevent a memory leak that occurs if you only use b and d and ignore m. This is demonstrated by test cases Chunky.splitSpaceLeak3 and Chunky.splitSpaceLeak4.


(Ord a, Num a) => C (T a) 
C a => C (T a)

This instance is not correct with respect to the equality check if the involved numbers contain zero chunks.

splitDefault :: (Ord b, Num b) => (a -> b) -> (b -> a) -> a -> a -> (a, (Bool, a))Source

Default implementation for wrapped types of Ord and Num class.

(-|) :: C a => a -> a -> aSource

x -| y == max 0 (x-y)

The default implementation is not efficient, because it compares the values and then subtracts, again, if safe. max 0 (x-y) is more elegant and efficient but not possible in the general case, since x-y may already yield a negative number.

zero :: C a => aSource

add :: C a => a -> a -> aSource

sum :: C a => [a] -> aSource

maximum :: C a => [a] -> aSource

Left biased maximum of a list of numbers that can also be empty. It holds

 maximum [] == zero

switchDifferenceNegative :: C a => a -> a -> (a -> b) -> (a -> b) -> bSource

In switchDifferenceNegative x y branchXminusY branchYminusX the function branchXminusY is applied to x-y if this difference is non-negative, otherwise branchYminusX is applied to y-x.

switchDifferenceOrdering :: C a => a -> a -> b -> (a -> b) -> (a -> b) -> bSource

In switchDifferenceOrdering x y branchZero branchXminusY branchYminusX