gadt-answers.hs

{-# LANGUAGE GADTs #-}
{-# LANGUAGE Rank2Types #-}

{-
  Lets start ^^ by enabling some language extensions. GADTs to encode our data types and
  Rank2Types for the existental we will need to hide our types when we don't care about
  the invariant.
 -}


{-
  Lets start with our key data type, we want to be able to represent
  keys that point to a String, an Int or a Bool.
 -}
data Key a where
  S :: String -> Key String
  I :: String -> Key Int
  B :: String -> Key Bool

{-
  Lets than create an "entry in a bag" that holds any well typed
  key-value pair. Note well that we explicitly want to hide the
  invariants that our keys maintain at this point.
 -}
data Entry =
  forall a. Entry (Key a) a

{-
  A simple representation of a Bag as a list of entries.
 -}
type Bag =
  [Entry]

flag :: Key Bool
flag = B "flag"

counter :: Key Int
counter = I "flag"

name :: Key String
name = S "name"

stuff :: Bag
stuff = [
    Entry flag True
  , Entry counter 0
  , Entry name "mth"
  ]

get :: Key a -> Bag -> Maybe a
get a [] =
  Nothing
get a ((Entry k v) : xs) =
  let next = get a xs
   in case (k, a) of
        (S s, S s') -> if s == s' then Just v else next
        (I i, I i') -> if i == i' then Just v else next
        (B i, B i') -> if i == i' then Just v else next
        _           -> next