Enumerations

An enumeration is a set of named constants that can apply to a particular operation. For example, when opening a file you typically want to specify whether the file is to be opened for reading, for writing or for both. A method that opens a file will take a parameter that specifies this. If this is implemented using an enumerated constant, then the parameter can be one of a specific set of (typically) integer values, for example, 1 = read, 2 = write, 3 = read and write. However, to avoid using ambiguous numbers in code, it is conventional to use names to represent particular values. These are known as enumerated constants or, more simply, as enums.

In .NET, enums are implemented as classes that inherit from the System.Enum base class. The class as a whole represents a set of enumerated constants; each of the constants is represented by a static field within the class.

Typically, an enumerated constant would be used as a parameter to a method or to specify the value of a property. For example, the DayOfWeek property of the DateTime object returns a value of Type System.DayOfWeek (it is incidental that both the Type and property are called DayOfWeek):

      ⎕USING←'' 'System'
      cal←⎕NEW DateTime(1981 09 23)
      cal.DayOfWeek
Wednesday
      cal.DayOfWeek.GetType
System.DayOfWeek
      System.DayOfWeek.⎕NL ¯2
Friday  Monday  Saturday  Sunday  Thursday  Tuesday  Wednesday

The function System.Convert.ToBase64String has some constructor overloads that take an argument of Type System.Base64FormattingOptions, which is an enum:

    System.Convert.ToBase64String
System.String ToBase64String(Byte[])
...
      System.Base64FormattingOptions.⎕NL ¯2
InsertLineBreaks  None

Hence:

      (⎕UCS 13 )∊ System.Convert.ToBase64String(⊂⍳100) System.Base64FormattingOptions.InsertLineBreaks
1
      (⎕UCS 13 )∊ System.Convert.ToBase64String(⊂⍳100) System.Base64FormattingOptions.None
0

An enum has a value that can be used in place of the enum itself when such usage is unambiguous. For example, the System.Base64FormattingOptions.InsertLineBreaks enum has an underlying value of 1:

      Convert.ToInt32 Base64FormattingOptions.InsertLineBreaks
1

This means that the scalar value 1 can be used as the second parameter to ToBase64String:

      (⎕UCS 13 )∊ System.Convert.ToBase64String(⍳100) 1
1

However, this practice is not recommended. Not only does it make the code less clear, but also if a value for a property or a parameter to a method can be one of several different enum types, APL cannot tell which is expected and the call will fail.