Haskell - Type conversion? - haskell

Question

I'm trying to convert data from a list, 'profile1', into a custom type called 'DataSubject'.
I'm passing this to a function 'makeDS' to attempt this conversion - however the following isn't working:
type Name = String
type Age = Int
type Iq = Int
type Language = String
data DataSubject = DS {name :: Name, age :: Age, iq :: Iq, language :: Language} deriving (Show)
data Contain = Name String | Age Int | Iq Int | Language String deriving (Show) --Use so list can take multiple types
profile1 = [Name "Bob", Age 22, Iq 100, Language "French"]
makeDS :: [Contain] -> DataSubject
makeDS t = DS {name = t!!0, age = t!!1, iq = t!!2, language = t!!3}
main = do
let x = makeDS profile1
putStrLn $ show x
Error:
Couldn't match type ‘Contain’ with ‘[Char]’
I'm just getting started with Haskell - could someone advise on my error? And if there's better ways of doing this?

Answer 1

In the definition of makeDS, the variable t is of type [Contain] (i.e. a list of Contain), so when you say t!!0 this will extract the first element of that list, which has type Contain. The problem is that the name field of DataSubject contains a String (which is an alias of [Char]). So you are trying to store a Contain in the place of [Char], which is not possible because the types are different. You need a different approach in you code.
One issue is that every single Contain value represents a single field of DataSubject. So if we are given a list of Contain, there is no guarantee that the values will be given in a specific order (e.g. Name first, followed by Age, etc) or even that all fields are provided. Even if you always provide all fields in a specific order in your code as convention, haskell cannot possibly know that. One solution that does not depend on order is to try to "build" the DataSubject object step-by-step, by starting with an "empty" DataSubject and then examining the list of Contain and adding the corresponding DataSubject field:
makeDS :: [Contain] -> DataSubject
makeDS = foldr updateDS emptyDS
where
updateDS (Name s) ds = ds {name = s}
updateDS (Age n) ds = ds {age = n}
updateDS (Iq n) ds = ds {iq = n}
updateDS (Language s) ds = ds {language = s}
emptyDS = DS {name = "", age = 0, iq = 0, language = ""}
So here, I defined emptyDS which is an "empty" DataSubject object and a function called updateDS which take a (single) Contain and a DataSubject and updates the DataSubject based on the field specified by Contain and then it returns it. Finally, I use a fold to run repeatedly update the DataSubject (starting with emptyDS) using updateDS.

Answer 2

You have a type mismatch. You have a list of Contain. So when you use
t !! 0
you get a Contain, not a String, which is necessary for name in DS. You need a function Contain -> Name, e.g.
containToName :: Contain -> Name
containToName (Name xs) = xs
containToName _ = error "not a name"
However, that's a partial function, since containToName (Age 12) will lead to an error.
Note that this has nothing to do with typeclasses. Now, if you want to use profile1, one way would be to just use
profile1 :: DataSubject
instead of
profile1 :: [Contain]
e.g.
profile1 :: DataSubject
profile1 = DS "Bob" 22 100 "French"
After all, there's nothing in the type [Contain] that will make sure that you have all the ingredients for a complete DataSubject.
And if there's better ways of doing this?
That depends on what you want to do. If you just want to handle DataSubjects, don't use an temporary list of Contain. If you want to handle user input (or similar), it gets a little bit more tricky.

Answer 3

The declaration of DataSubject says that we need a Name for the name field. And Name is the same as String. So in the expression DS {name = t!!0, ...}, we need t !! 0 to return a String. However, t !! 0 returns an element of t, and t has type [Contain]. So t !! 0 has type Contain, which is different from String.
To fix this type error, you need to convert the Contain to a String, maybe so:
DS { name = case t !! 0 of Name s => s, ... }