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84 lines
3.9 KiB
Go
84 lines
3.9 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 autogen.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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// Unit provides two main functionalities.
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//
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// 1)
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// It provides a number of types representing either an SI base unit
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// or a common combination of base units, named for the unit it
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// represents (Length, Mass, Pressure, etc.). Each type has
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// a float64 as the underlying unit, and its value represents the
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// number of that underlying unit (Kilogram, Meter, Pascal, etc.).
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// For example,
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// height := 1.6 * unit.Meter
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// acc := unit.Acceleration(9.8)
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// creates a variable named 'height' with a value of 1.6 meters, and
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// a variable named 'acc' with a value of 9.8 meters 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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// func UnitDensity(t unit.Temperature, pressure unit.Pressure) (unit.Density){
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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 := 5 * unit.Bar
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// rho := UnitDensity(p, t) // compile-time error
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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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// func Float64Density(temperature, pressure float64) (float64){
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// ...
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// }
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// func main(){
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// t := 300.0 // degrees kelvin
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// p := 50000.0 // Pascals
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// rho := Float64Density(p, t) // no error
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// }
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// Many types have constants defined representing named SI units (Meter,
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// Kilogram, etc. ) or SI derived units (Bar, 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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// l := 0.001 * unit.Meter
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// k := 1 * unit.Milli * unit.Meter
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// j := unit.Length(0.001)
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//
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// 2)
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// Unit provides the type "Unit", meant to represent a general dimensional
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// value. unit.Unit can be used to help prevent errors of dimensionality
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// when multiplying or dividing dimensional numbers. This package also
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// provides the "Uniter" interface which is satisfied by any type which can
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// be converted to a unit. New varibles of type Unit can be created with
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// the New function and the Dimensions map. For example, the code
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// acc := New(9.81, Dimensions{LengthDim:1, TimeDim: -2})
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// creates a variable "acc" which has a value of 9.81 m/s^2. Methods of
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// unit can be used to modify this value, for example:
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// acc.Mul(1.0 * unit.Kilogram).Mul(1 * unit.Meter)
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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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// var energy unit.Energy
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// err := (*energy).From(acc)
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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. NewDimension should not be
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// used, however, to create the unit of 'Slide', because in this case slide
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// is just a measurement of area. Instead, a constant could be defined.
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// const Slide unit.Area = 0.001875 // m^2
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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-meters 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.
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package unit // import "gonum.org/v1/gonum/unit"
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