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{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
-- From https://ocharles.org.uk/blog/posts/2014-12-13-multi-param-type-classes.html
import Control.Monad.Trans.Class (lift)
import Control.Monad.Trans.Reader (ReaderT)
import Data.Foldable (forM_)
import Data.IORef
class Store store m where
new :: a -> m (store a)
get :: store a -> m a
put :: store a -> a -> m ()
type Present = String
storePresents :: (Store store m, Monad m) => [Present] -> m (store [Present])
storePresents xs = do
store <- new []
forM_ xs $ \x -> do
old <- get store
put store (x : old)
return store
instance Store IORef IO where
new = newIORef
get = readIORef
put ioref a = modifyIORef ioref (const a)
-- ex ps = do
-- store <- storePresents ps
-- get (store :: IORef [Present])
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