code: private ByteBuffer convertValueByFunction(Tree functionCall, CfDef columnFamily, ByteBuffer columnName, boolean withUpdate)
      {
          String functionName = functionCall.getChild(0).getText();
          Tree argumentTree = functionCall.getChild(1);
          String functionArg  = (argumentTree == null) ? "" : CliUtils.unescapeSQLString(argumentTree.getText());
          AbstractType<?> validator = getTypeByFunction(functionName);
  
          try
          {
  
              ByteBuffer value;
  
              if (functionArg.isEmpty())
              {
                  if (validator instanceof TimeUUIDType)
                  {
                      value = ByteBuffer.wrap(UUIDGen.getTimeUUIDBytes());
                  }
                  else if (validator instanceof LexicalUUIDType)
                  {
                      value = ByteBuffer.wrap(UUIDGen.decompose(UUID.randomUUID()));
                  }
                  else if (validator instanceof BytesType)
                  {
                      value = ByteBuffer.wrap(new byte[0]);
                  }
                  else
                  {
                      throw new RuntimeException(String.format("Argument for '%s' could not be empty.", functionName));
                  }
              }
              else
              {
                  value = getBytesAccordingToType(functionArg, validator);
              }
  
              // performing ColumnDef local validator update
              if (withUpdate)
              {
                  updateColumnMetaData(columnFamily, columnName, validator.toString());
              }
  
              return value;
          }
          catch (Exception e)
          {
              throw new RuntimeException(e);
          }
      }
docs: Used to convert value (function argument, string) into byte[]
  @param functionCall - tree representing function call ^(FUNCTION_CALL function_name value)
  @param columnFamily - column family definition (CfDef)
  @param columnName   - column name as byte[] (used to update CfDef)
  @param withUpdate   - also updates column family metadata for given column
  @return byte[] - string value as byte[]