Description

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Version

• 1.4 The predefined quantities have been put into unit-definitions.scm
• 1.3 Bug fix in unit-convert
• 1.2 Changed unit-convert to return a single numeric value given a single conversion argument
• 1.1 Added information units [patch by Joshua Griffith]
• 1.0 Initial release

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The unitconv library is an implementation of unit conversion routines based on the paper by Gordon S. Novak:

Conversion of Units of Measurement.
IEEE Trans. on Software Engineering, vol. 21, no. 8 (Aug. 1995), pp. 651-661.

Correctness of unit conversion is established by the technique of dimensional analysis: the source and goal units must have the same dimensions. Following Novak, this extension defines a dimension as an 8-vector of integral powers of the following base quantities:

(define-base-quantity  Unity          0)
(define-base-quantity  Length         dimvals[0])
(define-base-quantity  Time           dimvals[1])
(define-base-quantity  Temperature    dimvals[2])
(define-base-quantity  Mass           dimvals[3])
(define-base-quantity  Current        dimvals[4])
(define-base-quantity  Luminosity     dimvals[5])
(define-base-quantity  Substance      dimvals[6])
(define-base-quantity  Currency       dimvals[7])
(define-base-quantity  Information    dimvals[8])

The unit conversion routine uses dimension integers to check that the requested unit conversion is legitimate. For example, the conversion from kilograms to meters illegal. Consequently, the dimensionality of each unit must be specified when the unit is declared.

;; Syntax is (define-unit name quantity factor abbreviation ...)

;; define units of length
(define-unit meter     Length 1.0       m meters)
(define-unit inch      Length 0.0254    in inches)

;; define units of mass and time
(define-unit kilogram  Mass   1.0       kg kilograms)
(define-unit second    Time   1.0       s seconds)

;; define two new derived quantities: acceleration and force
(define-quantity   Acceleration  (/ Length (** Time 2)))
(define-quantity   Force         (* Mass Acceleration))

;; define a unit of force
(define-unit newton       Force (/ (* kilogram meter) (* second second)) nt newtons)

Now only conversion between units of the same dimensionality is permitted:

(unit-convert meter inch 1) ->  39.3700787401575
(unit-convert meter inch 2 3 4) ->  (78.740157480315 118.110236220472 157.48031496063)

(unit-convert meter kilogram 1)
Error: (unitconv) unit-convert : given units are of different dimensions: 
 source= #(unit meter (m meters) [Length] 1.0) ; 
 dest=  #(unit kilogram (kg kilograms) [Mass] 1.0)

quantity-expr ::=  base-quantity
                 | derived-quantity
                 | (* quantity-expr quantity-expr)
                 | (* quantity-expr integer)
                 | (/ quantity-expr quantity-expr)
                 | (/ quantity-expr integer)
                 | (** quantity-expr integer)