prefix-units-0.2.0: A basic library for SI/binary prefix units

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Definitions and functions for parsing and formatting prefix units.

This module defines the type Unit and associated functions for parsing numbers containing a prefix unit (e.g. 100M) into corespondingly scaled values (for the above example, 100000000), and for formatting numbers.

The units definition is taken from the man page units(7) and the web sites and

Since a give prefix unit (e.g. m) can be interpreted in different ways, the module offers various ways to interpret this:

  • in a binary context (e.g. when talking about memory), this will be interpreted as 2^20 (see ParseBinary)
  • in a SI context dealing with multiples, this will be intepreted as 10^3 (see ParseKMGT)
  • in an exact parsing mode, this will be interpreded as the "milli" prefix, i.e. 10^-3 (see ParseExact)

The different parsing mode are offered as different contexts will have different "natural" units, and always forcing precise parsing (which also implies case-sensitivity) will lead to confusing user interfaces.

The internal calculations when converting values are done via the Rational type (with arbitrary precision), and precision loss happens only at the last step of converting to the target type; for float/doubles this is fromRational, for integral types this is round.

A few examples are given below:

>>> showValue FormatBinary 2048
>>> showValue FormatSiAll 0.0001
>>> showValue (FormatFixed Mebi) 1048576
>>> parseValue ParseExact "2.5Ki"::Either String Double
Right 2560.0
>>> parseValue ParseBinary "2M"::Either String Int
Right 2097152
>>> parseValue ParseExact "2ki"
Left "Unrecognised unit 'ki'"

The failure in the last example is due to the fact that ParseExact is case-sensitive.


Basic definitions


class Real a => RationalConvertible a where Source

Typeclass for handling values that can be converted to/from Rational.


convFromRational :: Rational -> a Source

Converts the value from Ratioal

siUnits :: [Unit] Source

List of all SI units.

siSupraunitary :: [Unit] Source

List of units which are supraunitary (their multiplier is greater than one).

siKMGT :: [Unit] Source

List of SI units which are greater or equal to Kilo.

binaryUnits :: [Unit] Source

List of binary units.

siBase :: Rational Source

The base for SI units.

binaryBase :: Rational Source

The base for binary units.

Unit-related functions

unitMultiplier :: Unit -> Rational Source

Returns the unit scaling "multiplier" (which can be either supra- or sub-unitary):

>>> unitMultiplier Micro
1 % 1000000
>>> unitMultiplier Mebi
1048576 % 1

unitName :: Unit -> String Source

Returns the unit full name.

unitSymbol :: Unit -> String Source

Returns the unit ASCII symbol.

fancySymbol :: Unit -> String Source

Returns the unit symbol, which for the Micro unit is not ASCII.

Formatting functions

data FormatMode Source

Defines the formatting modes.



Formats the value using any SI unit.


Formats the value using supraunitary SI units only (which means that e.g. 0.001 will remain as such instead of being formatted as 1m).


Formats the value using units greater or equal to Kilo.


Formats the value using binary units.


Formats the value as it is, without scaling.

FormatFixed Unit

Formats the value using the given unit.

recommendedUnit :: Real a => FormatMode -> a -> Maybe Unit Source

Computes the recommended unit for displaying a given value. The simple algorithm uses the first unit for which we have a supraunitary representation. In case we don't find any such value (e.g. for a zero value), then Nothing is returned. For FormatFixed, we always select the given unit, irrespective of the value.

formatValue Source


:: RationalConvertible a 
=> FormatMode

The desired FormatMode

-> a

The value to format

-> (a, Maybe Unit)

Scaled value and optional unit

Computes the scaled value and unit for a given value

showValue Source


:: (RationalConvertible a, Show a) 
=> FormatMode

The desired format mode.

-> a

The value to show

-> String

Resulting string

Generates a final string representation of a value.

Parsing functions

data ParseMode Source

Defines available parse modes.



Exact parser mode. This mode is fully case-sensitive.


Parses only units bigger than Kilo, respectively Kibi (for binary units). This allows the parser to be case-insensitive.


Parses binary units only. In this mode, both the exact and shortened forms are accepted (e.g. both "k" and "ki" will be converted into Kibi). Furthermore, the parsing is case-insensitive.

parseSymbol :: ParseMode -> String -> Either String Unit Source

Parses a unit from a string. The exact parsing mode determines the rules for parsing and the range of possible units.

parseValue Source


:: (Read a, RationalConvertible a) 
=> ParseMode

The desired parse mode

-> String

String to be parsed

-> Either String a

Either a Left error message, or a Right parsed value

Main parse routine.

Low-level generic functions

unitRange :: FormatMode -> Either Unit [Unit] Source

The available units range for various format modes.


data ParseOptions Source

Defines unit handling mode on parse.



Requires that the input string has a unit.

UnitDefault Unit

If unit is missing, use a default one.


The unit is optional, a missing one means the value is not scaled.

parseExactSymbol :: String -> Either String Unit Source

Parses a symbol in the exact mode. See ParseExact for details.

parseBinarySymbol :: String -> Either String Unit Source

Parses a binary symbol. See ParseBinary for details.

parseKMGTSymbol :: String -> Either String Unit Source

Parses the given symbol as one of the "big" units (kilo/kibi and above). This allows the parsing to be case-insensitive.

parseGeneric Source


:: (Read a, RationalConvertible a) 
=> ParseOptions

Unit options

-> [Unit]

Optional list of valid units

-> ParseMode

The desired parse mode

-> String

String to be parsed

-> Either String a 

Low-level parse routine. Takes two function arguments which fix the initial and final conversion, a parse mode and the string to be parsed.


showValueWith Source


:: (RationalConvertible a, Show a) 
=> (Unit -> String)

Function to convert the (optional) unit into a string, e.g. unitSymbol or fancySymbol

-> FormatMode

The desired format mode

-> a

The value to show

-> String

Resulting string

Simple helper to generate the full string representation of an integral value.