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CHAPTER 5 M THE MODELING LANGUAGE
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Employees, for example, are in a different category than Cars. In the context of the M language, you use types for defining or expressing these categories.
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Intrinsic and Derived Types
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There are two type categories in M: intrinsic types and derived types. Intrinsic types, like Integer, Text, or Logical (true/false), are pre-defined in M and are understood by the M compiler. Derived types must be explicitly defined somewhere in M code, and a derived type definition will invoke one or more intrinsic types and/or other derived types. Let s start with a very simple example of what I mean by a derived type. I ll stick with the subject matter used in the last chapter for the time being: Cars. Figure 5-2 shows what you might have for a very simple definition of a Car type.
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Figure 5-2. CarType module an example of a type definition The Car type definition in Figure 5-2 starts with the type keyword, followed by the name of the type. The type definition then follows, scoped within braces. In this example, I ve named the type Car no surprise there. And I ve given the type three values: Mfr, Model, and Year. Mfr and Model must both conform to the intrinsic type Text, and Year must conform to the intrinsic type Integer16. The colon (:) operator following Mfr, Model, and Year in the type definition is called the ascription operator. It designates the ascribed type for the value. So Mfr and Model have the ascribed intrinsic type of Text, while Year has the ascribed intrinsic type of Integer16. The attributes, or values, of a given type are not necessarily constrained to be of an intrinsic type, like Text or Integer. Other derived types may be part of a new type definition. These other derived types may be defined within the module (namespace) of the type that requires it, or they may be defined within the scope of another module if they are made visible through import/export directives. (I ll talk about import/export in the section titled Modules Revisited: Import and Export Directives. ) So let s take this to the next step by adding a value that has a derived, or non-intrinsic, type: Engine. I ll add a new derived type for Engine within the scope of the module shown in the last code snippet, and add an Engine value to the Car type definition (see Figure 5-3).
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CHAPTER 5 M THE MODELING LANGUAGE
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Figure 5-3. CarType module with an Engine type added If you compare this code with that shown in Figure 5-2, you ll notice I ve changed some of the value definitions (Integer to Unsigned) and added some constraints. Rather than defining the Year of the Car type to be an Integer16 (which could be of any reasonable or unreasonable value, and which would allow the year to be negative), I ve reset it to be an Unsigned16 integer in the range 1769 (when the first steam-powered car was made) to 2020. There are similar reality-based constraints on the value definitions for the Engine type: no less than 1 and no more than 12 cylinders and no more than 1,000 horsepower. I ve added an identity, or Id value for both the Car and Engine types. There are several reasons for doing this: You might have two distinct cars with the same manufacturer, model, year, and engine type, and no definitive way of distinguishing these two instances without having unique identities. The M language specification requires that if a derived type (Car in this context) includes another derived type as a value (such as Engine), the included type must have a unique identity. If the type is used to define an extent ( table), the type should provide a SQL unique identity that can map to the primary key of the table.
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The AutoNumber() function provides a way of establishing a unique identity for any type instance. With this example in hand, let s look at types from a more abstract level. Types are simply a way of defining a collection of values. The M language is structurally typed rather than nominally typed. This is a fancy way of saying that if a collection of values conforms to two types, even though the types have different names, the types are equivalent as far as the M compiler is concerned. If you were to define a type named Boat that had the same structure as defined in Figure 5-3 for Car, the M compiler would be indifferent between the two types.
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