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--- language:types2:data [2024/07/18 16:14] – rajit
+++ language:types2:data [2024/07/18 16:23] (current) – [Structures] rajit
@@ Line 1: / Line 1: @@
-====== Data types ======
+====== User-defined data types ======
-A data type is defined using ''deftype''. A data type corresponds
+A user-defined data type is defined using ''deftype''. These are classified into two categories:
-to an integer or Boolean value, although it could also be a composite
+   * //data types//, which correspond to representations of the built-in ''int'' and ''bool'' types.
-construct like a record or structure (from software programming
+   * //structures//, which are a collection of data types.
-languages). The syntax is similar to a process, and the constraints
-about declarations/etc. apply here as well.
+===== Data types =====
+A data type corresponds to an integer or Boolean value, even though it
+could also be a composite construct like a record or structure (from software
+programming languages). The syntax is similar to a process, and the constraints
+about declarations/etc. apply here as well.  These data types are handled as
+the underlying built-in ''int'' or ''bool'' that they correspond to at the CHP
+level of abstraction.
 Often data types have some additional structure that is not required for a
@@ Line 49: / Line 56: @@
 type is supposed to represent a circuit structure that is used to
 represent a data value.
+==== Data methods ====
+There are two methods that can be specified for a data type:
+   - a //set method//, used to write a value to the type;
+   - a //get method//, used to read the value of the type.
+One can think of these as type conversion methods invoked
+automatically to read or write the data type. When a normal
+data type is used, the special variable ''self'' is implicitly
+defined to be the built-in type that is implemented by the
+user-defined data type.
+<code act>
+deftype d1of2 <: int<1> (bool d0,d1)
+{
+   spec {
+    exclhi(d0,d1)
+   }
+   methods {
+     set {
+       [self=1->d1-;d0+ [] self=0->d0-;d1+]
+     }
+     get {
+       [d0->self:=1 [] d1->self:=0]
+     }
+   }
+}
+</code>
+In the example above, the ''set'' method says that the way to set a
+''d1of2'' data type to the value ''0'' is to set ''d0'' to
+''false'' and ''d1'' to ''true''. The special variable
+''self'' is used to specify the ''int<1>'' value of the type, and
+the methods specify conversion operations.
+The selection statement in the ''get'' method uses the deterministic
+selection operator ''[]'' (see [[language:langs:hse|the hse sublanguage]]). This is an implicit check
+that when the ''get'' method is invoked, signals ''d0'' and ''d1''
+cannot both be ''true''. We have also made this explicit in the
+specification body. Also, if both ''d0'' and ''d1'' are false
+(i.e. an illegal state in which to execute a get operation), the
+variable ''self'' is not assigned; the operation waits for at least
+one of ''d0'' or ''d1'' to be true. This is viewed as an error for a
+data type. (This is different in the case of a channel, where the
+semantics of the channel permit waiting.)
@@ Line 67: / Line 119: @@
 The distinction between a structure and another data type is that other data types are
 implementations of one of the built-in types like ''int'' or ''bool.''
+==== Pure structures ====
+A pure structure is one that only contains other pure structures or ''int'' and ''bool'' components.
+In other words, they are structures that don't include channels. Structures like this correspond
+to what is normally viewed as a record/struct in a normal software programming language.