Indexed Access Type - Dictionary Cost

Main.purs

module Main where

import Prelude

import Effect (Effect)
import Effect.Class.Console (log)
import TryPureScript as TryPureScript
import Prim.Row as Row

main :: Effect Unit
main = TryPureScript.render =<< TryPureScript.withConsole do
  log "some value"


-- Common boilerplate:
class Accessory :: Type -> Symbol -> Type -> Constraint
class Accessory r s t | r s -> t

type Access r s t = Accessory r s t => t
instance Row.Cons s t r' r => Accessory (Record r) s t

-- User code:
type Env =
  { readFile :: String -> String
  , writeFile :: String -> String -> String
  , log :: String -> String
  }

mkEnvAff :: Env
mkEnvAff = do
  let
    readFile :: Access Env "readFile" _
    readFile path = path

    writeFile :: Access Env "writeFile" _
    writeFile path content = path <> content

    log :: Access Env "log" _
    log msg = msg <> "foo"
  { readFile, writeFile, log }