Unit of Measurement Classes

You can only coerce back and forth between measurement classes within the same category. The return values for these classes are the name of the data type followed by a space and its value. The return value of:

set theMeters to 6000 as meters

is “meters 6000.0” and its class is meters. The following code illustrates what is and is not permitted in terms of using these classes. The end of the code example shows the results from Script Editor’s Event Log window:

set theMeters to 6000 as meters
set theFeet to theMeters as feet
log theFeet
set theReal to (theFeet as inches) as real
log theReal
set cubYards to theReal as cubic yards
log cubYards
set cubYards to cubYards as yards

Event log:
(*feet 1.96850394E+4*) -- meters to feet returns as class 'feet'
(*2.362204728E+5*)-- feet to inches to real returns as class 'real'
(*cubic yards 2.362204728E+5*) -- real to cubic yards returns as class 'cubic 
yards'
--> Can't make cubic yards 2.362204728E+5 into a yards. (* returns an error,
because you cannot coerce a 'cubic yards' class into a 'yards' class.
*)

You can convert meters to feet (6000 meters is equivalent to about 19,685 feet), because both of these classes are in the “length” category. The previous example then converts feet to inches (also permitted because they are both length units) and stores that result as a real value type (i.e., a floating-point number that represents the number of inches in 6,000 meters). Then, since the number (stored in the variable theReal) is a real value type rather than a length-unit type, it can be coerced to cubic yards. cubic yards is in the cubic-volume category. But cubic yards cannot be coerced to yards, which is in the length category, thus giving rise to the error in the prior example.

These groups of data types make it very easy to create utility functions for calculating measurements. The next example is a subroutine that converts meters to feet or feet to meters (see Chapter 8). The second parameter to the ConvertMe function is a boolean. If it’s true, then the conversion is from meters to feet. If false, then the conversion is from feet to meters. Another way to accomplish this task is to check the class of a single parameter (i.e., val). If its class is feet, then convert it to meters, and vice versa. If its class is neither, then write the subroutine to return an error code such as -1.

on ConvertMe(val, toFeet)
   if toFeet then -- 3.2808399 feet equals one meter
      set conResult to val * 3.2808399
      set replyToUser to "Converting " & val & " meters to feet 
equals:¬ 
      " & (round conResult) & " feet" -- use round scripting addition
      display dialog replyToUser   (* display the conversion result using 
display-dialog scripting addition *)
   else -- one foot equals 0.304799999537 meters
      set conResult to val * 0.304799999537
      set replyToUser to "Converting " & val & " feet to meters 
equals:¬
      " & (round conResult) & " meters" -- use round scripting addition
      display dialog replyToUser  (* display the conversion result using 
display-dialog scripting addition *)
   end if
end ConvertMe

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Allowed coercions

Syntax

Description

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Examples