I am writing a parser with the help of parsec and I have a problem

data Param = Param {paramName::String, argument :: Maybe String}
  deriving (Show)

paramExpr1 :: Parser Param
paramExpr1 = do
  paramKeyword
  void $ lexeme $ char '-'
  paramName <- word
  return $ Param paramName Nothing 

paramExpr3 :: Parser Param
paramExpr3 = do
  pN  <- paramExpr1 -- <- PROBLEM HERE
  return $ Param pN Nothing 

In short, I don’t understand how to get the named field, paramExpr1 will return Param and I would like to get paramName, but I don’t understand how

>Solution :

You can extract the field by using the field name as a function:

paramExpr3 :: Parser Param
paramExpr3 = do
  pN <- paramExpr1
  return $ Param (paramName pN) Nothing 

Alternatively, instead of creating a new value using the constructor Param, you can use the record update syntax and change the fields you want to change while leaving the others as they are.

paramExpr3 :: Parser Param
paramExpr3 = do
  pN <- paramExpr1
  return pN{ argument = Nothing }

More alternatives exist (e.g. using lenses), but these are the most basic approaches.