An example of the ST Monad to append items to a mutable array

STMonadUse.purs

import Data.Array 
import Data.Array.ST (empty, push, freeze)
import Data.Either (Either(..))
import Control.Monad.ST.Internal (ST, foreach)

-- Return Rights from an Array of Either's using the ST monad
rights :: forall a b. Array (Either a b) -> Array b
rights array =
  let
    -- This seems to be required because 
    inner :: forall c. ST c (Array b)
    inner = do
      -- Create a new empty array
      -- empty :: forall a. f a
      arr <- empty
      
      let
        insertIfValid e =
          case e of
            -- push :: forall h a. a -> STArray h a -> ST h Int
            Right v -> push v arr *> pure unit
            _ -> pure unit
            
      -- foreach :: forall r a. Array a -> (a -> ST r Unit) -> ST r Unit
      -- ST.foreach xs f runs the computation returned by the       function f for each of the inputs xs.
      _ <- foreach array insertIfValid
      
      freeze arr
  in
    run inner