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FUTURE_PLANS.md

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Use of lenses and classy mtl will be left to supplemental material. It would be nice to be able to use it lenses more but the additional explanation required might be a bit much and not very predictable ?

Further lessons

N - Death to Strings

Start up and Servant introduction.

  • Use Scotty/Warp for the "hello, world" application.
  • Explain that we're using strings for routes and this is bad(TM).
  • Move to Servant, explain why, show type driven dev to explain routes->function relationship.

N - Faking Global Vars with Science

Show that nothing can be changed in the current app without recompliation. Rework the application so we can change the port values and have general app config.

  • Read a file in app start up to allow for port config changes.
  • Explain that this can be cumbersome to explicitly pass the config around, there are better ways(TM).
  • Add mtl dependency.
  • Natural Transformation required from our new ReaderT to Servant (too much?).

N - Would you like to play a game?

Introduce handling input/output also preempt the inclusion of persistent storage.

  • Add rock-paper-scissors data type. (Don't create Enc/Dec instances yet)
  • Add route to accept an RPS move that always plays paper against the user.
  • Go over what the errors from Servant & GHC are telling us.
  • Add the required instances.
  • Change the function up so that it randomly selects a move, evaluates victory/defeat.

N - Type safe tantrums

Introduce error handling by breaking REST rules by having our application throw an error when it loses at Rock-Paper-Scissors.

  • Change RPS function to throw an error on defeat, creating required error data types.
  • Discuss the errors that appear RE return values and making Servant respond appropriately.
  • Introduce ExceptT, change the ReaderT from earlier to what we want it to be.
    • More discussion to be had here regarding the ordering of transformer stacks.
  • Discuss the errors that appear. Work through fixing these with type-holes in the Natural Transform.

N - Elephants

We'd like to be able to store a history of RPS games.

  • Create a datatype that has some game info, respective moves, outcome, time.
  • Introduce and integrate postgresql-simple:
    • Config for connection
    • Connection on the ReaderT
    • Using Connection from ReaderT in query function(s)
  • Add the required instances so we can read/write our datatype (provide a SQL file to initialise/purify the DB)
  • Add our route to the API to request games by id, demonstrate work flow of 'game not found'.
  • Work through the errors using type-holes to create our function.
  • Stringly queries acceptable for now - postgresql-simple

** Prompt discussion about what lurks beneath the surface of the IO a query functionality.

N - Except when exceptionally excepted

Handling, catching, and rethrowing exceptions. Motivate errors as values over exceptions.

  • Possible ways postgresql-simple can fail on us, note that the types provide no information

  • Introduce exception handling with catching and handling to our error values.

  • Generalise our current DB querying technique so that we take a query and handle some exceptional cases.

  • Discuss the relevance of handling these errors:

    • Shouldn't they just come up in testing?
    • I'm sure there are other points here, some Haskell applications are built not to care about these sorts of errors.

  • Discuss how you might implement logging ? Leave as exercise.

N - BOSS FIGHT - A

Replace the stringly DB layer with something better:

These packages will force a lot of other things to be picked up at the same time, to varying degrees:

  • Selda : Type level lists, type operators, data structures as tuples initially.
  • Tisch : Type level HLists, overloaded labels, type families, familiarity with Opaleye.
  • Opaleye : Arrows, full polymorphic records.
  • Groundhog : Alien data type definitions using their Template Haskell DSL, is an ORM more than a type safe SQL package (if that makes sense).

All packages use generics of some description, not sure how much air time they need? Passing mention and throw some links around, move on.

N - BOSS FIGHT - B

Integration of 'classy mtl' style application design.

  • Briefly explain the problem with using a transformer stack or newtype'd stack like we have.
    • Concrete types
    • Cannot generalise easily to abstract out larger moving parts
  • Discuss what we want is just constraints, PARAMETRICITY driven development!!
  • Build intuition for AsFoo & HasFoo
  • mtl classes are a start, but how do we make this easier?
  • Describe CanFoo and introduce ConstraintKinds