A simple crud DSL with an interpreter written as cofree commonad in Purescript

The store type is simply

type Store = Array User

where

newtype User = User
    { id :: Int
    , name :: String
    }

There are four commands in the DSL:

data Command a = Add User a
               | Remove Int a
               | ChangeName Int String a
               | GetUsers (Array User -> a)
               | SaveUser User a

The type of the DSL is

type StoreDSL a = Free Command a

There are two interpreters

• synchronous one

  newtype Run a = Run
      { addUser :: User -> a
      , remove :: Int ->  a
      , changeName :: Int -> String -> a
      , getUsers :: Unit -> Tuple (Array User) a
      , saveUser :: User -> a
      }
  
  type Interp a = Cofree Run a
  

  The pairing is given by

  pair :: forall x y. Command (x -> y) -> Run x -> y
  

  We pair the Free and Cofree using the explore function from Control.Comonad.Cofree module.

• asynchronous one with computations in the Aff monad

  newtype RunAff eff a = RunAff
      { addUser :: User -> Aff eff a
      , remove :: Int ->  Aff eff a
      , changeName :: Int -> String -> Aff eff a
      , getUsers :: Unit -> Aff eff (Tuple (Array User) a)
      , saveUser :: User -> Aff eff a
      }
  
  type AffInterp eff a = Cofree (RunAff eff) a
  

  The pairing is given by

  pairInAff :: forall eff x y. Command (x -> y) -> RunAff eff x -> Aff eff y
  

  Here we pair using a custom exploreAff function.