Syntax

do select type record-expression as alias
 (case type-name local scope)*
 (else local-scope)?
done

You can use the do select-type block to determine the type of a record at runtime.

  declare record publication
   field stream name
   field stream publisher
  
  declare record book
   extends publication
   field stream author variable
   field stream year-of-publication
   field stream ISBN
  
  declare record periodical
   extends publication
   field integer issues-per-year
   field stream editor-in-chief
   field stream ISPN
  
  declare record novel
   extends book
   field stream genre
  
  process
     local publication pubs variable
     local novel war-and-peace
     local periodical field-and-stream
  
     set new pubs to war-and-peace
     set new pubs to field-and-stream
  
     repeat over pubs
        do select-type pubs as p
           case book
              output "Book%n"
           case periodical
              output "Periodical%n"
           case publication
              output "Publication%n"
        done
     again

The types specified in the case statements must be the type of the expression in the select-type statement, or an extension of that type. This means that do select-type is useful only in cases where you have a shelf of a base type that may contain records of extended types and you need to determine the exact types of those records.

If the exact type of the tested expression is not specified in a case, then the closest base type will be selected. Thus the code above will print out:

  Book
  Periodical

The type of the first record on the pubs shelf is "novel" but there is no case for "novel". Novel is extended from "book", so case book is selected. You can change the result by adding a case novel to the program.

The do select-type statement includes an alias specified by as. This alias, which must be a legal OmniMark name, can be used in the case blocks to address the specific properties of the record.

The record expression specified in the do select-type statement is a base type of the type of the specified record. You cannot use that expression to access the record fields that belong only to the extended type. The alias, by contrast, is treated as a variable of the type specified in the case statement. Thus you can query all the properties of that specific type using the alias name:

  declare record point
   field integer x
   field integer y
  
  declare record pixel
   extends point
   field stream color
  
  declare record point-3d
   extends point
   field integer z
  
  process
   local pixel a
   local point b variable
  
   set a:x to 12
   set a:y to 15
   set a:color to "RED"
  
   set new b to a
  
   do select-type b as pt
      case pixel
         output pt:color || "%n"
      case point-3d
         output "d" % pt:z || "%n"
      case point
         output "d" % pt:x || "," || "d" % pt:y
      else
         output "I'm stumped%n"
   done

A do select-type statement may include an else clause which will be selected in case that none of the case statement match the type of the tested expression, or the types that it extends.

Related Syntax    cast

Related Concepts    Records, extended

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