Getting a value from a custom datatype-CodePudding

I have a custom data type in Haskell. I have a custom type from which I later want to get a property and output it. In Java I would just go through the properties with the . (dot) operator, however this doesn't work in Haskell. How would I go about doing this?

This is the code I have now

data Person = P Name Address
type Name = String
type Address = String

x :: Person 
x = (P "abc" "def")

y :: Name
y = x.Name

main :: IO ()
main = putStrLn $ "The name is: "    y

CodePudding user response：

You can use pattern matching:

y = case x of P name _ -> name
y = name where P name _ = x
y = let P name _ = x in name

You could write a field accessor function:

name (P n _) = n
y = name x

The suggestion in the comments of y = (\(P name _) -> name) x is doing essentially this; the definition of name above is syntax sugar for

name = \(P n _) -> n

which gives the solution from the comment once its definition is inlined.

Or you could redefine your type with record syntax and have the compiler write your accessor:

data Person = P
    { name :: Name
    , address :: Address
    }

y = name x

Actually, if you do this, I think I'd probably omit the type aliases.

data Person = P
    { name :: String
    , address :: String
    }

They are just repeating information already available in the field name, and not actually buying you much; for example, the compiler won't prevent you from accidentally reading a name from one person and storing it in the address of another.