Constructing a value of my own data type in Haskell

Question

I am trying to construct a new value of type MyData , which is a personal data type I defined, but I am getting an error. MyData has a constructor List which receives a list of MyData values. I am trying to extract two elements of MyData from the List type and create a new one using the Two And constructor. The parser works perfectly fine, I checked.

data MyData = Var String | Con Bool | Two TwoOp MyData MyData | List [MyData] 
 deriving Show
data Final= F MyData MyData deriving Show
data TwoOp = And | Or deriving Show

add_and (Right (F (List e1) (List e2))) i1 i2 = do
 a<-e1!!i1
 b<-e1!!i2
 return (Two And a b)

 val (Right (F e1 e2))=add_and (Right (F e1 e2)) 0 1
 parse_value string_expression= val (parse getfinal "" string_expression)

If something is not clear, please ask. I have spent many hours trying to solve it, but don't know how.

Here is the error I am getting:

 Couldn't match type `MyData' with `m MyData'
    Expected type: [m MyData]
      Actual type: [MyData]
    Relevant bindings include
      add_and :: Either t Final -> Int -> Int -> m MyData
        (bound at D:\PF\final.hs:53:1)
    In the first argument of `(!!)', namely `e1'
    In a stmt of a 'do' block: a <- e1 !! i1

D:\PF\final.hs:55:5:
    Couldn't match type `MyData' with `m MyData'
    Expected type: [m MyData]
      Actual type: [MyData]
    Relevant bindings include
      add_and :: Either t Final -> Int -> Int -> MyData
        (bound at D:\PF\final.hs:53:1)
    In the first argument of `(!!)', namely `e1'
    In a stmt of a 'do' block: b <- e1 !! i2
Failed, modules loaded: none.

Answer 1

You don't want to use <- to extract values from the list.

In this case you just want to use pattern matching:

add_and :: Final -> MyData
add_and (F (List (a:_)) (List (b:_))) = Two And a b
add_and _ = error "I don't know what to do here."

list1 :: MyData
list1 = List [ Var "abc" ]

list2 :: MyData  
list2 = List [ Con False ]

final :: Final
final = F list1 list2

main = print $ add_and final

When run in ghci:

*Main> main
Two And (Var "abc") (Con False)