GDL Exercise 1: Implementing a GDL Schema

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Exercise

Implement a schema that creates three categories of attributes and does not impose restrictions on where constructs can be located.

These attributes must be divided into the following categories:

  • Attributes that can appear at the root level and within constructs.

  • Attributes that can appear only at the root level.

  • Attributes that can appear only within constructs.

Do not define any keywords in your schema; just include the framework for the future keywords.

Note   By using templates, you can create a virtual schema--that is, a schema that does not define any GDL entries. The base schema that is defined in this way exerts its influence no matter how this schema is extended in the future.

Solution

The following code example demonstrates one way to complete this exercise.

*Template:  ATTRIBUTE
{
    *Type:  ATTRIBUTE
    *Virtual:  TRUE
}
*Template:  ROOT_ATTRIB
{
    *Inherits: ATTRIBUTE
    *Virtual:  TRUE
}
*Template:     CONSTRUCT_ATTRIB  *%  May not appear at Root level
{
    *Inherits: ATTRIBUTE
    *Virtual:  TRUE
}
*Template:     FREEFLOAT
{
    *Inherits: ATTRIBUTE
    *Virtual:  TRUE
}
*Template:  CONSTRUCTS
{
    *Type: CONSTRUCT
    *Members:  ( CONSTRUCTS, FREEFLOAT, CONSTRUCT_ATTRIB)
    *Virtual:  TRUE
}

*Template:  ROOT
{
            *Type: CONSTRUCT
            *AllowNewMembers: FALSE
            *Name:  "root"
            *Instances:  <ANY>
            *Members:  (ROOT_ATTRIB, FREEFLOAT, CONSTRUCTS)
}

Note   You can place the templates in the preceding example in a MasterTemplate.gdl file for use by the next exercise.