Question

I need to add my own data type to a list which is in a function, here’s my code:

type Car = (String, String, Int, String)

testDatabase :: [Car] 
testDatabase = [("Ford", "Petrol", 2006, "Sport")]

addNewCar :: Car 
addNewCar newCar = newCar:testDatabase

Here’s the error I get:

ERROR file:.\template.hs:20 - Type error in explicitly typed binding
*** Term : addNewCar
*** Type : ([Char],[Char],Int,[Char]) -> [([Char],[Char],Int,[Char])]
*** Does not match : Car

(sorry its a rubbish explanation im just struggling a tad with Haskell). Thank you in advance!!

Ash!

Answer 1

The inferred type of addNewCar is ([Char],[Char],Int,[Char]) -> [([Char],[Char],Int,[Char]), which is the same as Car -> [Car]. This type says that addNewCar is a function which takes a car and returns a list of cars. This is exactly the type you want.

However your type signature says that addNewCar is simply a value of type Car. This is wrong and clashes with the inferred type. That’s why you get the error. So to fix this, simply remove the type signature or change it to addNewCar :: Car -> [Car].