Equipping your Library for Backprop

So you want your users to be able to use your numerical library with backprop, huh?

This page is specifically for library authors who want to allow their users to use their library operations and API with backprop. End-users of the backprop library should not have to worry about the contents of this page.

Equipping your library with backprop involves providing "backprop-aware" versions of your library functions. In fact, it is possible to make a library fully by providing only backprop versions of your functions, since you can use a backprop-aware function as a normal function with evalBP. Alternatively, you can re-export all of your functions in a separate module with "backprop-aware" versions.

Know Thy Types

The most significant effort will be in lifting your library's functions. If you have a function:

myFunc :: a -> b

Then its lifted version would have type:

myFunc :: Reifies s W => BVar s a -> BVar s b

That is, instead of a function directly taking an a and returning a b, it's a function taking a BVar containing an a, and returning a BVar containing a b.

Functions taking multiple arguments can be translated pretty straightforwardly:

func1   ::                       a ->        b ->        c
func1BP :: Reifies s W => BVar s a -> BVar s b -> BVar s c

And also functions returning multiple arguments:

func2   ::                       a -> (       b,        c)
func2BP :: Reifies s W => BVar s a -> (BVar s b, BVar s c)

It is recommended (for ease of use with -XTypeApplications) that Reifies s W be the final constraint in all code you write.

Note that almost all operations involving BVar'd items require that the contents have a Backprop instance. Alternative API's to backprop that require Num instances instead (or explicitly specified addition functions) are available in Numeric.Backprop.Num and Numeric.Backprop.Explicit.

The Easy Way

BVar based functions are just normal functions, so they can be applied normally and passed as first-class values. If possible, if you can utilize functions that are already BVar'd/lifted, then you can just define your API in terms of those lifted functions. This is also how users are expected to be able to use your library: just use the lifted functions you provide, in order to make their own lifted functions using normal function application and composition.

Lifting operations manually

However, if no lifted primitive functions are available, then you do have to do some legwork to provide information on gradient computation for your types. Ideally, you would only need to do this for some minimal set of your operations, and then define the rest of them in terms of the functions you have already lifted.

A full tutorial on lifting your library functions can be found here. It describes the usage of the liftOp and op family of functions to fully lift your single-argument single-result and multiple-argument single-result functions to be backpropagatable.

Returning multiple items

As an extension of the manual gradient tutorial, we can consider functions that return multiple items.

You can always return tuples inside BVars:

    :: (Backprop a, Reifies s W)
    => Int
    -> BVar s [a]
    -> BVar s ([a], [a])
splitAt n = liftOp1 . op1 $ \xs ->
    let (ys, zs) = Data.List.splitAt n xs
    in  ((ys, zs), \(dys,dzs) -> dys ++ dzs)
                      -- assumes dys and dzs have the same lengths as ys and zs

This works as expected. However, it is recommended, for the benefit of your users, that you return a tuple of BVars instead of a BVar of tuples:

    :: (Backprop a, Reifies s W)
    => Int
    -> BVar s [a]
    -> (BVar s [a], BVar s [a])
splitAt' n xs = (yszs ^^. _1, yszs ^^. _2)
    yszs = liftOp1 (op1 $ \xs' ->
        let (ys, zs) = Data.List.splitAt n xs'
        in  ((ys, zs), \(dys,dzs) -> dys ++ dzs)
      ) xs

using _1 and _2 from the microlens or lens packages. This might also be cleaner if you take advantage of the T2 or T3 pattern synonyms:

    :: (Backprop a, Reifies s W)
    => Int
    -> BVar s [a]
    -> (BVar s [a], BVar s [a])
splitAt'' n xs = (ys, zs)
    T2 ys zs = liftOp1 (op1 $ \xs' ->
        let (ys, zs) = Data.List.splitAt n xs'
        in  ((ys, zs), \(dys,dzs) -> dys ++ dzs)
      ) xs


If your function witnesses an isomorphism, there are handy combinators for making this easy to write. This is especially useful in the case of data constructors:

newtype Foo = MkFoo { getFoo :: Double }
  deriving Generic

instance Backprop Foo

    :: Reifies s W
    => BVar s Double
    -> BVar s Foo
mkFoo = isoVar MkFoo getFoo

data Bar = MkBar { bar1 :: Double, bar2 :: Float }
  deriving Generic

instance Backprop Bar

    :: Reifies s W
    => BVar s Double
    -> BVar s Float
    -> BVar s Bar
mkBar = isoVar2 MkBar (\b -> (bar1 b, bar2 b))

Note also that if you have a newtype with one constructor (or any other two Coercible types), you can simply use coerceVar:

    :: BVar s Double
    -> BVar s Foo
mkFoo' = coerceVar          -- requires no `Reifies s W` constraint


If you do decide to go to the extreme, and provide only a BVar-based interface to your library (and no non-BVar based one), then you might have a situation where you have a function where you cannot define the gradient -- maybe no gradient exists, or you haven't put in the time to write one. In this case, you can use noGrad and noGrad1:

    :: (Num a, Backprop a, Reifies s W)
    => BVar s a
    -> BVar s a
negateNoGrad = liftOp1 (noGrad1 negate)

This function can still be used with evalBP to get the correct answer. It can even be used with gradBP if the result is never used in the final answer.

However, if it is used in the final answer, then computing the gradient will throw a runtime exception.

Be sure to warn your users! Like any partial function, this is not recommended unless in extreme circumstances.

Monadic Operations

This should all work if your operations are all "pure". However, what about the cases where your operations have to be performed in some Applicative or Monadic context?

For example, what if add :: X -> X -> IO X ?

One option you can do is to newtype-wrap your operations, and then give those a backprop instance:

newtype IOX = IOX (IO X)

instance Backprop IOX where
    zero (IOX x) = IOX (fmap zeroForX x)
    -- or, depending on the type of `zeroForX`:
    -- zero (IOX x) = IOX (zeroForX =<< x)

    add (IOX x) (IOX y) = IOX $ do
      x' <- x
      y' <- y
      addForX x' y'

    one (IOX x) = IOX (fmap oneForX x)

And you can define your functions in terms of this:

    :: Reifies s W
    => BVar s IOX
    -> BVar s IOX
    -> BVar s IOX
addX = liftOp2 . op2 $ \(IOX x) (IOX y) ->
    ( IOX (do x' <- x; y' <- y; addForX x' y')
    , \dzdy -> (dzdy, dzdy)

This should work fine as long as you never "branch" on any results of your actions. You must not ever need to peek inside the results of the action in order to decide what operations to do next. In other words, this works if the operations you need to perform are all known and fixed before-hand, before any actions are performed. So, this means no access to the Eq or Ord instances of BVars (unless your monad has Eq or Ord instances defined).

A newtype wrapper is provided to give you this behavior automatically -- it's ABP, from Numeric.Backprop and Numeric.Backprop.Class.

type IOX = ABP IO X

However, this will not work if you need to do things like compare contents, etc. to decide what operations to use.

At the moment, this is not supported. Please open an issue if this becomes an issue!

Supporting Data Types

Your library will probably have data types that you expect your users to use. To equip your data types for backpropagation, you can take a few steps.

Backprop Class

First of all, all of your library's types should have instances of the Backprop typeclass. This allows values of your type to be used in backpropagatable functions. See the Backprop typeclass section of this documentation for more information on writing a Backprop instance for your types.

In short:

  1. If your type is a type with a single constructor whose fields are all instances of Backprop, you can just write instance Backprop MyType, and the instance is generated automatically (as long as your type has a Generic instance)

    data MyType = MkMyType Double [Float] (R 10) (L 20 10) (V.Vector Double)
      deriving Generic
    instance Backprop MyType
  2. If your type is an instance of Num, you can use zeroNum, addNum, and oneNum to get free definitions of the typeclass methods.

    instance Backprop Double where
        zero = zeroNum
        add  = addNum
        one  = oneNum
  3. If your type is made using a Functor instance, you can use zeroFunctor and oneFunctor:

    instance Backprop a => Backprop (V.Vector a) where
        zero = zeroFunctor
        add  = undefined        -- ??
        one  = oneFunctor
  4. If your type has an IsList instance, you can use addIsList:

    instance Backprop a => Backprop (V.Vector a) where
        zero = zeroFunctor
        add  = addIsList
        one  = oneFunctor

For more details, see the aforementioned documentation or the actual typeclass haddock documentation.


If you have product types, users should be able to access values inside BVars of your data type. There are two main ways to provide access: the lens-based interface and the higher-kinded-data-based interface.

The lens-based interface gives your users "getter" and "setter" functions for fields, and the higher-kinded-data-based interface lets your users pattern match on your data type's original constructor to get fields and construct values.

Lens-Based Interface

If you are defining a product type, like

data MyType = MT { _mtDouble  :: Double
                 , _mtInt     :: Int
                 , _mtDoubles :: [Double]

Users who have a BVar s MyType can't normally access the fields inside, because you can't directly pattern match normally, and the record accessors are MyType -> Int (unlifted). As a library maintainer, you can provide them lenses to the fields, either generated automatically using the lens or microlens-th packages:

makeLenses ''MyType

or manually by hand:

mtInt' :: Functor f => (Int -> f Int) -> MyType -> f MyType
mtInt' f mt = (\i -> mt { _mtInt = i }) <$> f (_mtInt mt)

Now, users can use ^. or view from the lens or microlens packages to retrieve your fields:

(^. mtDouble)  ::        MyType ->        Double

And (^^.) and viewVar from backprop to retrieve fields from a BVar:

(^^. mtDouble) :: BVar s MyType -> BVar s Double

They can also use set or .~ to modify fields, and setVar and .~~ to modify and "set" fields in a BVar:

set    mtDouble ::        Double ->        MyType ->        MyType
setVar mtDouble :: BVar s Double -> BVar s MyType -> BVar s MyType

Likewise, over and %~ can be used to apply a function to the contents of a field, and overVar and %~~ can be used to apply backpropagatable functions to over fields of a value in a BVar.

Higher-Kinded Data Interface

The alternative "Higher-Kinded Data" technique, inspired by this article, allows your users to directly pattern match on BVars of your types to get their contents.

Doing this requires modifying the definition of your data types slightly. Instead of MyType above, we can make a type family that can be re-used for all of your data types:

type family HKD f a where
    HKD Identity a = a
    HKD f        a = f a

and define your data types in terms of this type family (remembering to derive Generic):

data MyType2' f = MT2 { mt2Double  :: HKD f Double
                      , mt2Int     :: HKD f Int
                      , mt2Doubles :: HKD f [Double]
  deriving Generic

Now your original data type can be recovered with MyType2' Identity, and can be pattern matched directly in the same way as the original type (the Identity disappears):

type MyType2 = MyType2' Identity

deriving instance Show MyType2
instance Backprop MyType2

getMT2Double :: MyType2 -> Double
getMT2Double (MT2 d _ _) = d

But now, users can pattern match on a BVar s MyType2 to get BVars of the contents, with splitBV or the BV pattern synonym:

    :: Reifies s W
    => BVar s MyType2
    -> BVar s Double
getMT2DoubleBVar (splitBV -> MT2 d _ _) = d

Under splitBV, your users can pattern match on the MT2 constructor and get the contents as BVars.

Note that HKD access through pattern matching is potentially less performant than access using lens (by about 10-20%).

Users can also use joinBV (or the BV pattern synonym in constructor mode) to re-construct a BVar of MyType2 in terms of BVars of its contents using the MT2 constructor:

    :: Reifies s W
    => BVar s Double
    -> BVar s Int
    -> BVar s [Double]
    -> BVar s MyType2
makeMyType2 d i ds = joinBV $ MT2 d i ds