– made simpler

In “Symbolic expressions can be Automatically differentiated too” I demonstrated that forward mode Automatic Differentiation (AD) is not as mysterious as it is often made out to be. In fact it is quite simple.

On the other hand, reverse mode AD seems unfortunately to be unavoidably more complicated than forward mode, but in this article I hope to show that it is still a bit simpler than it is often made out to be. If you read introductions to reverse mode AD you will find scary passages like

reverse accumulation requires the storage of the intermediate variables wi as well as the instructions that produced them in a data structure known as a Wengert list (or “tape”)

and

The nodes in the adjoint graph represent multiplication by the derivatives of the functions calculated by the nodes in the primal. For instance, addition in the primal causes fanout in the adjoint; fanout in the primal causes addition in the adjoint; a unary function y = f(x) in the primal causes x̄ = ȳ f′(x) in the adjoint; etc.

This doesn’t help anyone to learn reverse mode AD. It *is* a bit complicated but it *isn’t* scary.

There is only any benefit in using reverse mode AD when differentiating an expression with a large number of input variables. If you only have a small number of input variables then forward mode will probably be faster. With that in mind let’s set up the relevant data structures and API.

```
{-# LANGUAGE LambdaCase #-}
import qualified Data.Map.Strict as Map
import qualified Data.Maybe as Maybe
import Data.List (foldl')
-- The type of coordinates. This is for clarity of display
-- purposes only.
type Coord = Int
-- An expression type that has labels 'e' on subexpressions
-- and labels 'v' on variables.
--
-- The expression X1 * (X2 + (X1 * 2)) would be represented as
--
-- Var ((), 1)
-- `Product`
-- (Var ((), 2)
-- `Sum`
-- (Var ((), 1) `Product` (One `Sum` One)))
data E e v = Var (v, Coord)
| One
| Zero
| Negate (e, E e v)
| Sum (e, E e v) (e, E e v)
| Product (e, E e v) (e, E e v)
| Exp (e, E e v)
deriving Show
```

Compare this to the forward mode expression datatype which had only one variable (“`X`

”) and didn’t allow labelling subexpressions or variables. We will use the labels in the reverse mode algorithm.

For the sake of simplicity we’ll consider vectors of length 1000 represented by `Map`

s.

```
-- The vector type is implemented as a Map from coordinates
-- to Double. In this example, for concreteness, we will consider
-- vectors of length 1000.
type V = Map.Map Coord Double
-- The zero vector
zero :: V
zero = Map.fromList (zip [1..1000] [0,0..])
-- Add two vectors
plus :: V -> V -> V
plus = Map.unionWith (+)
-- Multiply a vector by a scalar
times :: Double -> V -> V
times a = Map.map (a *)
-- Negate a vector
negateV :: V -> V
negateV = Map.map negate
-- The component of a vector in a given coordinate direction.
-- For example, the "component along" 2 of (3,4,5,6,...) is 4.
componentAlong :: Coord -> V -> Double
componentAlong i v = Maybe.fromMaybe 0 (Map.lookup i v)
-- A vector which has one non-zero entry, value x in the i
-- direction. For example, "5 `inDirection` 3" is (0,0,5,0,...).
inDirection :: Double -> Coord -> V
inDirection x i = Map.fromList [(i, x)]
-- Add a quantity to the given component. For example,
-- "plusComponent 2 10 (3,4,5,6,...)" is "(3,14,5,6,...)".
plusComponent :: Coord -> Double -> V -> V
plusComponent = Map.insertWith (+)
```

We can use this API to implement forward mode AD for functions with multiple inputs.

```
forwardMode :: V -> E e v -> (Double, V)
forwardMode v = ev where
ev = \case
Var (_, i) -> (componentAlong i v,
1 `inDirection` i)
One -> (1, zero)
Zero -> (0, zero)
Negate (_, e) -> let (ex, ed) = ev e
in (-ex, negateV ed)
Sum (_, e) (_, e') -> let (ex, ed) = ev e
(ex', ed') = ev e'
in (ex + ex', ed `plus` ed')
Product (_, e) (_, e') -> let (ex, ed) = ev e
(ex', ed') = ev e'
in (ex * ex', (ex `times` ed')
`plus`
(ex' `times` ed))
Exp (_, e) -> let (ex, ed) = ev e
in (exp ex, exp ex `times` ed)
```

Compare this definition to the one of `diffEval`

in the the previous article. You will see that it has exactly the same structure. We can also define a sample expression to test it on which corresponds precisely to `bigExpression`

from the previous article.

```
f :: E () () -> E () ()
f x = exp_ (x_ `minus` one)
where a `minus` b = a `Sum` ((), Negate b)
one = ((), One)
x_ = ((), x)
exp_ a = Exp ((), a)
bigExpression :: E () ()
bigExpression = iterate f x1 !! 1000
where x1 = (Var ((), 1))
```

```
exampleForward =
mapM_ (print
. componentAlong 1
. snd
. flip forwardMode bigExpression
. (`inDirection` 1))
[0.00009, 1, 1.00001]
```

```
> exampleForward
3.2478565715995278e-6
1.0
1.0100754777229357
-- That was slow
```

Unfortunately, `forwardMode`

is terribly slow. In fact it takes time proportional to *n*, where *n* is the number of inputs. We’re using vectors of length 1,000 in this article and if we used vectors of length 10,000 it would be ten times slower. The reason for this asymptotic complexity is that each time we combine the derivatives of subexpressions, for example in

`ed `plus` ed'`

we are doing *O(n)* work.

Reverse mode is faster when there are many inputs. First I’ll explain what reverse mode is and then I’ll explain why it’s faster. Unfortunately reverse mode is not as simple to explain as forward mode. It is simplest to understand if split into three parts.

Given a point in `V`

at which to evaluate the derivative of our expression first we decorate every subexpression with its *value* at that point.

```
evalDecorate :: V -> E e v -> (Double, E Double v)
evalDecorate v = ev where
ev = \case
Var (a, i) -> (componentAlong i v, Var (a, i))
One -> (1, One)
Zero -> (0, Zero)
Negate (_, e) -> let (x, d1) = ev e
in (-x, Negate (x, d1))
Sum (_, e) (_, e') -> let (x, d1) = ev e
(y, d2) = ev e'
in (x + y, Sum (x, d1) (y, d2))
Product (_, e) (_, e') -> let (x, d1) = ev e
(y, d2) = ev e'
in (x * y, Product (x, d1) (y, d2))
Exp (_, e) -> let (x, d1) = ev e
in (exp x, Exp (x, d1))
```

The first component of the return value is the value of the whole expression. This evaluation is half of what forward mode AD does. In reverse mode we calculate the value but we do not combine it with the derivative yet. Instead we keep the whole decorated expression tree around for a second pass.

By way of example, let us evaluate the partial derivatives of

`f = X1 * (X2 + (X1 * 2))`

at `(X1, X2) = (3,4)`

. We expect to find that

`f = 30`

`f_X1 = X2 + 4 * X1 = 16`

`f_X2 = X1 = 3`

After being decorated with values of subexpressions, `f`

becomes

`[X1; 3] * [[X2; 4] + [[X1; 3] * [2; 2]; 6]; 10]`

(From here we see indeed that `f = 3 * 10 = 30`

.)

The second pass is the only conceptually hard part of reverse mode. We propagate a “sensitivity” value from the root of the expression tree to the variables. It tells us how sensitive the value of our expression is to changes of the variables.

```
sensitivityDecorate :: Double -> E Double v -> E () Double
sensitivityDecorate = ev where
ev s = \case
Var (_, x) -> Var (s, x)
One -> One
Zero -> Zero
Negate (_, e) -> Negate ((), ev (-s) e)
Sum (_, e) (_, e') -> Sum ((), ev s e)
((), ev s e')
Product (x, e) (y, e') -> Product ((), ev (s * y) e)
((), ev (s * x) e')
Exp (x, e) -> Exp ((), ev (s * exp x) e)
```

The calculation simply follows the normal rules of calculus. The negation operation has sensitivity `-1`

to its argument, the sum operation has sensitivity `1`

to both its arguments and the product `x1 * x2`

has sensitivity `b`

to `x1`

when the value of `x2`

is `b`

, and sensitivity `a`

to `x2`

when the value of `x1`

is `a`

.

Continuing with our example, after sensitivity decoration we obtain

`[X1; 10] * ([X2; 3] + ([X1; 3 * 2] * 2))`

which is

`[X1; 10] * ([X2; 3] + ([X1; 6] * 2))`

The third step is trivial and is to walk the leaves and gather them into a list.

```
listOfVars :: [(v, Coord)] -> E e v -> [(v, Coord)]
listOfVars = ev where
ev l = \case
Var t -> t : l
One -> l
Zero -> l
Negate (_, e) -> l `ev` e
Sum (_, e) (_, e') -> l `ev` e `ev` e'
Product (_, e) (_, e') -> l `ev` e `ev` e'
Exp (_, e) -> l `ev` e
```

Despite the functionality of `listOfVars`

being trivial it is *absolutely critical* that we gather the leaves by consing onto an accumulating argument. It is very expensive to use structurally recursive list appends.

Our example gives us the following list of leaves

`[X1; 10], [X2; 3], [X1; 6]`

Then we simply sum componentwise, giving

`[X1; 16], [X2; 3]`

as anticipated. Combining the passes gives the complete reverse mode algorithm.

```
reverseMode :: V -> E e v -> V
reverseMode v = foldl' (\d (s, x) -> plusComponent x s d) zero
. listOfVars []
. sensitivityDecorate 1
. snd
. evalDecorate v
```

On our previously defined example expression with vectors of size 1000, reverse mode is *much* faster than forward mode.

```
exampleReverse =
mapM_ (print
. componentAlong 1
. (\x -> reverseMode x bigExpression)
. (`inDirection` 1))
[0.00009, 1, 1.00001]
```

```
> exampleReverse
3.2478565715995362e-6
1.0
1.010075477722936
-- Yes, it was fast
```

It is faintly reassuring that the answers we get are numerically slightly different than for forward mode. The algebraic derivative is the same, of course, but the numerical operations are performed in a different order which leads to tiny differences in the floating point value.

Why is reverse mode faster than forward mode? We noted previously that forward mode must do *O(n)* work at each subexpression to combine derivatives. Reverse mode avoids this by walking the leaves. In reverse mode all the information you need about the derivatives is contained in the leaves.

What are the downsides of reverse mode? Firstly it’s probably slower than forward mode for small numbers of inputs, i.e. on small vectors. Perhaps more importantly though the `evalDecorate`

pass forces the whole expression into memory which may be unacceptable for large expressions.

Where is the Wengert list? Presumably it’s the expression tree decorated with values of subexpressions which is then decorated with sensitivities. It seems nice to have this tree than a Wengert list (or “tape”)! What’s all this about adjoint graphs and duals and primals? I really have no clue.

Reverse mode AD seems to be unavoidably more complicated than forward mode but it is not actually particularly complicated and certainly not as hard to understand as it seems to be from much of the literature you will read about it.

Thanks to Edward Kmett who patiently answered my questions on reverse mode so I was finally able to understand it.