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Csongor Kiss

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Browsing Hackage the other day, I came across the Tardis Monad. Reading its description, it turns out that the Tardis monad is capable of sending state back in time. Yep. Back in time.


No, it’s not the reification of some hypothetical time-travelling particle, rather a really clever way of exploiting Haskell’s laziness.

In this rather lengthy post, I’ll showcase some interesting consequences of lazy evaluation and the way to work ourselves up from simple examples to ’time travelling’ craziness through different levels of abstraction.

The repMax problem

Imagine you had a list, and you wanted to replace all the elements of the list with the largest element, by only passing the list once. You might say something like “Easier said than done, how do I know the largest element without having passed the list before?”

Let’s start from the beginning: – First, you ask the future for the largest element of the list, (don’t worry, this will make sense in a bit) let’s call this value rep (as in the value we replace stuff with).

Walking through the list, you do two things:

  • replace the current element with rep
  • ’return’ the larger of the current element and the largest element of the remaining list.

When only one element remains, replace it with rep, and return what was there originally. (this is the base case)

Right, at the moment, we haven’t acquired the skill of seeing the future, so we just write the rest of the function with that bit left out.

repMax :: [Int] -> Int -> (Int, [Int])
repMax [] rep = (rep, [])
repMax [x] rep = (x, [rep])
repMax (l : ls) rep = (m', rep : ls')
  where (m, ls') = repMax ls rep
        m' = max m l

So, it takes a list, and the rep element, and returns (Int, [Int])

repMax [1,2,3,4,5,3] 6 gives us (5, [6,6,6,6,6,6]) which is exactly what we wanted: the elements are replaced with rep and we also have the largest element. Now, all we need to do is use that largest element as rep:

doRepMax :: [Int] -> [Int]
doRepMax xs = xs'
  where (largest, xs') = repMax xs largest

Wait, what?

This can be done thanks to lazy evaluation. Haskell systems use so-called ’thunks’ for values that are yet to be evaluated. When you say (min 5 6), the expression will form a thunk and not be evaluated until it really needs to be. Here, rep can be thought of as a reference to a thunk. When we tell GHC to put largest in all slots of the list, it will in fact put a reference to the same thunk in those slots, not the actual data. As we pass the list, this thunk is building up with nested max expressions. For [1,2,3,4], will end up with a thunk: max 1 (max 2 (max 3 4)). A reference to this thunk will be placed everywhere in the list. By the time we finished traversing the list, the thunk will be finished too, and can be evaluated. (Before finishing, the thunk has the form similar to max 1 (_something_) where _something_ is the max of the rest of the list. This obivously can not be evaluated at this point)

How about generalising this idea to other data structures?

There’s an old saying in the world of lists

“Everything’s a fold”.

Indeed, we could easily rewrite our doRepMax function using a fold:

foldmax :: (Ord a, Num a) => [a] -> [a]
foldmax ls = ls'
    (ls', largest) 
      = foldl (\(b, c) a -> (largest : b, max a c)) ([], 0) ls

Brilliant! Now we can use this technique on everything that is Foldable! Or can we?

Taking a look at the type signature of the generalised foldl (from Data.Foldable): Data.Foldable.foldl :: Foldable t => (b -> a -> b) -> b -> t a -> b we realise that the returned value’s structure b is independent from that of the input t a. The reason we could get away with this in our fold example was that we knew we were dealing with a list, so we used the : operator explicitly to restore the structure.

No problem! There exists a type class that does just what we want, that is it lets us fold it while keeping its structure. This magical class is called Traversable.

{-# LANGUAGE DeriveFunctor,
             DeriveTraversable #-}

data Tree a = Empty | Leaf a | Node (Tree a) a (Tree a) 
  deriving (Show, Functor, Foldable, Traversable)

– Thankfully, GHC is clever enough to derive Traversable for us from this data definiton. (But it wouldn’t be too difficult to do by hand anyway)

Traversable data structures can do a really neat trick (among many others): mapAccumR :: Traversable t => (a -> b -> (a, c)) -> a -> t b -> (a, t c)

This function is like combining a map with a fold (and so all Traversables also need to be Functors and Foldables). We take a function (a -> b -> (a, c)), an initial a and a Traversable of bs (t b).

The elements will be changed with their respective cs. (the one calculated by (a -> b -> (a, c))) So c is a perfect place for us to put our rep (the largest element in this case)

Apart from the final Traversable t c, it also returns the accumulated as (that’s where we return the largest).

generalMax :: (Traversable t, Num a, Ord a) => t a -> t a
generalMax t = xs'
    (largest, xs')
      = mapAccumR (\a b -> (max a b, largest)) 0 t

This generalisation gives us new options! What we’ve been doing so far is we’ve used a, b and c as the same types (as, say Ints).

For instance, if we want to replace all the elements with the average of them, then we can accumulate the sum and the count of elements in a tuple (a will then take the role of this tuple) and c will be the sum divided by the count, for which we’re going to ask the future again!

generalAvg :: (Traversable t, Integral a) => t a -> t a
generalAvg t = xs'
    avg = s `div` c
    ((s, c), xs') 
      = mapAccumR (\(s', c') b -> ((s' + b, c' + 1), avg)) (0,0) t

And so on, we can do all sorts of interesting things in a single traversal of our data structures.

States travelling back in time

What are states anyway?

In Haskell, whenever we want to write functions that operate on some sort of environment or state, we write these functions in the following form: statefulFunction :: b -> c -> d -> s -> (a, s) that is, we take some arguments (b, c, d here), a state s, and return a new, possibly modified state along with some value a. Now, this involves writing a lot of boilerplate code, both in the type signatures and in the actual code that is using the state.

For example, using the state as a counter:

statefulFunction arg1 arg2 arg3 counter =
  (arg1 + arg2 + arg3, counter + 1)
bindStatefulFunctions ::
  (s -> (a, s)) ->
  (a -> s -> (b, s)) ->
  s -> (b, s)
bindStatefulFunctions f1 f2 = \initialState ->
  let (result, updatedState) = f1 initialState
  in f2 result updatedState

Note that f2 takes an extra a, that’s the output of the first function. That’s why this function is called bind, we bind the output of the first function to the input of the second while passing the modified state.

The State monad essentially does something like the above code, but hides it all and makes the state passing implicit. Also, being a monad, gives us the all so convenient do notation!

State s a is basically just a type synonym for s -> (a, s), so our previous example could be written as statefulFunction :: b -> c -> d -> State s a

and bindStatefulFunctions we get for free from State (known as >>= for monads)

Now we can do:

statefulFunction arg1 arg2 arg3 = do
  counter <- get
  put (counter + 1)
  return (arg1 + arg2 + arg3)

(Did you know that Haskell is also the best imperative language?) Notice how the state is not explicitly passed as an argument (thus our function is partially applied), but is bound to counter by the get function. Put then puts the updated counter back in the state. Return then just makes sure that what we get out of is wrapped back in the State monad.

The nice thing about the State monad is that all the computations we do within it are essentially just partially applied functions, so they can’t be evaluated until provided with an initial state, which will then magically flow through the pipeline of computations, each doing their respective modifications in the meantime.

mapAccumR does a series of stateful computations (in nature, but it’s not using the State monad), where it takes a value and a state, then returns a new value with a modified state. (Accum refers to the fact that this state can be used as an accumulator as we traverse the data)

mapAccumR :: Traversable t => (a -> b -> (a, c)) -> a -> t b -> (a, t c)

a is that state here, that is what we used to store the largest element. This state, however, travels forward in time, so to speak, as we go through the list. The trick we do only happens at the end, when we feed it its own output. We can do so thanks to lazy evaluation.

So the State monad passes its s from computation to computation, that’s how these computations are bound.

Imagine using the same laziness self-feeding trick, but for passing the state:

reverseBind stateful1 stateful2
  = \s -> (x', s'')
  where (x, s'') = stateful1 s'
        (x', s') = stateful2 x s

So first we run stateful1 with the state modified by stateful2! Then we run stateful2 with stateful1’s output. Finally, we return the state after running stateful1 along with the value x' from stateful2. Note that because of the way this binding is done, stateful1’s ouput state will actually be the past of stateful1. (That is, whatever we do with the state in stateful1, will be visible to the computations preceding stateful1, just like how stateful2’s effects are seen in stateful1. Lazy evaluation rocks!)

Coming from an imperative background, this can be thought of as stateful1 putting forward references to the values it uses from the state, and once those values are actually calculated in the future, stateful1 will be able to do whatever it wanted. These references are not explicit though as they would be in C (using pointers, for example), but implicitly placed there by GHC as thunks.

That also means whatever we do with these values has to be done lazily. (an example below)

The above code is a modified version of the monadic binding found in the rev-state package (which is in turn a modification of the original State monad by reversing the flow of state).

Finally, the time machine, TARDIS

So we have the State monad, of which the state flows forwards, then we have the Rev-State, which sends the state backwards. So what do we get if we combine these two? Yes, a time machine! Also known as the Tardis monad: it is in fact a combination of the State and Rev-State monads with some nice functions to deal with the bidirectional states.

I say states, because naturally, we have data coming from the future and data coming from the past, and those make two (a backwards travelling and a forwards travelling state).

These could be of different types, say we can send Strings back in time and Ints to the future.

A single-pass assembler: an example

Writing an assembler is relatively straightforward. We go through a list of assembly instructions and turn them into their binary equivalent for the given CPU architecture.

However, there are some instructions that we can’t immediately convert. One of such instructions is a label for branching. (jumps) For these labels, we need a symbol table.

import qualified Data.Map.Strict as M

type Addr = Int
type SymTable = M.Map String Addr -- map label names to their addresses

data Instr = Add
            | Mov
            | ToLabel String
            | ToAddr Addr
            | Label String
            | Err
            deriving (Show)

Instr is a rather rudimentary representation of assembly instructions, but it does the job for us now.

What we want to have is a function that takes a list of Instrs and returns a list of [(Addr, Instr)] and also replace all the ToLabels with ToAddrs that point to the address of the label. If the label is never defined, we put an Err there. (In real life, you would use some ExceptT monad transformer to handle such errors.)

runAssembler :: [Instr] -> [(Addr, Instr)]

Jumping to a label that is already defined is easy, we look it up in our SymTable and convert ToLabel to ToAddr. This sounds like an application of the State monad, doesn’t it? When we encounter a label definition, just add it to the state (SymTable). Done!

The problem arises from the fact that some labels might be defined after they are used. The ‘else’ block of an if statement will typically be done like this. Implementing this in C, you could remember these positions and at the end, fill in the gaps with the knowledge you have acquired. Thunks, anyone?

I’ll just use a Rev-State monad and send these definitions back in time. Simple enough, right?

So at this point, we can see that we will need both types of these states: one that’s travelling forward and one that is going backwards. And that is exactly what the Tardis monad is!

Labels will not be turned into any binary, instead the next actual instruction’s address will be used.

type Assembler a = Tardis SymTable SymTable a

Right, our runAssembler function will run some assemble function in the Tardis monad. (That is, it will give it the initial states and extract the final value at the end).

runAssembler asm = instructions
  where (instructions, _)
          = runTardis (assemble 0 asm) (M.empty, M.empty)

The assemble function turns a list of instructions to [(Addr, Instr)] in the Assembler monad (which is a synonym for Tardis SymTable SymTable). What’s that 0 doing there, you ask?

We need to keep track of the address we will use for the next instruction. This is because of labels. When we encounter a regular instruction, we put that at the provided address, then increment that address by 1. If a label comes around, we put it in the State then continue without incrementing the address.

assemble :: Addr -> [Instr] -> Assembler [(Addr, Instr)]
assemble _ [] = return []
-- label found, update state then go on
assemble addr (Label label : is') = do
  modifyBackwards (M.insert label addr) -- send to past
  modifyForwards (M.insert label addr)  -- send to future
  assemble addr is' -- assemble the rest of the instructions
-- jump to label found, replace with
-- jump to address
-- then do the rest starting at (addr + 1)
assemble addr (ToLabel label : is') = do
  bw <- getFuture
  fw <- getPast
  let union = M.union bw fw -- take union of the two symbol tables
      this = case M.lookup label union of
                Just a' -> (addr, ToAddr a')
                Nothing -> (addr, Err)
  rest <- assemble (addr + 1) is'
  return $ this : rest
-- regular instruction found,
-- assign it to the address
-- then do the rest starting at (addr + 1)
assemble addr (instr : is') = do
  rest <- assemble (addr + 1) is'
  return $ (addr, instr) : rest

Now we come up with some test instructions:

input :: [Instr]
input = [Add,
         ToLabel "my_label",
         Label "my_label",
         Label "second_label",
         ToLabel "second_label",

…and we can try running the assembler on this data:

> runAssembler input

> [(0,Add),(1,Add),(2,ToAddr 5),(3,Mov),(4,Mov),(5,Mov),(6,ToAddr 5),(7,Mov)]

Yay! Just what we wanted!

IO doesn’t mix with the future! (The past is fine)

Be careful about what you do with the state coming from the future. Everything has to be lazily passed through.

You might be tempted to use the TardisT monad transformer to interleave IO effects in your time-travelling code. Most IO computations, however are strict.

Let’s say you want to get the label from the future and print its address. IO’s print will try to evaluate its argument (which is a partial thunk at this point). It will block the thread until the evaluation is completed, which will result in the program breaking, as the thread block prevents it from progressing further. In this case, I’d advise the use of a Writer monad which has a lazy mechanism, and the results can be printed at the end using IO.


Thanks for reading this lengthy post, in which we saw how we can mimic the use of references in pure Haskell code (altough time-travel is an arguably better name for this). This comes at a price though: accumulating unevaluated thunks can use up quite a bit of memory, so be careful if you want to use these techniques in a memory critical environment.

If you find any bugs or mistakes, please make sure to let me know!