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Nested strict data in Haskell


Every so often someone bemoans the space leaks that can arise due to Haskell’s laziness. A frequently touted remedy is to make data stricter by turning on BangPatterns, by defining data structures with explicitly strict fields, or by creating implicitly strict fields with the StrictData extension. Each of these approaches leaves something to be desired. In this article I’ll explain how the approaches work, what they leave to be desired, and a suggest a reasonably general alternative. The alternative seems lightweight enough for Haskell programmers to adopt when they define strict data structures.

The problem

Consider the function pairFoldBad:

pairFoldBad :: (Integer, Integer)
pairFoldBad = foldl' f (0, 0) [1..million]
  where f (count, theSum) x = (count + 1, theSum + x)

Strict foldl' was the correct thing to use here, rather than lazy foldl, so why is the function bad? Because each pair component (count and theSum) is not necessarily an evaluated Integer, merely a thunk which can be evaluated to an Integer. Each time f processes a list element x the thunk count has a + 1 operation added on top of it and the thunk theSum has a + x operation added on top of it. After foldl' has finished, the return value of pairFoldBad will be a pair of two thunks, each one million elements deep! In other words, it has a space leak.

Solution with bang patterns

A typical solution is to use bang patterns to make sure count and theSum are evaluated on the way in to f, as in pairFoldBangs.

pairFoldBangs :: (Integer, Integer)
pairFoldBangs = foldl' f (0, 0) [1..million]
  where f (!count, !theSum) x = (count + 1, theSum + x)

Each time around the loop f returns two thunks of depth 1. The subsequent call to f takes them as arguments. The bang patterns (i.e. the ! symbols) evaluate each of the thunks to evaluated Integers. The overall return value of the foldl' is a pair of depth 1 thunks.

This does the job. It’s a little bit weird that f produces thunks of depth 1 because that means the foldl' produces thunks of depth 1 and we really want evaluated Integers. They are evaluated immediately to Integers as soon as we use them and there’s no space leak but it feels like we’re doing something not exactly right.

An alternative that produces a pair of evaluated Integers is pairFoldBangsAwkward. It ensures that the pair components are evaluated on the way out (i.e. when the pair is created) not on the way in (i.e. when the pair is inspected).

pairFoldBangsAwkward :: (Integer, Integer)
pairFoldBangsAwkward = foldl' f (0, 0) [1..million]
  where f (count, theSum) x = let !count'  = count + 1
                                  !theSum' = theSum + x
                              in (count', theSum')

This form is rather unwieldy though. No less unwieldy is use of the strict function application operator $!:

  where f (count, theSum) x = ((,) $! count + 1) $! theSum + x

The major drawback of using BangPatterns to solve this problem is that we have to remember to do so! The type system does not guide us to write our program correctly. The program is type correct even if we omit all the bang patterns.

Solution with strict data type

To get some help from the type system we can switch out Haskell’s standard pair type for one we define ourselves, with strict fields:

data StrictPair a b = StrictPair !a !b

Then when we write pairFoldStrictPair as below there is no space leak.

pairFoldStrictPair :: StrictPair Integer Integer
pairFoldStrictPair = foldl' f (StrictPair 0 0) [1..million]
  where f (StrictPair count theSum) x = StrictPair (count + 1) (theSum + x)

Why is there no space leak? This code looks exactly the same as the original problematic code pairFoldBad, except we are using the StrictPair type we defined ourselves instead of Haskell’s built-in pair. Why is it different? It is different because whenever a value is constructed using a constructor with a strict field (i.e. a field with a ! in front of it in the data declaration, such as the fields of StrictPair above) the compiler inserts code to evaluate that field. In the case of pairFoldStrictPair the code that is generated is the same as if we had written the desugared form pairFoldStrictPair_Desugared below.

pairFoldStrictPair_Desugared :: StrictPair Integer Integer
pairFoldStrictPair_Desugared = foldl' f (StrictPair 0 0) [1..million]
  where f (StrictPair count theSum) x = let !count'  = count + 1
                                            !theSum' = theSum + x
                                        in StrictPair count' theSum'

This is helpful: we now cannot avoid being strict. If we use the StrictPair type then we can’t forget to evaluate the components.

The major drawback of defining strict data types to replace the more familiar lazy ones is that they really are completely different types with completely different associated libraries (if any). We can’t use the standard fst and snd functions on StrictPair, for example (although libraries do exist that provide this functionality). It is necessary to explicitly convert back and forth between (,) and StrictPair.

A problem with strict nested fields

A further problem with strict data fields is that users often think that they provide more benefit than the reality. For example, from the above we know not to write maybeFoldBad:

maybeFoldBad :: (Integer, Maybe Integer)
maybeFoldBad = foldl' f (0, Nothing) [1..million]
  where f (i, Nothing) x = (i + 1, Just x)
        f (i, Just j)  x = (i + 2, Just (j + x))

Perhaps we should try writing it with a StrictPair instead:

maybeFoldStillBad :: StrictPair Integer (Maybe Integer)
maybeFoldStillBad = foldl' f (StrictPair 0 Nothing) [1..million]
  where f (StrictPair i Nothing)  x = StrictPair (i + 1) (Just x)
        f (StrictPair i (Just j)) x = StrictPair (i + 2) (Just (j + x))

This is still no good! The problem is that although the Maybe Integer in the second component of the StrictPair is strictly evaluated that only means that evaluating the constructor of the StrictPair evaluates the constructor of the Maybe. The payload of the Just is not evaluated so we build up a layer of thunk each time around the loop.

It is common in the Haskell world to see strict data field definitions like

data MyData = MyData { field1 :: !String
                     , field2 :: ![Double]
                     , field3 :: !(Maybe Bool)

Those strict fields probably don’t do what the author hoped! They are almost entirely pointless. The bang annotations on the String and list mean that those fields are only evaluated one cons cell deep. The tail of the data structure is left unevaluated, as is the first element. Similarly the Maybe Bool is only evaluated to a Nothing or Just. If it’s the latter then its payload is unevaluated.

Having noted this caveat we can find a way of addressing the problem in our case. maybeFoldBangs is just too painful to write by hand, and besides, we might leave out a bang accidentally. Instead we can repeat the strict data type creation process and define StrictMaybe (indeed this has already been done for us) and write maybeFoldStrictMaybe, a function without space leaks.

maybeFoldBangs :: (Integer, Maybe Integer)
maybeFoldBangs = foldl' f (0, Nothing) [1..million]
  where f (!i, Nothing) x = (i + 1, Just x)
        f (!i, Just !j) x = (i + 2, Just (j + x))

data StrictMaybe a = StrictNothing | StrictJust !a

maybeFoldStrictMaybe :: StrictPair Integer (StrictMaybe Integer)
maybeFoldStrictMaybe = foldl' f (StrictPair 0 StrictNothing) [1..million]
  where f (StrictPair i StrictNothing)  x = StrictPair (i + 1) (StrictJust x)
        f (StrictPair i (StrictJust j)) x = StrictPair (i + 2) (StrictJust (j + x))

This works fine, but we’re going down a path where we will have to deal with two universes of data types: one lazy universe and one strict universe.

Unifying strict and lazy data types

Can we do better than two distinct universes? Yes, I think we can! Let’s define a newtype Strict with which we will represent the invariant: “when it is evaluated all its immediate children are evaluated too”. By way of convenience we can define a typeclass Strictly to allow us to create Strict values and a pattern synonym Strict to allow us to extract values from the newtype (we should be careful with the actual constructor because it should be used only in ways which preserve the invariant).

-- Any value of `Strict` should satisfy the invariant that when it is
-- evaluated then all its immediate children are evaluated too.
-- The constructor is "unsafe" in the sense that if you don't ensure
-- the invariant holds when you use it then you will violate the
-- expectations of the consumer.
newtype Strict a = MkStrictUnsafe a
pattern Strict a <- MkStrictUnsafe a

class Strictly a where
  strict :: a -> Strict a

instance Strictly (a, b) where
  -- This is a safe use of MkStrictUnsafe because it satisfies the
  -- invariant!  We know a and b are evaluated at the point we
  -- construct the pair.
  strict (!a, !b) = MkStrictUnsafe (a, b)

Now let’s see an example of using our “Strict pair” to write a pair fold. In pairFoldStrict the Strict type guides us to write correct, space leak free, code, which was the benefit of StrictPair. On the other hand we don’t have the downside of StrictPair: there is no new data type. We can interoperate freely with the existing ecosystem!

pairFoldStrict :: Strict (Integer, Integer)
pairFoldStrict = foldl' f (strict (0, 0)) [1..million]
  where f (Strict (count, theSum)) x = strict (count + 1, theSum + x)

We can also freely compose Strict types. After defining a standard Strictly instance for Maybe the fold with Maybe can be written, space leak free, as maybeFoldStrict.

instance Strictly (Maybe a) where
  strict = \case
    -- This is a safe use of MkStrictUnsafe because it satisfies
    -- the invariant.  Nothing has no children.  Just has one child
    -- which we know is evaluated when we construct the Strict Maybe.
    Nothing -> MkStrictUnsafe Nothing
    Just !x -> MkStrictUnsafe (Just x)

maybeFoldStrict :: Strict (Integer, Strict (Maybe Integer))
maybeFoldStrict = foldl' f (strict (0, strict Nothing)) [1..million]
  where f (Strict (i, Strict Nothing))  x = strict (i + 1, strict (Just x))
        f (Strict (i, Strict (Just j))) x = strict (i + 2, strict (Just (j + x)))

What could Strict buy us in practice?

Strict could buy us the ability to conveniently define strict nested data types without requiring a parallel universe of strict types. We now know that writing

data MyData = MyData
    { field1 :: !(Either Int Bool)
    , field2 :: !(Maybe Double, Data.Map.Strict.Map Int Float)

doesn’t make a data type as strict as we probably hoped. Instead of the parallel universe version

data MyData = MyData
    { field1 :: !(StrictEither Int Bool)
    , field2 :: !(StrictPair (StrictMaybe Double)
                             (Data.Map.Strict.Map Int Float))

we can use Strict with the existing universe of data types

data MyData = MyData
    { field1 :: !(Strict (Either Int Bool))
    , field2 :: !(Strict (Strict (Maybe Double),
                          Data.Map.Strict.Map Int Float))

Performance impact

Efficient construction

If strict is inlined then the compiler ought to be able to determine whether constructor arguments have already been evaluated and thus avoid redundantly evaluating them again.

Efficient destruction

New 2020-11-04

Although inlining strict allows us to avoid redundant evaluations when constructing I don’t think the simple form above avoids redundant evaluation when destructing. For example, if we write

case strictMaybe of
    Strict (Just x) -> let !x' = x in f x'

then we would like the compiler to be able to elide the evaluation of x, as below, because, given our implementation, x has already been evaluated.

case strictMaybe of
    Strict (Just x) -> f x

To achieve efficient destruction we need to use a more complicated, and somewhat hairy, setup. The class becomes

class Strictly a where
  data Strict a
  strict :: a -> Strict a
  matchStrict :: Strict a -> a
  unstrict :: Strict a -> a

and the Maybe a instance becomes

instance Strictly (Maybe a) where
  newtype Strict (Maybe a) = StrictMaybe (Data.Strict.Maybe a)
  strict x = unsafeCoerce $ case x of
    Nothing -> x
    Just !_ -> x
  matchStrict (StrictMaybe x) = case x of
    Data.Strict.Just j  -> Just j
    Data.Strict.Nothing -> Nothing
  unstrict = unsafeCoerce

pattern Strict :: Strictly a => a -> Strict a
pattern Strict x <- (matchStrict->x)

Note that Strict (Maybe a) is now a separate data type to Maybe a, but representationally equivalent, so we can unsafeCoerce between them at zero run time cost. How do we use this class and its methods?

The new setup is hairy because it requires us to be very careful with unsafePerformIO. However, I believe it gives us a convenient API to strict data types at zero additional run time cost.

What can’t Strict buy us?

I don’t see how Strict could help much with large lazy data structures such as lists (including Strings). The only way that I can see to use Strict with standard lists whilst satisfying its invariant would be to walk the whole list, which is prohibitively inefficient. Instead I recommend not using large lazy data structures anywhere one desires strictness. Instead use strict data structures such as strict Text, ByteString, Map, Vector or Array (I’m not sure of the strictness characteristics of Seq and I haven’t validated the strictness guarantees of Vector or Array. That will have to be another article.)


Defining strict fields that contain lazy types is almost completely pointless:

data MyData = MyData { field1 :: !String
                     , field2 :: ![Double]
                     , field3 :: !(Maybe Bool)

As an alternative, it is an open question whether Strict, as defined in this article, can prove general enough to achieve widespread use or whether the solution is a parallel universe of strict data types.

Have you seen or used anything like Strict before? If so please contact me.