Protobuf3: How to describe map of repeated string?

The Official documentation about map type says:

map<key_type, value_type> map_field = N;

...where the key_type can be any integral or string type (so, any scalar type except for floating point types and bytes). The value_type can be any type.

I want to define a map<string, repeated string> field, but it seems illegal on my libprotoc 3.0.0, which complains Expected ">". So I wonder if there is any way to put repeated string into map.

A Possible workaround could be:

message ListOfString {
    repeated string value = 1;
}

// Then define:
map<string, ListOfString> mapToRepeatedString = 1;

But ListOfString here looks redundant.

Solution

I had the same need, and got the same error. I do not believe this is possible. Here is the relevant BNF definitions from the language specification.

https://developers.google.com/protocol-buffers/docs/reference/proto3-spec

messageType = [ "." ] { ident "." } messageName
mapField = "map" "<" keyType "," type ">" mapName "=" fieldNumber [ "["fieldOptions "]" ] ";"
type = "double" | "float" | "int32" | "int64" | "uint32" | "uint64"
  | "sint32" | "sint64" | "fixed32" | "fixed64" | "sfixed32" | "sfixed64"
  | "bool" | "string" | "bytes" | messageType | enumType
messageName = ident
ident = letter { letter | decimalDigit | "_" }
field = [ "repeated" ] type fieldName "=" fieldNumber [ "[" fieldOptions "]" ] ";"

"repeated" keyword only appears in the field definition. The map definition requires a "type", which does not include the repeated keyword.

That means there are a few options.

You could create a wrapper around the repeated value as you indicated.

There is the older way people defined define maps, which is more burdensome but is equivalent. This is the backwards compatible example from the language guide. https://developers.google.com/protocol-buffers/docs/proto3#maps

    message MapFieldEntry {
      key_type key = 1;
      repeated value_type value = 2;
    }
    repeated MapFieldEntry map_field = N;

You would need to convert the data to a map yourself, but this should be fairly trivial in most languages. In Java:

    List<MapFieldEntry> map_field = // Existing list from protobuf.
    Map<key_type, List<value_type>> = map_field.stream()
        .collect(Collectors.toMap(kv -> kv.key, kv -> kv.value));

Use google.protobuf.ListValue https://developers.google.com/protocol-buffers/docs/reference/google.protobuf#listvalue This is an untyped list collection from their well known types.