Composition of row polymorphic functions in PureScript

row-polymorphic-composition.purs

module Main where

import Prelude
import Data.Record
import Control.Monad.Eff (Eff)
import Control.Monad.Eff.Console (CONSOLE, log)

fooHandler :: forall r. { foo :: String | r } -> String
fooHandler args = "(foo-handler " <> args.foo <> ")"

barHandler :: forall r. { bar :: String | r } -> String
barHandler args = "(bar-handler " <> args.bar <> ")"

composeHandlers :: forall a. (a -> String) -> (a -> String) -> a -> String
composeHandlers f g x = "(compose-handler " <> (f x) <> " " <> (g x) <> ")"

-- composedHandler :: forall r. { foo :: String, bar :: String } -> String
composedHandler = composeHandlers fooHandler barHandler

-- injectBar :: forall r. {  | r } -> { bar :: String | r }
injectBar record = unionMerge record { bar: "injectedBar" }

-- extendedHandler :: forall r. { foo:: String | r } -> String
extendedHandler = map composedHandler injectBar

args = { foo: "fooValue", baz: "bazValue" }

-- result :: String
result = extendedHandler args

main :: forall e. Eff (console :: CONSOLE | e) Unit
main = do
  log $ result