Added structure-valued expressions

p4-16/spec/P4-16-spec.mdk

@@ -3677,6 +3677,50 @@ tuple<bit<32>, bool> x = { 10, false };
 
 The empty list expression has type `tuple<>` - a tuple with no components.
 
+## Structure-valued expressions { #sec-structure-expressions}
+
+One can write expressions that evaluate to a structure or header.  The
+syntax of these expressions is given by:
+
+~ Begin P4Grammar
+expression ...
+    | '{' kvList '}'
+    | '(' typeRef ')' expression
+    ;
+
+kvList
+    : kvPair
+    | kvList "," kvPair
+    ;
+
+kvPair
+    : name "=" expression
+    ;
+~ End P4Grammar
+
+For a structure-valued expression `typeRef` is the name of a `struct`
+or `header` type.  The `typeRef` can be omitted if it can be inferred
+from context, e.g., when initializing a variable with a `struct` type.
+The following example shows a structure-valued expression used in an
+equality comparison expression:
+
+~ Begin P4Example
+struct S {
+    bit<32> a;
+    bit<32> b;
+}
+
+S s;
+...
+// Compare s with a structure-valued expression
+bool b = s == (S) { a = 1, b = 2 };
+~ End P4Example
+
+Structure-valued expressions can be used in the right-hand side of
+assignments, in comparisons, in field selection expressions, and as
+arguments to functions, method or actions.  Structure-valued
+expressions are not left values.
+
 ## Operations on sets { #sec-set-exprs }
 
 Some P4 expressions denote sets of values (`set<T>`, for some type `T`;
@@ -3824,35 +3868,17 @@ their corresponding fields are equal.
 
 ## Structure initializers { #sec-structure-initializers }
 
-Structures can be initialized using structure initializers, which
-specify explicitly the intialized fields.  A structure initializer
-expression evaluates to a struct; it can be used on the right-hand
-side of an initialization of a variable with a struct type.
-
-~ Begin P4Grammar
-expression ...
-    | '{' kvList '}'
-    ;
-
-kvList
-    : kvPair
-    | kvList "," kvPair
-    ;
-
-kvPair
-    : name "=" expression
-    ;
-~ End P4Grammar
-
-The following example shows a structure initialized using a
-structure initializer:
+Structures can be initialized using structure-valued expression
+([#sec-structure-expressions]).  The following example shows a
+structure initialized using a structure-valued expression:
 
 ~ Begin P4Example
 struct S {
     bit<32> a;
     bit<32> b;
 }
 const S x = { a = 10, b = 20 };
+const S x = (S){ a = 10, b = 20 }; // equivalent
 ~ End P4Example
 
 The compiler must raise an error if a field name appears more than