ExplicitState.hs

type IP a = State -> (State,a)
data State

andThen :: IP a -> (a -> IP b) -> IP b
f `andThen` g = \s0 -> let (s1,a) = f s0
                           (s2,b) = g a s1
                       in  (s2,b)

instance Monad IP where
  (>>=) = andThen
  return x = \s -> (s,x)

safePrint line = do
  noInk <- outOfInk
  if noInk
    then return -1
    else do print line
            return 0

outOfInk = \s -> (s,False)

Output:

Prelude> :r
[1 of 1] Compiling Main             ( ExplicitState.hs, interpreted )

ExplicitState.hs:9:10:
    Illegal instance declaration for `Monad IP'
      (All instance types must be of the form (T t1 ... tn)
       where T is not a synonym.
       Use -XTypeSynonymInstances if you want to disable this.)
    In the instance declaration for `Monad IP'
Failed, modules loaded: none.

And with {-# LANGUAGE TypeSynonymInstances #-}:

Prelude> :r
[1 of 1] Compiling Main             ( ExplicitState.hs, interpreted )

ExplicitState.hs:11:10:
    Type synonym `IP' should have 1 argument, but has been given none
    In the instance declaration for `Monad IP'
Failed, modules loaded: none.