Matching data types in Haskell. Conversion error-CodePudding

I am trying to write the function trade_report :: String -> [Transaction] -> String which will take an element in a list and return the values associated with the element.

For context in what the Type transaction is:

type Transaction = (Char, Int, Int, String, Int) 

test_log :: [Transaction]
test_log = [('B', 100, 1104,  "VTI",  1),
            ('B', 200,   36, "ONEQ",  3),
            ('B',  50, 1223,  "VTI",  5),
            ('S', 150, 1240,  "VTI",  9),
            ('B', 100,  229, "IWRD", 10),
            ('S', 200,   32, "ONEQ", 11), 
            ('S', 100,  210, "IWRD", 12)
            ]

CodePudding user response：

get_trades returns a list of Transaction values. transaction_to_string only takes a single Transaction value. You need to use map.

> :t transaction_to_string
transaction_to_string :: Transaction -> String
> :t map transaction_to_string
map transaction_to_string :: [Transaction] -> [String]

Once you have your list of String values, you can join them together with \n characters using Data.List.intercalate.

> import Data.List
> intercalate "\n" ["foo", "bar"]
"foo\nbar"