07 06 introducing new types

Similarly to typedef, the keyword type can be used to introduce a new type.

    | optAnnotations TYPE typeRef name

type bit<32> U32;
U32 x = (U32)0;

While similar to typedef, the type keyword introduces a new type which is not a synonym with the original type: values of the original type and the newly introduced type cannot be mixed in expressions.

Currently the types that can be created by the type keyword are restricted to one of: bit<>, int<>, bool, or types defined using type from such types.

One important use of such types is in describing P4 values that need to be exchanged with the control plane through communication channels (e.g., through the control-plane API or through network packets sent to the control plane). For example, a P4 architecture may define a type for the switch ports:

type bit<9> PortId_t;

This declaration will prevent PortId_t values from being used in arithmetic expressions without casts. Moreover, this declaration may enable special manipulation or such values by software that lies outside of the datapath (e.g., a target-specific toolchain could include software that automatically converts values of type PortId_t to a different representation when exchanged with the control-plane software).