Use a struct type as T in value_set<T>

[hanw]

Is the following P4 code correct syntax when the type T in PVS is a struct?

struct my_packet {
  bit<32> data;
}

struct value_set_t {
  bit<32> field;
}

parser MyParser(packet_in b, out my_packet p, inout my_metadata m, inout standard_metadata_t s) {

    value_set<value_set_t>(4) pvs;

    state start {
        b.extract(p.data);
        transition select({ p.data }) {
            pvs: accept;
            {0x810} : foo;
        }
    }

    state foo {
        transition accept;
    }
}

When I compile with p4test at commit d50facb, type unification failed at the constant key expression {0x810}.

error: SelectExpression: Cannot unify tuple type Tuple(1) with non tuple-type int
        transition select({ p.data.da }) {

Complete test program below:
pvs.p4.gz

[hanw]

An alternative syntax would be

parser MyParser(packet_in b, out my_packet p, inout my_metadata m, inout standard_metadata_t s) {

    value_set<value_set_t>(4) pvs;

    state start {
        b.extract(p.data);
        transition select(p.data) {
            pvs: accept;
            0x810 : foo;
        }
    }

    state foo {
        transition accept;
    }
}

I am not sure which one should we support.

[jafingerhut]

Do you get a similar error message if the struct contains 2 fields, and the transition select expression is a list of 2 expressions with type bit<W>?

Or does the error only occur for a struct with 1 field, and a list of 1 value?

[mihaibudiu]

We should support the second variant: the most natural one for the user, even if the compiler has to do a bit more work. I will tag these as enhancements, because they are in line with the current spec; we should update the spec to allow struct types for value sets.

[hanw]

Updated text and example in spec to allow struct type for the value set as part of p4lang/p4-spec#602 p4lang/p4-spec@278afae