quantities-0.1.0: Unit conversion and manipulation library.

Safe HaskellSafe-Inferred




This package is used to create and manipulate physical quantities, which are a numerical value associated with a unit of measurement.

In this package, values with units are represented with the Quantity type. Included is an expression parser and a huge list of predefined quantities with which to parse strings into a Quantity datatype. Once created, a quantity can be converted to different units or queried for its dimensionality. A user can also operate on quantities arithmetically, and doing so uses automatic unit conversion and simplifcation.



Currently, one constructor is supported to create quantities: fromString. There is an included expression parser that can parse values and strings corresponding to builtin units. To view defined unit types, look at the source code for defaultDefString.

fromString :: String -> Either QuantityError QuantitySource

Create a Quantity by parsing a string. Uses an UndefinedUnitError for undefined units.

>>> fromString "25 m/s"
Right 25.0 meter / second
>>> fromString "fakeunit"
Left (UndefinedUnitError "fakeunit")

unitsFromString :: String -> Either QuantityError CompositeUnitSource

Parse units from a string. Equivalent to fmap units . fromString

>>> unitsFromString "N * s"
Right [newton,second]

data Quantity Source

Combination of magnitude and units.

magnitude :: Quantity -> DoubleSource

Numerical magnitude of quantity.

>>> magnitude <$> fromString "100 N * m"
Right 100.0

units :: Quantity -> CompositeUnitSource

Units associated with quantity.

>>> units <$> fromString "3.4 m/s^2"
Right [meter,second ** -2.0]

type CompositeUnit = [SimpleUnit]Source

Collection of SimpleUnits. Represents combination of simple units.

data SimpleUnit Source

Representation of single unit. For example: "mm^2" is represented as

 SimpleUnit { symbol = "meter", prefix = "milli", power = 2.0 }


These functions are used to convert quantities from one unit type to another.

convert :: Quantity -> CompositeUnit -> Either QuantityError QuantitySource

Convert quantity to given units.

>>> convert <$> fromString "m" <*> unitsFromString "ft"
Right (Right 3.280839895013123 foot)

convertBase :: Quantity -> QuantitySource

Convert a quantity to its base units.

>>> convertBase <$> fromString "newton"
Right 1000.0 gram meter / second ** 2.0

dimensionality :: Quantity -> CompositeUnitSource

Computes dimensionality of quantity.

>>> dimensionality <$> fromString "newton"
Right [length,mass,time ** -2.0]

Quantity arithmetic

Once created, quantities can be manipulated using the included arithmetic functions.

>>> let (Right x) = fromString "m/s"
>>> let (Right y) = fromString "mile/hr"
>>> x `multiplyQuants` y
1.0 meter mile / hour / second
>>> x `divideQuants` y
1.0 hour meter / mile / second
>>> x `addQuants` y
Right 1.4470399999999999 meter / second
>>> x `subtractQuants` y
Right 0.55296 meter / second
>>> x `exptQuants` 1.5
1.0 meter ** 1.5 / second ** 1.5

The functions multiplyQuants, divideQuants, and exptQuants change units, and the units of the result are reduced to simplest terms.

>>> x `divideQuants` x
>>> fmap (multiplyQuants x) $ fromString "s"
Right 1.0 meter
>>> x `exptQuants` 0

addQuants :: Quantity -> Quantity -> Either QuantityError QuantitySource

Adds two quantities. Second quantity is converted to units of first quantity.

subtractQuants :: Quantity -> Quantity -> Either QuantityError QuantitySource

Subtract two quantities. Second quantity is converted to units of first quantity.

multiplyQuants :: Quantity -> Quantity -> QuantitySource

Multiplies two quantities.

divideQuants :: Quantity -> Quantity -> QuantitySource

Divides two quantities.

exptQuants :: Quantity -> Double -> QuantitySource

Exponentiates a quantity with a double.

Error type

data QuantityError Source

Custom error type


UndefinedUnitError String

Used when trying to parse an undefined unit.

DimensionalityError CompositeUnit CompositeUnit

Used when converting units that do not have the same dimensionality (example: convert meter to second).

UnitAlreadyDefinedError String

Used internally when defining units and a unit is already defined.

PrefixAlreadyDefinedError String

Used internally when defining units and a prefix is already defined.

ParserError String

Used when a string cannot be parsed.

type QuantityComputation = Either QuantityErrorSource

Computation monad that propagates QuantityErrors. Some examples:

 computation :: QuantityComputation Quantity
 computation = do
   x <- fromString "mile/hr"
   y <- unitsFromString "m/s"
   convert x y

Returns Right 0.44704 meter / second

 computation :: QuantityComputation Quantity
 computation = do
   x <- fromString "BADUNIT"
   convertBase x

Returns Left (UndefinedUnitError BADUNIT)