Question

I have the following definitions

data AST
    = Number Integer
    | Identifier String
    | String String
    -- | Operation BinOp AST AST
    | Query String
    deriving (Show, Eq)

data Tuple = Tuple {cmd :: String, 
                    cid :: AST,
                    argumentList :: [AST],
                    queryList :: [AST]} deriving (Show, Eq)

and when I try to print for example cid I get the error

No instance for (Show (Tuple -> AST)) arising from a use of `print'

that is surprising me since both AST and Tuple are members of the show class. How can I make cid, argumentList and queryList printable?

Answer 1

cid is just a function which is used to access the “cid” field of your Tuple. It is impossible to print it, just as it’s impossible to print fst. However, this would be correct:

t :: Tuple
t = ...

main = print (cid t)