FixedList.hs

{-# LANGUAGE PatternSynonyms, ViewPatterns #-}

import Data.List
import Data.Foldable
import Control.Monad.State
import Data.Distributive

fromList :: (Applicative m, Distributive m, Traversable m) => [a] -> Maybe (m a)
fromList xs = do
  (ys, []) <- runStateT (traverse (\_ -> StateT uncons) (pure ())) xs
  return ys

pattern FixedList :: (Distributive m, Traversable m, Applicative m) => m a -> [a]
pattern FixedList m <- (fromList -> Just m) where
  FixedList m = toList m