Copyright  Copyright (C) 20062015 Bjorn Buckwalter 

License  BSD3 
Maintainer  bjorn@buckwalter.se 
Stability  Stable 
Portability  GHC only 
Safe Haskell  None 
Language  Haskell2010 
Extensions 

Summary
In this module we provide data types for performing arithmetic with physical quantities and units. Information about the physical dimensions of the quantities/units is embedded in their types and the validity of operations is verified by the type checker at compile time. The boxing and unboxing of numerical values as quantities is done by multiplication and division of units, of which an incomplete set is provided.
We limit ourselves to "Newtonian" physics. We do not attempt to accommodate relativistic physics in which e.g. addition of length and time would be valid.
As far as possible and/or practical the conventions and guidelines of NIST's "Guide for the Use of the International System of Units (SI)" [1] are followed. Occasionally we will reference specific sections from the guide and deviations will be explained.
Disclaimer
Merely an engineer, the author doubtlessly uses a language and notation that makes mathematicians and physicist cringe. He does not mind constructive criticism (or pull requests).
The sets of functions and units defined herein are incomplete and reflect only the author's needs to date. Again, patches are welcome.
Usage
Preliminaries
This module requires GHC 7.8 or later. We utilize Data Kinds, TypeNats, Closed Type Families, etc. Clients of the module are generally not required to use these extensions.
Clients probably will want to use the NegativeLiterals extension.
Examples
We have defined operators and units that allow us to define and work with physical quantities. A physical quantity is defined by multiplying a number with a unit (the type signature is optional).
v :: Velocity Prelude.Double v = 90 *~ (kilo meter / hour)
It follows naturally that the numerical value of a quantity is obtained by division by a unit.
numval :: Prelude.Double numval = v /~ (meter / second)
The notion of a quantity as the product of a numerical value and a unit is supported by 7.1 "Value and numerical value of a quantity" of [1]. While the above syntax is fairly natural it is unfortunate that it must violate a number of the guidelines in [1], in particular 9.3 "Spelling unit names with prefixes", 9.4 "Spelling unit names obtained by multiplication", 9.5 "Spelling unit names obtained by division".
As a more elaborate example of how to use the module we define a function for calculating the escape velocity of a celestial body [2].
escapeVelocity :: (Floating a) => Mass a > Length a > Velocity a escapeVelocity m r = sqrt (two * g * m / r) where two = 2 *~ one g = 6.6720e11 *~ (newton * meter ^ pos2 / kilo gram ^ pos2)
The following is an example GHC session where the above function is used to calculate the escape velocity of Earth in kilometer per second.
>>>
:set +t
>>>
let me = 5.9742e24 *~ kilo gram  Mass of Earth.
me :: Quantity DMass GHC.Float.Double>>>
let re = 6372.792 *~ kilo meter  Mean radius of Earth.
re :: Quantity DLength GHC.Float.Double>>>
let ve = escapeVelocity me re  Escape velocity of Earth.
ve :: Velocity GHC.Float.Double>>>
ve /~ (kilo meter / second)
11.184537332296259 it :: GHC.Float.Double
For completeness we should also show an example of the error messages we will get from GHC when performing invalid arithmetic. In the best case GHC will be able to use the type synonyms we have defined in its error messages.
>>>
x = 1 *~ meter + 1 *~ second
Couldn't match type 'Numeric.NumType.DK.Integers.Zero with 'Numeric.NumType.DK.Integers.Pos1 Expected type: Unit 'Metric DLength a Actual type: Unit 'Metric DTime a In the second argument of `(*~)', namely `second' In the second argument of `(+)', namely `1 *~ second'
In other cases the error messages aren't very friendly.
>>>
x = 1 *~ meter / (1 *~ second) + 1 *~ kilo gram
Couldn't match type 'Numeric.NumType.DK.Integers.Zero with 'Numeric.NumType.DK.Integers.Neg1 Expected type: Quantity DMass a Actual type: Dimensional ('Numeric.Units.Dimensional.Variants.DQuantity Numeric.Units.Dimensional.Variants.* 'Numeric.Units.Dimensional.Variants.DQuantity) (DLength / DTime) a In the first argument of `(+)', namely `1 *~ meter / (1 *~ second)' In the expression: 1 *~ meter / (1 *~ second) + 1 *~ kilo gram In an equation for `x': x = 1 *~ meter / (1 *~ second) + 1 *~ kilo gram
It is the author's experience that the usefullness of the compiler error messages is more often than not limited to pinpointing the location of errors.
Notes
Future work
While there is an insane amount of units in use around the world it is reasonable to provide at least all SI units. Units outside of SI will most likely be added on an asneeded basis.
There are also plenty of elementary functions to add. The Floating
class can be used as reference.
Additional physics models could be implemented. See [3] for ideas.
Related work
Henning Thielemann numeric prelude has a physical units library, however, checking of dimensions is dynamic rather than static. Aaron Denney has created a toy example of statically checked physical dimensions covering only length and time. HaskellWiki has pointers [4] to these.
Also see Samuel Hoffstaetter's blog post [5] which uses techniques similar to this library.
Libraries with similar functionality exist for other programming languages and may serve as inspiration. The author has found the Java library JScience [6] and the Fortress programming language [7] particularly noteworthy.
References
 http://physics.nist.gov/Pubs/SP811/
 http://en.wikipedia.org/wiki/Escape_velocity
 http://jscience.org/api/org/jscience/physics/models/packagesummary.html
 http://www.haskell.org/haskellwiki/Physical_units
 http://liftm.wordpress.com/2007/06/03/scientificdimensiontypearithmeticandphysicalunitsinhaskell/
 http://jscience.org/
 http://research.sun.com/projects/plrg/fortress.pdf
 type Unit m = Dimensional (DUnit m)
 type Quantity = Dimensional DQuantity
 data Metricality
 data Dimension = Dim TypeInt TypeInt TypeInt TypeInt TypeInt TypeInt TypeInt
 type family a * b
 type family a / d
 type family d ^ x
 type family Root d x
 type Recip d = DOne / d
 data Dimension' = Dim' !Int !Int !Int !Int !Int !Int !Int
 class HasDimension a where
 dimension :: a > Dimension'
 type KnownDimension d = HasDimension (Proxy d)
 (*~) :: Num a => a > Unit m d a > Quantity d a
 (/~) :: Fractional a => Quantity d a > Unit m d a > a
 (^) :: (Fractional a, KnownTypeInt i, KnownVariant v, KnownVariant (Weaken v)) => Dimensional v d1 a > Proxy i > Dimensional (Weaken v) (d1 ^ i) a
 (^/) :: (KnownTypeInt n, Floating a) => Quantity d a > Proxy n > Quantity (Root d n) a
 (**) :: Floating a => Dimensionless a > Dimensionless a > Dimensionless a
 (*) :: (KnownVariant v1, KnownVariant v2, KnownVariant (v1 * v2), Num a) => Dimensional v1 d1 a > Dimensional v2 d2 a > Dimensional (v1 * v2) (d1 * d2) a
 (/) :: (KnownVariant v1, KnownVariant v2, KnownVariant (v1 * v2), Fractional a) => Dimensional v1 d1 a > Dimensional v2 d2 a > Dimensional (v1 * v2) (d1 / d2) a
 (+) :: Num a => Quantity d a > Quantity d a > Quantity d a
 () :: Num a => Quantity d a > Quantity d a > Quantity d a
 negate :: Num a => Quantity d a > Quantity d a
 abs :: Num a => Quantity d a > Quantity d a
 nroot :: (KnownTypeInt n, Floating a) => Proxy n > Quantity d a > Quantity (Root d n) a
 sqrt :: Floating a => Quantity d a > Quantity (Root d Pos2) a
 cbrt :: Floating a => Quantity d a > Quantity (Root d Pos3) a
 exp :: Floating a => Dimensionless a > Dimensionless a
 log :: Floating a => Dimensionless a > Dimensionless a
 sin :: Floating a => Dimensionless a > Dimensionless a
 cos :: Floating a => Dimensionless a > Dimensionless a
 tan :: Floating a => Dimensionless a > Dimensionless a
 asin :: Floating a => Dimensionless a > Dimensionless a
 acos :: Floating a => Dimensionless a > Dimensionless a
 atan :: Floating a => Dimensionless a > Dimensionless a
 sinh :: Floating a => Dimensionless a > Dimensionless a
 cosh :: Floating a => Dimensionless a > Dimensionless a
 tanh :: Floating a => Dimensionless a > Dimensionless a
 asinh :: Floating a => Dimensionless a > Dimensionless a
 acosh :: Floating a => Dimensionless a > Dimensionless a
 atanh :: Floating a => Dimensionless a > Dimensionless a
 atan2 :: RealFloat a => Quantity d a > Quantity d a > Dimensionless a
 (*~~) :: (Functor f, Num a) => f a > Unit m d a > f (Quantity d a)
 (/~~) :: (Functor f, Fractional a) => f (Quantity d a) > Unit m d a > f a
 sum :: (Num a, Foldable f) => f (Quantity d a) > Quantity d a
 mean :: (Fractional a, Foldable f) => f (Quantity d a) > Quantity d a
 dimensionlessLength :: (Num a, Foldable f) => f (Dimensional v d a) > Dimensionless a
 nFromTo :: (Fractional a, Integral b) => Quantity d a > Quantity d a > b > [Quantity d a]
 type DOne = Dim Zero Zero Zero Zero Zero Zero Zero
 type DLength = Dim Pos1 Zero Zero Zero Zero Zero Zero
 type DMass = Dim Zero Pos1 Zero Zero Zero Zero Zero
 type DTime = Dim Zero Zero Pos1 Zero Zero Zero Zero
 type DElectricCurrent = Dim Zero Zero Zero Pos1 Zero Zero Zero
 type DThermodynamicTemperature = Dim Zero Zero Zero Zero Pos1 Zero Zero
 type DAmountOfSubstance = Dim Zero Zero Zero Zero Zero Pos1 Zero
 type DLuminousIntensity = Dim Zero Zero Zero Zero Zero Zero Pos1
 type Dimensionless = Quantity DOne
 type Length = Quantity DLength
 type Mass = Quantity DMass
 type Time = Quantity DTime
 type ElectricCurrent = Quantity DElectricCurrent
 type ThermodynamicTemperature = Quantity DThermodynamicTemperature
 type AmountOfSubstance = Quantity DAmountOfSubstance
 type LuminousIntensity = Quantity DLuminousIntensity
 _0 :: Num a => Quantity d a
 _1 :: Num a => Dimensionless a
 _2 :: Num a => Dimensionless a
 _3 :: Num a => Dimensionless a
 _4 :: Num a => Dimensionless a
 _5 :: Num a => Dimensionless a
 _6 :: Num a => Dimensionless a
 _7 :: Num a => Dimensionless a
 _8 :: Num a => Dimensionless a
 _9 :: Num a => Dimensionless a
 pi :: Floating a => Dimensionless a
 tau :: Floating a => Dimensionless a
 siUnit :: forall d a. (KnownDimension d, Num a) => Unit NonMetric d a
 one :: Num a => Unit NonMetric DOne a
 mkUnitR :: Floating a => UnitName m > ExactPi > Unit m1 d a > Unit m d a
 mkUnitQ :: Fractional a => UnitName m > Rational > Unit m1 d a > Unit m d a
 mkUnitZ :: Num a => UnitName m > Integer > Unit m1 d a > Unit m d a
 name :: Unit m d a > UnitName m
 exactValue :: Unit m d a > ExactPi
 weaken :: Unit m d a > Unit NonMetric d a
 strengthen :: Unit m d a > Maybe (Unit Metric d a)
 exactify :: Unit m d a > Unit m d ExactPi
 showIn :: (KnownDimension d, Show a, Fractional a) => Unit m d a > Quantity d a > String
 class KnownVariant v where
 data Dimensional v :: Dimension > * > *
 dmap :: (a1 > a2) > Dimensional v d a1 > Dimensional v d a2
 changeRep :: (KnownVariant v, Real a, Fractional b) => Dimensional v d a > Dimensional v d b
 changeRepApproximate :: (KnownVariant v, Floating b) => Dimensional v d ExactPi > Dimensional v d b
Types
Our primary objective is to define a data type that can be used to represent (while still differentiating between) units and quantities. There are two reasons for consolidating units and quantities in one data type. The first being to allow code reuse as they are largely subject to the same operations. The second being that it allows reuse of operators (and functions) between the two without resorting to occasionally cumbersome type classes.
The relationship between (the value of) a Quantity
, its numerical
value and its Unit
is described in 7.1 "Value and numerical value
of a quantity" of [1]. In short a Quantity
is the product of a
number and a Unit
. We define the *~
operator as a convenient
way to declare quantities as such a product.
type Unit m = Dimensional (DUnit m) Source
A unit of measurement.
type Quantity = Dimensional DQuantity Source
A dimensional quantity.
data Metricality Source
Encodes whether a unit is a metric unit, that is, whether it can be combined with a metric prefix to form a related unit.
Physical Dimensions
The phantom type variable d encompasses the physical dimension of
a Dimensional
. As detailed in [5] there are seven base dimensions,
which can be combined in integer powers to a given physical dimension.
We represent physical dimensions as the powers of the seven base
dimensions that make up the given dimension. The powers are represented
using NumTypes. For convenience we collect all seven base dimensions
in a data kind Dimension
.
We could have chosen to provide type variables for the seven base
dimensions in Dimensional
instead of creating a new data kind
Dimension
. However, that would have made any type signatures involving
Dimensional
very cumbersome. By encompassing the physical dimension
in a single type variable we can "hide" the cumbersome type arithmetic
behind convenient type classes as will be seen later.
Represents a physical dimension in the basis of the 7 SI base dimensions, where the respective dimensions are represented by type variables using the following convention.
 l: Length
 m: Mass
 t: Time
 i: Electric current
 th: Thermodynamic temperature
 n: Amount of substance
 j: Luminous intensity
For the equivalent termlevel representation, see Dimension'
(KnownTypeInt l, KnownTypeInt m, KnownTypeInt t, KnownTypeInt i, KnownTypeInt th, KnownTypeInt n, KnownTypeInt j) => HasDimension (Proxy Dimension (Dim l m t i th n j)) Source 
Dimension Arithmetic
When performing arithmetic on units and quantities the arithmetics must be applied to both the numerical values of the Dimensionals but also to their physical dimensions. The type level arithmetic on physical dimensions is governed by closed type families expressed as type operators.
We could provide the Mul
and Div
classes with full functional
dependencies but that would be of limited utility as there is no
limited use for "backwards" type inference. Efforts are underway to
develop a typechecker plugin that does enable these scenarios, e.g.
for linear algebra.
type family a * b infixl 7 Source
Multiplication of dimensions corresponds to adding of the base dimensions' exponents.
type family a / d infixl 7 Source
Division of dimensions corresponds to subtraction of the base dimensions' exponents.
type family d ^ x infixr 8 Source
Powers of dimensions corresponds to multiplication of the base dimensions' exponents by the exponent.
We limit ourselves to integer powers of Dimensionals as fractional powers make little physical sense.
Roots of dimensions corresponds to division of the base dimensions' exponents by the order(?) of the root.
See sqrt
, cbrt
, and nroot
for the corresponding termlevel operations.
type Recip d = DOne / d Source
The reciprocal of a dimension is defined as the result of dividing DOne
by it,
or of negating each of the base dimensions' exponents.
Term Level Representation of Dimensions
To facilitate parsing and prettyprinting functions that may wish to operate on termlevel representations of dimension, we provide a means for converting from typelevel dimensions to termlevel dimensions.
data Dimension' Source
A physical dimension, encoded as 7 integers, representing a factorization of the dimension into the
7 SI base dimensions. By convention they are stored in the same order as
in the Dimension
data kind.
Eq Dimension' Source  
Ord Dimension' Source  
Show Dimension' Source  
Monoid Dimension' Source  The monoid of dimensions under multiplication. 
HasDimension Dimension' Source 
class HasDimension a where Source
Dimensional values inhabit this class, which allows access to a termlevel representation of their dimension.
dimension :: a > Dimension' Source
Obtains a termlevel representation of a value's dimension.
HasDimension Dimension' Source  
HasDimension AnyUnit Source  
HasDimension (AnyQuantity v) Source  
(KnownTypeInt l, KnownTypeInt m, KnownTypeInt t, KnownTypeInt i, KnownTypeInt th, KnownTypeInt n, KnownTypeInt j) => HasDimension (Proxy Dimension (Dim l m t i th n j)) Source  
KnownDimension d => HasDimension (Dimensional v d a) Source 
type KnownDimension d = HasDimension (Proxy d) Source
A KnownDimension is one for which we can construct a termlevel representation.
Each validly constructed type of kind Dimension
has a KnownDimension
instance.
While KnownDimension
is a constraint synonym, the presence of
in
a context allows use of KnownDimension
d
.dimension
:: Proxy
d > Dimension'
Dimensional Arithmetic
(*~) :: Num a => a > Unit m d a > Quantity d a infixl 7 Source
Forms a Quantity
by multipliying a number and a unit.
(/~) :: Fractional a => Quantity d a > Unit m d a > a infixl 7 Source
(^) :: (Fractional a, KnownTypeInt i, KnownVariant v, KnownVariant (Weaken v)) => Dimensional v d1 a > Proxy i > Dimensional (Weaken v) (d1 ^ i) a infixr 8 Source
Raises a Quantity
or Unit
to an integer power.
Because the power chosen impacts the Dimension
of the result, it is necessary to supply a typelevel representation
of the exponent in the form of a Proxy
to some TypeInt
. Convenience values pos1
, pos2
, neg1
, ...
are supplied by the Numeric.NumType.DK.Integers module. The most commonly used ones are
also reexported by Numeric.Units.Dimensional.Prelude.
The intimidating type signature captures the similarity between these operations
and ensures that composite Unit
s are NotPrefixable
.
(^/) :: (KnownTypeInt n, Floating a) => Quantity d a > Proxy n > Quantity (Root d n) a infixr 8 Source
Computes the nth root of a Quantity
using **
.
The Root
type family will prevent application of this operator where the result would have a fractional dimension or where n is zero.
Because the root chosen impacts the Dimension
of the result, it is necessary to supply a typelevel representation
of the root in the form of a Proxy
to some TypeInt
. Convenience values pos1
, pos2
, neg1
, ...
are supplied by the Numeric.NumType.DK.Integers module. The most commonly used ones are
also reexported by Numeric.Units.Dimensional.Prelude.
Also available in prefix form, see nroot
.
(**) :: Floating a => Dimensionless a > Dimensionless a > Dimensionless a infixr 8 Source
Raises a dimensionless quantity to a floating power using **
.
(*) :: (KnownVariant v1, KnownVariant v2, KnownVariant (v1 * v2), Num a) => Dimensional v1 d1 a > Dimensional v2 d2 a > Dimensional (v1 * v2) (d1 * d2) a infixl 7 Source
(/) :: (KnownVariant v1, KnownVariant v2, KnownVariant (v1 * v2), Fractional a) => Dimensional v1 d1 a > Dimensional v2 d2 a > Dimensional (v1 * v2) (d1 / d2) a infixl 7 Source
() :: Num a => Quantity d a > Quantity d a > Quantity d a infixl 6 Source
Subtracts one Quantity
from another.
nroot :: (KnownTypeInt n, Floating a) => Proxy n > Quantity d a > Quantity (Root d n) a Source
Computes the nth root of a Quantity
using **
.
The Root
type family will prevent application of this operator where the result would have a fractional dimension or where n is zero.
Because the root chosen impacts the Dimension
of the result, it is necessary to supply a typelevel representation
of the root in the form of a Proxy
to some TypeInt
. Convenience values pos1
, pos2
, neg1
, ...
are supplied by the Numeric.NumType.DK.Integers module. The most commonly used ones are
also reexported by Numeric.Units.Dimensional.Prelude.
Also available in operator form, see ^/
.
Transcendental Functions
exp :: Floating a => Dimensionless a > Dimensionless a Source
log :: Floating a => Dimensionless a > Dimensionless a Source
sin :: Floating a => Dimensionless a > Dimensionless a Source
cos :: Floating a => Dimensionless a > Dimensionless a Source
tan :: Floating a => Dimensionless a > Dimensionless a Source
asin :: Floating a => Dimensionless a > Dimensionless a Source
acos :: Floating a => Dimensionless a > Dimensionless a Source
atan :: Floating a => Dimensionless a > Dimensionless a Source
sinh :: Floating a => Dimensionless a > Dimensionless a Source
cosh :: Floating a => Dimensionless a > Dimensionless a Source
tanh :: Floating a => Dimensionless a > Dimensionless a Source
asinh :: Floating a => Dimensionless a > Dimensionless a Source
acosh :: Floating a => Dimensionless a > Dimensionless a Source
atanh :: Floating a => Dimensionless a > Dimensionless a Source
atan2 :: RealFloat a => Quantity d a > Quantity d a > Dimensionless a Source
The standard two argument arctangent function. Since it interprets its two arguments in comparison with one another, the input may have any dimension.
Operations on Collections
Here we define operators and functions to make working with homogenuous lists of dimensionals more convenient.
We define two convenience operators for applying units to all elements of a functor (e.g. a list).
(*~~) :: (Functor f, Num a) => f a > Unit m d a > f (Quantity d a) infixl 7 Source
Applies *~
to all values in a functor.
(/~~) :: (Functor f, Fractional a) => f (Quantity d a) > Unit m d a > f a infixl 7 Source
Applies /~
to all values in a functor.
sum :: (Num a, Foldable f) => f (Quantity d a) > Quantity d a Source
The sum of all elements in a list.
mean :: (Fractional a, Foldable f) => f (Quantity d a) > Quantity d a Source
The arithmetic mean of all elements in a list.
dimensionlessLength :: (Num a, Foldable f) => f (Dimensional v d a) > Dimensionless a Source
The length of the foldable data structure as a Dimensionless
.
This can be useful for purposes of e.g. calculating averages.
:: (Fractional a, Integral b)  
=> Quantity d a  The initial value. 
> Quantity d a  The final value. 
> b  The number of intermediate values. If less than one, no intermediate values will result. 
> [Quantity d a] 
Returns a list of quantities between given bounds.
Dimension Synonyms
Using our Dimension
data kind we define some type synonyms for convenience.
We start with the base dimensions, others can be found in Numeric.Units.Dimensional.Quantities.
type DOne = Dim Zero Zero Zero Zero Zero Zero Zero Source
The typelevel dimensions of dimensionless values.
Quantity Synonyms
Using the above type synonyms we can define type synonyms for quantities of particular physical dimensions.
Again we limit ourselves to the base dimensions, others can be found in Numeric.Units.Dimensional.Quantities.
type Dimensionless = Quantity DOne Source
Constants
For convenience we define some constants for small integer values
that often show up in formulae. We also throw in pi
and tau
for
good measure.
_0 :: Num a => Quantity d a Source
The constant for zero is polymorphic, allowing
it to express zero Length
or Capacitance
or Velocity
etc, in addition
to the Dimensionless
value zero.
_1 :: Num a => Dimensionless a Source
_2 :: Num a => Dimensionless a Source
_3 :: Num a => Dimensionless a Source
_4 :: Num a => Dimensionless a Source
_5 :: Num a => Dimensionless a Source
_6 :: Num a => Dimensionless a Source
_7 :: Num a => Dimensionless a Source
_8 :: Num a => Dimensionless a Source
_9 :: Num a => Dimensionless a Source
pi :: Floating a => Dimensionless a Source
tau :: Floating a => Dimensionless a Source
Twice pi
.
For background on tau
see http://tauday.com/taumanifesto (but also
feel free to review http://www.thepimanifesto.com).
Constructing Units
siUnit :: forall d a. (KnownDimension d, Num a) => Unit NonMetric d a Source
A polymorphic Unit
which can be used in place of the coherent
SI base unit of any dimension. This allows polymorphic quantity
creation and destruction without exposing the Dimensional
constructor.
one :: Num a => Unit NonMetric DOne a Source
The unit one
has dimension DOne
and is the base unit of dimensionless values.
As detailed in 7.10 "Values of quantities expressed simply as numbers:
the unit one, symbol 1" of [1] the unit one generally does not
appear in expressions. However, for us it is necessary to use one
as we would any other unit to perform the "boxing" of dimensionless values.
mkUnitR :: Floating a => UnitName m > ExactPi > Unit m1 d a > Unit m d a Source
Forms a new atomic Unit
by specifying its UnitName
and its definition as a multiple of another Unit
.
Use this variant when the scale factor of the resulting unit is irrational or Approximate
. See mkUnitQ
for when it is rational
and mkUnitZ
for when it is an integer.
Note that supplying zero as a definining quantity is invalid, as the library relies upon units forming a group under multiplication.
Supplying negative defining quantities is allowed and handled gracefully, but is discouraged on the grounds that it may be unexpected by other readers.
Unit Metadata
exactValue :: Unit m d a > ExactPi Source
weaken :: Unit m d a > Unit NonMetric d a Source
Discards potentially unwanted type level information about a Unit
.
Pretty Printing
showIn :: (KnownDimension d, Show a, Fractional a) => Unit m d a > Quantity d a > String Source
On Functor
, and Conversion Between Number Representations
We intentionally decline to provide a Functor
instance for Dimensional
because its use breaks the
abstraction of physical dimensions.
If you feel your work requires this instance, it is provided as an orphan in Numeric.Units.Dimensional.Functor.
class KnownVariant v where Source
A physical quantity or unit.
We call this data type Dimensional
to capture the notion that the
units and quantities it represents have physical dimensions.
The type variable a
is the only nonphantom type variable and
represents the numerical value of a quantity or the scale (w.r.t.
SI units) of a unit. For SI units the scale will always be 1. For
nonSI units the scale is the ratio of the unit to the SI unit with
the same physical dimension.
Since a
is the only nonphantom type we were able to define
Dimensional
as a newtype, avoiding boxing at runtime.
extractValue, extractName, injectValue, dmap
data Dimensional v :: Dimension > * > * Source
dmap :: (a1 > a2) > Dimensional v d a1 > Dimensional v d a2 Source
Maps over the underlying representation of a dimensional value. The caller is responsible for ensuring that the supplied function respects the dimensional abstraction. This means that the function must preserve numerical values, or linearly scale them while preserving the origin.
changeRep :: (KnownVariant v, Real a, Fractional b) => Dimensional v d a > Dimensional v d b Source
Convenient conversion between numerical types while retaining dimensional information.
changeRepApproximate :: (KnownVariant v, Floating b) => Dimensional v d ExactPi > Dimensional v d b Source
Convenient conversion from exactly represented values while retaining dimensional information.