Tying the Knot with a State monad

Question

I\'m working on a Haskell project that involves tying a big knot: I\'m parsing a serialized representation of a graph, where each node is at some offset into the file, and may r

Answer 1

Regarding the implementation, I would make it a composition of a Reader monad (for the future) and a State monad (for past/present). The reason is that you set your future only once (in tie) and then don't change it.

{-# LANGUAGE DoRec, GeneralizedNewtypeDeriving #-}

import Control.Monad.State
import Control.Monad.Reader
import Control.Applicative

newtype RecStateT s m a = RecStateT (StateT s (ReaderT s m) a) deriving
  ( Alternative
  , Applicative
  , Functor
  , Monad
  , MonadPlus
  )

tie :: MonadFix m => RecStateT s m a -> s -> m (a, s)
tie (RecStateT m) s = do
  rec (a, s') <- flip runReaderT s' $ flip runStateT s m
  return (a, s')

getPast :: Monad m => RecStateT s m s
getPast = RecStateT get

getFuture :: Monad m => RecStateT s m s
getFuture = RecStateT ask

putPresent :: Monad m => s -> RecStateT s m ()
putPresent = RecStateT . put

Regarding your second question, it'd help to know your dataflow (i.e. to have a minimal example of your code). It's not true that strict patterns always lead to loops. It's true that you need to be careful so as not to create a non-producing loop, but the exact restrictions depend on what and how you're building.