units

The units package provides a mechanism for compile-time dimensional analysis in Haskell programs. It defines an embedded type system based on units-of-measure. The units and dimensions defined are fully extensible, and need not relate to physical properties. This package exports definitions only for Dimensionless and Number. The set of units and dimensions from the International System (SI) are exported from the companion package units-defs.

This package supports independent notions of dimension and unit. Examples of dimensions include length and mass. Examples of unit include meter and gram. Every unit measures a particular dimension, but a given dimension may be measured by many different units. For example, both meters and feet measure length.

The package supports defining multiple inter-convertible units of the same dimension, such as Meter and Foot. When extracting a numerical value from a quantity, the desired unit must be specified, and the value is converted into that unit.

The laws of nature have dimensions, and they hold true regardless of the units used. For example, the gravitational force between two bodies is (gravitational constant) * (mass 1) * (mass 2) / (distance between body 1 and 2)^2, regardless of whether the distance is given in meters or feet or centimeters. In other words, every law of nature is unit-polymorphic.

The units package supports unit-polymorphic programs through the coherent system of units (CSU) mechanism. A CSU is essentially a mapping from dimensions to the units. All dimensioned quantities (generally just called quantities) are expressed using the Qu type. The Qu type constructor takes a (perhaps compound) dimension, a CSU and a numerical value type as arguments. Internally, the quantity is stored as a number in the units as specified in the CSU – this may matter if you are worried about rounding errors. In the sequence of computations that works within one CSU, there is no unit conversion. Unit conversions are needed only when putting values in and out of quantities, or converting between two different CSUs.

Checking out units

Units has a git submodule, so you’ll want to use git clone --recursive. Example:

git clone --recursive https://github.com/goldfirere/units

User contributions

It is easy to imagine any number of built-in facilities that would go well with this package (sets of definitions of units for various systems, vector operations, a suite of polymorphic functions that are commonly needed but hard to define, etc.). Yet, I (Richard) don’t have the time to imagine or write all of these. If you write code that is sufficiently general and might want to be included with this package (but you don’t necessarily want to create your own new package), please write me!

Modules

The units package exports several key modules. Note that you will generally import only one of Data.Metrology, Data.Metrology.Poly, or Data.Metrology.Vector.

• Data.Metrology.Poly

  This is the main exported module. It exports all the necessary functionality for you to build your own set of units and operate with them.

• Data.Metrology

  This re-exports most of the definitions from Data.Metrology.Poly, but restricts a few operators to work only with the default LCSU, as this is simpler for new users to units.

• Data.Metrology.Vector

  This also re-exports a similar set of definitions as Data.Metrology.Poly, but provides numerical operations based on vector-space instead of the standard numerical classes.

• Data.Metrology.Linear

  This exports a set of definitions for interoperability with the linear package. This is /not/ a top-level import; generally, import this with Data.Metrology.Poly as well.

• Data.Metrology.Internal

  This module contains mostly-internal definitions that may appear in GHC’s error messages. Users will generally not need to use these definitions in their code. However, by exporting this module from within Data.Metrology.Poly, we can reduce the module-prefix clutter in error messages.

• Data.Metrology.Unsafe

  This module exports the constructor for the central datatype that stores quantities. With this constructor, you can arbitrarily change units! Use at your peril.

• Data.Metrology.Show

  This module defines a Show instance for quantities, printing out the number stored along with its canonical dimension. This behavior may not be the best for every setting, so it is exported separately. Importing this module reduces the guaranteed unit-safety of your code, because it allows you to inspect (in a round-about way) how your quantities are stored.

• Data.Metrology.Parser

  This module allows users to create custom unit parsers. The user specifies a set of prefixes and a set of units to parse, and then a quasi-quoting parser is generated. See the module documentation for details.

• Data.Metrology.TH

  This module exports several functions, written with Template Haskell, that make programming with units somewhat easier. In particular, see declareMonoUnit, which gets rid of a lot of the boilerplate if you don’t want unit polymorphism.

• Data.Metrology.Quantity

  This module defines a Quantity class to enable easy, safe conversions with non-units types. See the module for more documentation.

Examples

We will build up a full working example in several sections. It is awkward to explain the details of the pieces until the whole example is built, so please read on to see how it all works. For more complete(-ish) examples, see this test case (for examples of how to use units) and units-defs (for examples of how to define units).

Dimension definitions

When setting up your well-typed units-of-measure program, the first step is to define the dimensions you will be working in. (If your application involves physical quantities, you may want to check Data.Dimensions.SI in the units-defs package first.)

data LengthDim = LengthDim  -- each dimension is a datatype that acts as its own proxy
instance Dimension LengthDim

data TimeDim = TimeDim
instance Dimension TimeDim

We can now build up dimensions from these base dimensions:

type VelocityDim = LengthDim :/ TimeDim

Unit definitions

We then define units to work with these dimensions. Here, we define two different inter-convertible units for length. (Note that just about all of this boilerplate can be generated by functions in the Data.Metrology.TH module.)

data Meter = Meter
instance Unit Meter where           -- declare Meter as a Unit
  type BaseUnit Meter = Canonical   -- Meters are "canonical"
  type DimOfUnit Meter = LengthDim  -- Meters measure Lengths
instance Show Meter where           -- Show instances are optional but useful
  show _ = "m"                      -- do *not* examine the argument!

data Foot = Foot
instance Unit Foot where
  type BaseUnit Foot = Meter        -- Foot is defined in terms of Meter
  conversionRatio _ = 0.3048        -- do *not* examine the argument!
                                    -- We don't need to specify the `DimOfUnit`;
                                    -- it's implied by the `BaseUnit`.
instance Show Foot where
  show _ = "ft"

data Second = Second
instance Unit Second where
  type BaseUnit Second = Canonical
  type DimOfUnit Second = TimeDim
instance Show Second where
  show _ = "s"

A unit assignment

To perform computations with units, we must define a so-called local coherent set of units, or LCSU. This is a mapping from dimensions to units, and it informs exactly how the quantities are stored. For example:

type LCSU = MkLCSU '[(LengthDim, Meter), (TimeDim, Second)]

This definition says that we wish to store lengths in meters and times in seconds. Note that, even though Meter is defined as the Canonical length, we could have used Foot in our LCSU. Canonical units are used only in conversion between units, not the choice of how to store a quantity.

Value types

To use all these pieces to build the actual type that will store quantities, we use one of the MkQu_xxx type synonyms, as follows:

type Length = MkQu_DLN LengthDim LCSU Double
  -- Length stores lengths in our defined LCSU, using `Double` as the numerical type
type Length' = MkQu_ULN Foot LCSU Double
  -- same as Length. Note the `U` in `MkQu_ULN`, allowing it to take a unit

type Time = MkQu_DLN TimeDim LCSU Double

Some computations

We now show some example computations on the defined types:

extend :: Length -> Length            -- a function over lengths
extend x = redim $ x |+| (1 % Meter)

inMeters :: Length -> Double          -- extract the # of meters
inMeters = (# Meter)                  -- more on this later

conversion :: Length                  -- mixing units
conversion = (4 % Meter) |+| (10 % Foot)

vel :: Length %/ Time                 -- The `%*` and `%/` operators allow
                                      -- you to combine types
vel = (3 % Meter) |/| (2 % Second)

Explanation

Let’s pick this apart. The data LengthDim = LengthDim declaration creates both the type LengthDim and a term-level proxy for it. It would be possible to get away without the proxies and use lots of type annotations, but who would want to? We must define an instance of Dimension to declare that LengthDim is a dimension. Why suffix with Dim? To distinguish the length dimension from the length type. Generally, the type is mentioned more often and should be the shorter name.

We then create a TimeDim to operate alongside the LengthDim. Using the :/ combinator, we can create a VelocityDim out of the two dimensions defined so far. See below for more information on unit combinators.

Then, we make some units, using similar data definitions. We define an instance of Unit to make Meter into a proper unit. The Unit class is primarily responsible for handling unit conversions. In the case of Meter, we define that as the canonical unit of length, meaning that all lengths will internally be stored in meters. It also means that we don’t need to define a conversion ratio for meters. You will also see that we say that Meters measure the dimension LengthDim, through the DimOfUnit declaration.

We also include a Show instance for Meter so that lengths can be printed easily. If you don’t need to show your lengths, there is no need for this instance.

When defining Foot, we say that its BaseUnit is Meter, meaning that Foot is inter-convertible with Meter. This declaration also says that the dimension measured by a Foot must be the same as the dimension for a Meter. We must then define the conversion ratio, which is the number of meters in a foot. Note that the conversionRatio method must take a parameter to fix its type parameter, but it must not inspect that parameter. Internally, it will be passed undefined quite often.

The definition for Second is quite similar to that for Meter.

The next section of code constructs an “LCSU” – a local coherent set of units. The idea is that we wish to be able to choose a set of units which are to be used in the internal, concrete representation. An LCSU is just an association list giving a concrete unit for each dimension in your domain. The particular LCSU here says that length is stored in meters and time is stored in seconds. It would be invalid to specify an LCSU with repeats for either dimension or unit.

With all this laid out, we can make the types that store values. units exports several MkQu_xxx type synonyms that vary in the arguments they expect. MkQu_DLN, for example, takes a dimension, an LCSU, and a numerical type. With the definition above, Length is now a type suitable for storing lengths.

Note that Length and Length' are the same type. The MkQu machinery notices that these two are inter-convertible and will produce the same dimensioned quantity.

Note that, as you can see in the function examples at the end, it is necessary to specify the choice of unit when creating a quantity or extracting from a quantity. Thus, other than thinking about the vagaries of floating point wibbles and the Show instance, it is completely irrelevant which unit the concrete unit in the LCSU. The type Length defined here could be used equally well in a program that deals exclusively in feet as it could in a program with meters.

As a tangential note: I have experimented both with definitions like data Meter = Meter and data Meter = Meters (note the s at the end). The second often flows more nicely in code, but the annoyance of having to remember whether I was at the type level or the term level led me to use the former in my work.

Other features

Prefixes

Here is how to define the “kilo” prefix:

data Kilo = Kilo
instance UnitPrefix Kilo where
  multiplier _ = 1000

kilo :: unit -> Kilo :@ unit
kilo = (Kilo :@)

We define a prefix in much the same way as an ordinary unit, with a datatype and a constructor to serve as a proxy. Instead of the Unit class, though, we use the UnitPrefix class, which contains a multiplier method. As with other methods, this may not inspect its argument.

Due to the way units are encoded, it is necessary to explicitly apply prefixes with the :@ combinator (available at both the type and term level). It is often convenient to then define a function like kilo to make the code flow more naturally:

longWayAway :: Length
longWayAway = 150 % kilo Meter

longWayAwayInMeters :: Double
longWayAwayInMeters = longWayAway # Meter  -- 150000.0

Unit combinators

There are several ways of combining units to create other units. Units can be multiplied and divided with the operators :* and :/, at either the term or type level. For example:

type MetersPerSecond = Meter :/ Second
type Velocity1 = MkQu_ULN MetersPerSecond LCSU Double

speed :: Velocity1
speed = 20 % (Meter :/ Second)

The units package also provides combinators “%*” and “%/” to combine the types of quantities.

type Velocity2 = Length %/ Time    -- same type as Velocity1

There are also exponentiation combinators :^ (for units) and %^ (for quantities) to raise to a power. To represent the power, the units package exports Zero, positive numbers One through Five, and negative numbers MOne through MFive. At the term level, precede the number with a p (mnemonic: “power”). For example:

type MetersSquared = Meter :^ Two
type Area1 = MkQu_ULN MetersSquared LCSU Double
type Area2 = Length %^ Two        -- same type as Area1

roomSize :: Area1
roomSize = 100 % (Meter :^ pTwo)

roomSize' :: Area1
roomSize' = 100 % (Meter :* Meter)

Note that addition and subtraction on units does not make physical sense, so those operations are not provided.

Dimension-safe cast

The haddock documentation shows the term-level quantity combinators. The only one deserving special mention is redim, the dimension-safe cast operator. Expressions written with the units package can have their types inferred. This works just fine in practice, but the types are terrible, unfortunately. Much better is to use top-level annotations (using abbreviations like Length and Time) for your functions. However, it may happen that the inferred type of your expression and the given type of your function may not exactly match up. This is because quantities have a looser notion of type equality than Haskell does. For example, “meter * second” should be the same as “second * meter”, even though these are in different order. The redim function checks (at compile time) to make sure its input type and output type represent the same underlying dimension and then performs a cast from one to the other. This cast is completely free at runtime. When providing type annotations, it is good practice to start your function with a redim $ to prevent the possibility of type errors. For example, say we redefine velocity a different way:

type Velocity3 = (MkQu_ULN Number LCSU Double) %/ Time %* Length
addVels :: Velocity1 -> Velocity1 -> Velocity3
addVels v1 v2 = redim $ v1 |+| v2

This is a bit contrived, but it demonstrates the point. Without the redim, the addVels function would not type-check. Because redim needs to know its result type to type-check, it should only be used at the top level, such as here, where there is a type annotation to guide it.

Note that redim is always dimension-safe – it will not convert a time to a length!

Monomorphic behavior

units provides a facility for ignoring LCSUs, if your application does not need to worry about numerical precision. The facility is through the type family DefaultUnitOfDim. For example, with the definitions above, we could say

type instance DefaultUnitOfDim LengthDim = Meter
type instance DefaultUnitOfDim TimeDim   = Second

and then use the DefaultLCSU for our LCSU. To make the use of the default LCSU even easier, the MkQu_xxx operators that don’t mention an LCSU all use the default one. So, we can say

type Length = MkQu_D LengthDim

and get to work. (This uses Double as the underlying numerical representation.)

The module Data.Metrology.SI from the units-defs package exports type instances for DefaultUnitOfDim for the SI types, meaning that you can use definitions like this right away.

More examples

Check out some of the test examples we have written to get more of a feel for how this all works, here.