Default values in Haskell data types

Question

When you define a class in a object-oriented language it usually sets the default values for the member variables. Is there any mechanism in Haskell to do the same thing in record types? And a follow up question: If we don't know from the very start all the values for a data constructor but we obtain them from IO interaction can we build the type using something like the builder pattern from OOP?

Thanks in advance

Answer 1

A common idiom is to define a default value.

data A = A { foo :: Int , bar :: String }

defaultA :: A
defaultA = A{foo = 0, bar = ""}

This can be then (purely) "updated" later on with real values.

doSomething :: Bool -> A
doSomething True  = defaultA{foo = 32}
doSomething False = defaultA{bar = "hello!"}

Pseudocode example:

data Options = O{ textColor :: Bool, textSize :: Int, ... }

defaultOptions :: Options
defaultOptions = O{...}

doStuff :: Options -> IO ()
doStuff opt = ...

main :: IO ()
main = do
     ...
     -- B&W, but use default text size
     doStuff defaultOptions{ color = False }

If there are no sensible default values, you can wrap the field values in Maybe.

If you feel adventurous, you can even use a more advanced approach to statically separate "intermediate" options values, which can lack a few fields, from "finalized" ones, which must have all the fields. (I'd not recommend this to Haskell beginners, though.)