Sum of records bottom in Haskell

sum-of-records-bottom-haskell.md

Let me guess there is a data PL as a sum type of records:

data PL = Functional { name :: String, pure :: Bool }
        | ObjectOriented { name :: String, multipleInheritance :: Bool }

The type of pure is:

λ> :type pure
pure :: PL -> Bool

You may have already noticed the problem.

λ> let haskell = Functional "Haskell" True
λ> pure haskell
True
λ> let cpp = ObjectOriented "CPP" True
λ> pure cpp
*** Exception: No match in record selector pure

The problem is that it's a runtime error. Because pure has the type PL -> Bool, there is no way to enforce pure to be applied only to Functionals in compile time.

A possible solution will be like below.

data Fuctional = Functional { name :: String, pure :: Bool }
data ObjectOriented = ObjectOriented { name :: String, multipleInheritance :: Bool }

data PL = Functional' Functional
        | ObjectOriented' ObjectOriented

Then the type of pure will be Fuctional -> Bool.

Thanks!