presto-main/src/main/java/io/prestosql/operator/scalar/MapToJsonCast.java/MapToJsonCast/MapToJsonCast
class MapToJsonCast:    private MapToJsonCast()
    {
        super(OperatorType.CAST,
                ImmutableList.of(
                        castableToTypeParameter("K", VARCHAR.getTypeSignature()),
                        castableToTypeParameter("V", JSON.getTypeSignature())),
                ImmutableList.of(),
                JSON.getTypeSignature(),
                ImmutableList.of(mapType(new TypeSignature("K"), new TypeSignature("V"))),
                false);
    }