Num package:hledger-web

Basic numeric class. The Haskell Report defines no laws for Num. However, (+) and (*) are customarily expected to define a ring and have the following properties:

• Associativity of (+) (x + y) + z = x + (y + z)
• Commutativity of (+) x + y = y + x
• fromInteger 0 is the additive identity x + fromInteger 0 = x
• negate gives the additive inverse x + negate x = fromInteger 0
• Associativity of (*) (x * y) * z = x * (y * z)
• fromInteger 1 is the multiplicative identity x * fromInteger 1 = x and fromInteger 1 * x = x
• Distributivity of (*) with respect to (+) a * (b + c) = (a * b) + (a * c) and (b + c) * a = (b * a) + (c * a)

Note that it isn't customarily expected that a type instance of both Num and Ord implement an ordered ring. Indeed, in base only Integer and Rational do.

Class Enum defines operations on sequentially ordered types. The enumFrom... methods are used in Haskell's translation of arithmetic sequences. Instances of Enum may be derived for any enumeration type (types whose constructors have no fields). The nullary constructors are assumed to be numbered left-to-right by fromEnum from 0 through n-1. See Chapter 10 of the Haskell Report for more details. For any type that is an instance of class Bounded as well as Enum, the following should hold:

• The calls succ maxBound and pred minBound should result in a runtime error.
• fromEnum and toEnum should give a runtime error if the result value is not representable in the result type. For example, toEnum 7 :: Bool is an error.
• enumFrom and enumFromThen should be defined with an implicit bound, thus:

enumFrom     x   = enumFromTo     x maxBound
enumFromThen x y = enumFromThenTo x y bound
where
bound | fromEnum y >= fromEnum x = maxBound
| otherwise                = minBound

Used in Haskell's translation of [n..] with [n..] = enumFrom n, a possible implementation being enumFrom n = n : enumFrom (succ n). For example:

• enumFrom 4 :: [Integer] = [4,5,6,7,...]

• enumFrom 6 :: [Int] = [6,7,8,9,...,maxBound ::
  Int]

Used in Haskell's translation of [n,n'..] with [n,n'..] = enumFromThen n n', a possible implementation being enumFromThen n n' = n : n' : worker (f x) (f x n'), worker s v = v : worker s (s v), x = fromEnum n' - fromEnum n and f n y | n > 0 = f (n - 1) (succ y) | n < 0 = f (n + 1) (pred y) | otherwise = y For example:

• enumFromThen 4 6 :: [Integer] = [4,6,8,10...]

• enumFromThen 6 2 :: [Int] = [6,2,-2,-6,...,minBound ::
  Int]

Used in Haskell's translation of [n,n'..m] with [n,n'..m] = enumFromThenTo n n' m, a possible implementation being enumFromThenTo n n' m = worker (f x) (c x) n m, x = fromEnum n' - fromEnum n, c x = bool (>=) ((x 0) f n y | n > 0 = f (n - 1) (succ y) | n < 0 = f (n + 1) (pred y) | otherwise = y and worker s c v m | c v m = v : worker s c (s v) m | otherwise = [] For example:

• enumFromThenTo 4 2 -6 :: [Integer] =
  [4,2,0,-2,-4,-6]

• enumFromThenTo 6 8 2 :: [Int] = []

Used in Haskell's translation of [n..m] with [n..m] = enumFromTo n m, a possible implementation being enumFromTo n m | n <= m = n : enumFromTo (succ n) m | otherwise = []. For example:

• enumFromTo 6 10 :: [Int] = [6,7,8,9,10]

• enumFromTo 42 1 :: [Integer] = []

Convert to an Int. It is implementation-dependent what fromEnum returns when applied to a value that is too large to fit in an Int.

Creates an OptionList from an Enum, using its Show instance for the user-facing value.

Sign of a number. The functions abs and signum should satisfy the law:

abs x * signum x == x

For real numbers, the signum is either -1 (negative), 0 (zero) or 1 (positive).

Convert from an Int.