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.

coot/purescript-dsl-example

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