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119 lines
4.7 KiB
Go
119 lines
4.7 KiB
Go
// Copyright ©2013 The Gonum Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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//go:generate go run generate_unit.go
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// Package unit provides a set of types and constants that facilitate
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// the use of the International System of Units (SI).
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//
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// The unit package provides two main functionalities: compile-time type-safe
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// base SI units and common derived units; and a system for dynamically
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// extensible user-defined units.
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//
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// Static SI units
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//
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// This package provides a number of types representing either an SI base
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// unit or a common combination of base units, named for the physical quantity
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// it represents (Length, Mass, Pressure, etc.). Each type is defined from
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// float64. The value of the float64 represents the quantity of that unit as
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// expressed in SI base units (kilogram, metre, Pascal, etc.). For example,
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//
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// height := 1.6 * unit.Metre
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// acc := unit.Acceleration(9.8)
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//
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// creates a variable named 'height' with a value of 1.6 metres, and
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// a variable named 'acc' with a value of 9.8 metres per second squared.
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// These types can be used to add compile-time safety to code. For
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// example,
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//
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// func unitVolume(t unit.Temperature, p unit.Pressure) unit.Volume {
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// ...
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// }
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//
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// func main(){
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// t := 300 * unit.Kelvin
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// p := 500 * unit.Kilo * unit.Pascal
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// v := unitVolume(p, t) // compile-time error
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// }
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//
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// gives a compile-time error (temperature type does not match pressure type)
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// while the corresponding code using float64 runs without error.
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//
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// func float64Volume(temperature, pressure float64) float64 {
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// ...
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// }
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//
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// func main(){
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// t := 300.0 // Kelvin
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// p := 500000.0 // Pascals
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// v := float64Volume(p, t) // no error
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// }
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//
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// Many types have constants defined representing named SI units (Metre,
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// Kilogram, etc. ) or SI derived units (Pascal, Hz, etc.). The unit package
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// additionally provides untyped constants for SI prefixes, so the following
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// are all equivalent.
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//
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// l := 0.001 * unit.Metre
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// k := 1 * unit.Milli * unit.Metre
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// j := unit.Length(0.001)
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//
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// Additional SI-derived static units can also be defined by adding types that
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// satisfy the Uniter interface described below.
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//
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// Dynamic user-extensible unit system
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//
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// The unit package also provides the Unit type, a representation of a general
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// dimensional value. Unit can be used to help prevent errors of dimensionality
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// when multiplying or dividing dimensional numbers defined a run time. New
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// variables of type Unit can be created with the New function and the
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// Dimensions map. For example, the code
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//
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// rate := unit.New(1 * unit.Milli, Dimensions{MoleDim: 1, TimeDim: -1})
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//
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// creates a variable "rate" which has a value of 1e-3 mol/s. Methods of
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// unit can be used to modify this value, for example:
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//
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// rate.Mul(1 * unit.Centi * unit.Metre).Div(1 * unit.Milli * unit.Volt)
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//
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// To convert the unit back into a typed float64 value, the From methods
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// of the dimensional types should be used. From will return an error if the
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// dimensions do not match.
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//
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// var energy unit.Energy
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// err := energy.From(acc)
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//
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// Domain-specific problems may need custom dimensions, and for this purpose
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// NewDimension should be used to help avoid accidental overlap between
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// packages. For example, results from a blood test may be measured in
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// "White blood cells per slide". In this case, NewDimension should be
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// used to create a 'WhiteBloodCell' dimension. NewDimension takes in a
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// string which will be used for printing that dimension, and will return
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// a unique dimension number.
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//
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// wbc := unit.NewDimension("WhiteBloodCell")
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//
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// NewDimension should not be used, however, to create the unit of 'Slide',
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// because in this case slide is just a measurement of liquid volume. Instead,
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// a constant could be defined.
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//
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// const Slide unit.Volume = 0.1 * unit.Micro * unit.Litre
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//
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// Note that unit cannot catch all errors related to dimensionality.
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// Different physical ideas are sometimes expressed with the same dimensions
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// and unit is incapable of catching these mismatches. For example, energy and
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// torque are both expressed as force times distance (Newton-metres in SI),
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// but it is wrong to say that a torque of 10 N·m is the same as 10 J, even
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// though the dimensions agree. Despite this, using the defined types to
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// represent units can help to catch errors at compile-time. For example,
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// using unit.Torque allows you to define a statically typed function like so
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//
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// func LeverLength(apply unit.Force, want unit.Torque) unit.Length {
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// return unit.Length(float64(want)/float64(apply))
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// }
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//
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// This will prevent an energy value being provided to LeverLength in place
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// of a torque value.
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package unit // import "gonum.org/v1/gonum/unit"
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