Total Map

Often one may have an enum type representing all possible keys of something, and wishes to store some data associated with it. In this case you have two options - use a function or a Map from containers. A function works, but can be difficult to update and has not Eq or Show instances. A map solves this issue, but gives no guarantee that a key has associated data - all functions become partial. This library offers a different way of solving this problem.

A TotalMap k a is a total mapping from a key of type k to a value of type a; each k will have exactly one a. It permits many instances, including Show and Eq.

Example

Let us create and example. We start with some imports and some language pragmas.

{-# LANGUAGE DeriveAnyClass #-}
{-# LANGUAGE DeriveGeneric #-}

import TotalMap

import Control.Lens
import Control.Monad (void)
import Data.Functor.Rep (tabulate)
import qualified GHC.Generics as GHC (Generic)
import Generics.SOP

TotalMap uses generics-sop internally, so we require it as an import. We also require some typeclasses introduced by lens.

For our example, we shall create a dummy program for sending out peoples fortunes based on their star sign. We shall create a star sign type.

data StarSign
  = Aries
  | Taurus
  | Gemini
  | Cancer
  | Leo
  | Virgo
  | Scorpio
  | Sagittarius
  | Capricorn
  | Aquarius
  | Pisces
  deriving (Eq, Show, GHC.Generic, Generic)

Note the derivation of both GHC.Generic and Generic. These are required to guarantee that StarSign’s constructors take no imports.

We have a list of people, and we can partition them based on their star sign.

data Date = Date
  { month :: Int
  , day :: Int
  } deriving (Eq, Show)

data Person = Person
  { name :: String
  , email :: String
  , birthDate :: Date
  } deriving (Eq, Show)

signFromDate :: Date -> StarSign
signFromDate = undefined

peopleSign :: [Person] -> TotalMap StarSign [Person]
peopleSign ps = tabulate $ \s -> filter ((==) s . signFromDate . birthDate) ps

We could send people an email with their fortune.

sendFortune :: String -> String -> StarSign -> IO ()
sendFortune = undefined

sendFortunes :: [Person] -> IO ()
sendFortunes =
  void .
  itraverse (\sign -> mapM_ (\p -> sendFortune (name p) (email p) sign)) .
  peopleSign

Future

• Come up with a better example
• Is Lens necessary. It is useful, but it is a huge dependency.

The following is required to make tests compile.

main :: IO ()
main = return ()