Decimal-0.4.1: Decimal numbers with variable precision

Safe HaskellSafe-Inferred




Decimal numbers are represented as m*10^(-e) where m and e are integers. The exponent e is an unsigned Word8. Hence the smallest value that can be represented is 10^-255.

Unary arithmetic results have the exponent of the argument. Addition and subtraction results have an exponent equal to the maximum of the exponents of the arguments. Other operators have exponents sufficient to show the exact result, up to a limit of 255:

 0.15 * 0.15 :: Decimal    = 0.0225
 (1/3) :: Decimal          = 0.33333333333333...
 decimalPlaces (1/3)       = 255

While (/) is defined, you don't normally want to use it. Instead The functions divide and allocate will split a decimal amount into lists of results which are guaranteed to sum to the original number. This is a useful property when doing financial arithmetic.

The arithmetic on mantissas is always done using Integer, regardless of the type of DecimalRaw being manipulated. In practice it is strongly recommended that Decimal be used, with other types being used only where necessary (e.g. to conform to a network protocol). For instance (1/3) :: DecimalRaw Int does not give the right answer.


Decimal Values

data Integral i => DecimalRaw i Source

Raw decimal arithmetic type constructor. A decimal value consists of an integer mantissa and a negative exponent which is interpreted as the number of decimal places. The value stored in a Decimal d is therefore equal to:

 decimalMantissa d / (10 ^ decimalPlaces d)

The Show instance will add trailing zeros, so show $ Decimal 3 1500 will return "1.500". Conversely the Read instance will use the decimal places to determine the precision.

Regardless of the type of the arguments, all mantissa arithmetic is done using Integer types, so application developers do not need to worry about overflow in the internal algorithms. However the result of each operator will be converted to the mantissa type without checking for overflow.



type Decimal = DecimalRaw IntegerSource

Arbitrary precision decimal type. Programs should do decimal arithmetic with this type and only convert to other instances of DecimalRaw where required by an external interface.

Using this type is also faster because it avoids repeated conversions to and from Integer.

realFracToDecimal :: (Integral i, RealFrac r) => Word8 -> r -> DecimalRaw iSource

Convert a real fractional value into a Decimal of the appropriate precision.

decimalConvert :: (Integral a, Integral b, Bounded b) => DecimalRaw a -> Maybe (DecimalRaw b)Source

Convert a DecimalRaw from one base to another. Returns Nothing if this would cause arithmetic overflow.

unsafeDecimalConvert :: (Integral a, Integral b) => DecimalRaw a -> DecimalRaw bSource

Convert a DecimalRaw from one base representation to another. Does not check for overflow in the new representation. Only use after using roundTo to put an upper value on the exponent, or to convert to a larger representation.

roundTo :: Integral i => Word8 -> DecimalRaw i -> DecimalRaw iSource

Round a DecimalRaw to a specified number of decimal places. If the value ends in 5 then it is rounded away from zero.

(*.) :: (Integral i, RealFrac r) => DecimalRaw i -> r -> DecimalRaw iSource

Multiply a DecimalRaw by a RealFrac value.

divide :: Decimal -> Int -> [(Int, Decimal)]Source

Divide a DecimalRaw value into one or more portions. The portions will be approximately equal, and the sum of the portions is guaranteed to be the original value.

The portions are represented as a list of pairs. The first part of each pair is the number of portions, and the second part is the portion value. Hence 10 dollars divided 3 ways will produce [(2, 3.33), (1, 3.34)].

allocate :: Decimal -> [Integer] -> [Decimal]Source

Allocate a DecimalRaw value proportionately with the values in a list. The allocated portions are guaranteed to add up to the original value.

Some of the allocations may be zero or negative, but the sum of the list must not be zero. The allocation is intended to be as close as possible to the following:

 let result = allocate d parts
 in all (== d / sum parts) $ zipWith (/) result parts

eitherFromRational :: Integral i => Rational -> Either String (DecimalRaw i)Source

Try to convert Rational to Decimal with absolute precision return string with fail description if not converted

normalizeDecimal :: Integral i => DecimalRaw i -> DecimalRaw iSource

Reduce the exponent of the decimal number to the minimal possible value