Support for deriving Functor, Foldable, and Traversable instances

GHC 6.12.1 introduces an extension to the deriving mechanism allowing for automatic derivation of Functor, Foldable, and Traversable instances using the DeriveFunctor, DeriveFoldable, and DeriveTraversable extensions, respectively. Twan van Laarhoven ​first proposed this feature in 2007, and ​opened a related GHC Trac ticket in 2009.

Example

{-# LANGUAGE DeriveFunctor, DeriveFoldable, DeriveTraversable #-}

data Example a = Ex a Char (Example a) (Example Char)
  deriving (Functor, Foldable, Traversable)

The derived code would look something like this:

instance Functor Example where
    fmap f (Ex a1 a2 a3 a4) = Ex (f a1) a2 (fmap f a3) a4

instance Foldable Example where
    foldr f z (Ex a1 a2 a3 a4) = f a1 (foldr f z a3)
    foldMap f (Ex a1 a2 a3 a4) = mappend (f a1) (mappend mempty (mappend (foldMap f a3) mempty))

instance Traversable Example where
    traverse f (Ex a1 a2 a3 a4) = Ex <$> (f a) <*> pure a2 <*> traverse f a3 <*> pure a4

Algorithm description

DeriveFunctor, DeriveFoldable, and DeriveTraversable all operate using the same underlying mechanism. GHC inspects the arguments of each constructor and derives some operation to perform on each argument, which depends of the type of the argument itself. In a Functor instance, for example fmap would be applied to occurrences of the last type parameter, but id would be applied to other type parameters. Typically, there are five cases to consider. (Suppose we have a data type data A a = ....)

1. Terms whose type does not mention a
2. Terms whose type mentions a
3. Occurrences of a
4. Tuple values
5. Function values

After this is done, the new terms are combined in some way. For instance, Functor instances combine terms in a derived fmap definition by applying the appropriate constructor to all terms, whereas in Foldable instances, a derived foldMap definition would mappend the terms together.

DeriveFunctor

A comment in ​TcGenDeriv.hs lays out the basic structure of DeriveFunctor, which derives an implementation for fmap.

For the data type:

  data T a = T1 Int a | T2 (T a)

We generate the instance:

  instance Functor T where
      fmap f (T1 b1 a) = T1 b1 (f a)
      fmap f (T2 ta)   = T2 (fmap f ta)

Notice that we don't simply apply 'fmap' to the constructor arguments.
Rather
  - Do nothing to an argument whose type doesn't mention 'a'
  - Apply 'f' to an argument of type 'a'
  - Apply 'fmap f' to other arguments
That's why we have to recurse deeply into the constructor argument types,
rather than just one level, as we typically do.

What about types with more than one type parameter?  In general, we only
derive Functor for the last position:

  data S a b = S1 [b] | S2 (a, T a b)
  instance Functor (S a) where
    fmap f (S1 bs)    = S1 (fmap f bs)
    fmap f (S2 (p,q)) = S2 (a, fmap f q)

However, we have special cases for
         - tuples
         - functions

More formally, we write the derivation of fmap code over type variable
'a for type 'b as ($fmap 'a 'b).  In this general notation the derived
instance for T is:

  instance Functor T where
      fmap f (T1 x1 x2) = T1 ($(fmap 'a 'b1) x1) ($(fmap 'a 'a) x2)
      fmap f (T2 x1)    = T2 ($(fmap 'a '(T a)) x1)

  $(fmap 'a 'b)          =  \x -> x     -- when b does not contain a
  $(fmap 'a 'a)          =  f
  $(fmap 'a '(b1,b2))    =  \x -> case x of (x1,x2) -> ($(fmap 'a 'b1) x1, $(fmap 'a 'b2) x2)
  $(fmap 'a '(T b1 b2))  =  fmap $(fmap 'a 'b2)   -- when a only occurs in the last parameter, b2
  $(fmap 'a '(b -> c))   =  \x b -> $(fmap 'a' 'c) (x ($(cofmap 'a 'b) b))

For functions, the type parameter 'a can occur in a contravariant position,
which means we need to derive a function like:

  cofmap :: (a -> b) -> (f b -> f a)

This is pretty much the same as $fmap, only without the $(cofmap 'a 'a) case:

  $(cofmap 'a 'b)          =  \x -> x     -- when b does not contain a
  $(cofmap 'a 'a)          =  error "type variable in contravariant position"
  $(cofmap 'a '(b1,b2))    =  \x -> case x of (x1,x2) -> ($(cofmap 'a 'b1) x1, $(cofmap 'a 'b2) x2)
  $(cofmap 'a '[b])        =  map $(cofmap 'a 'b)
  $(cofmap 'a '(T b1 b2))  =  fmap $(cofmap 'a 'b2)   -- when a only occurs in the last parameter, b2
  $(cofmap 'a '(b -> c))   =  \x b -> $(cofmap 'a' 'c) (x ($(fmap 'a 'c) b))

DeriveFunctor is special in that it can recurse into function types, whereas DeriveFoldable and DeriveTraversable cannot (see the section on covariant and contravariant positions).

DeriveFoldable

Another comment in ​TcGenDeriv.hs reveals the underlying mechanism behind DeriveFoldable:

Deriving Foldable instances works the same way as Functor instances,
only Foldable instances are not possible for function types at all.
Here the derived instance for the type T above is:

  instance Foldable T where
      foldr f z (T1 x1 x2 x3) = $(foldr 'a 'b1) x1 ( $(foldr 'a 'a) x2 ( $(foldr 'a 'b2) x3 z ) )

The cases are:

  $(foldr 'a 'b)         =  \x z -> z     -- when b does not contain a
  $(foldr 'a 'a)         =  f
  $(foldr 'a '(b1,b2))   =  \x z -> case x of (x1,x2) -> $(foldr 'a 'b1) x1 ( $(foldr 'a 'b2) x2 z )
  $(foldr 'a '(T b1 b2)) =  \x z -> foldr $(foldr 'a 'b2) z x  -- when a only occurs in the last parameter, b2

Note that the arguments to the real foldr function are the wrong way around,
since (f :: a -> b -> b), while (foldr f :: b -> t a -> b).

In addition to foldr, DeriveFoldable also generates a definition for foldMap as of GHC 7.8.1 (addressing ​#7436). The pseudo-definition for $(foldMap) would look something like this:

  $(foldMap 'a 'b)         = \x -> mempty     -- when b does not contain a
  $(foldMap 'a 'a)         = f
  $(foldMap 'a '(b1,b2))   = \x -> case x of (x1, x2) -> mappend ($(foldMap 'a 'b1) x1) ($(foldMap 'a 'b2) x2)
  $(foldMap 'a '(T b1 b2)) = \x -> foldMap $(foldMap 'a 'b2) x -- when a only occurs in the last parameter, b2

DeriveTraversable

From ​TcGenDeriv.hs:

Again, Traversable is much like Functor and Foldable.

The cases are:

  $(traverse 'a 'b)          =  pure     -- when b does not contain a
  $(traverse 'a 'a)          =  f
  $(traverse 'a '(b1,b2))    =  \x -> case x of (x1,x2) -> (,) <$> $(traverse 'a 'b1) x1 <*> $(traverse 'a 'b2) x2
  $(traverse 'a '(T b1 b2))  =  traverse $(traverse 'a 'b2)  -- when a only occurs in the last parameter, b2

Note that the generated code is not as efficient as it could be. For instance:

  data T a = T Int a  deriving Traversable

gives the function: traverse f (T x y) = T <$> pure x <*> f y
instead of:         traverse f (T x y) = T x <$> f y

Covariant and contravariant positions

One challenge of deriving Functor instances for arbitrary data types is handling function types. To illustrate this, note that these all can have derived Functor instances:

data CovFun1 a = CovFun1 (Int -> a)
data CovFun2 a = CovFun2 ((a -> Int) -> a)
data CovFun3 a = CovFun3 (((Int -> a) -> Int) -> a)

but none of these can:

data ContraFun1 a = ContraFun1 (a -> Int)
data ContraFun2 a = ContraFun2 ((Int -> a) -> Int)
data ContraFun3 a = ContraFun3 (((a -> Int) -> a) -> Int)

In CovFun1, CovFun2, and CovFun3, all occurrences of the type variable a are in covariant positions (i.e., the a values are produced), whereas in ContraFun1, ContraFun2, and ContraFun3, all occurrences of a are in contravariant positions (i.e., the a values are consumed). If we have a function f :: a -> b, we can't apply f to an a value in a contravariant position, which precludes a Functor instance.

Most type variables appear in covariant positions. Functions are special in that the lefthand side of a function arrow reverses variance. If a function type a -> b appears in a covariant position (e.g., CovFun1 above), then a is in a contravariant position and b is in a covariant position. Similarly, if a -> b appears in a contravariant position (e.g., CovFun2 above), then a is in a covariant position and b is in a contravariant position.

If we annotate covariant positions with p (for positive) and contravariant positions with n (for negative), then we can examine the above examples with the following pseudo-type signatures:

CovFun1/ContraFun1 :: n -> p
CovFun2/ContraFun2 :: (p -> n) -> p
CovFun3/ContraFun3 :: ((n -> p) -> n) -> p

Since ContraFun1, ContraFun2, and ContraFun3 all use the last type parameter in at least one n position, GHC would reject a derived Functor instance for each of them.

Requirements for legal instances

This mechanism cannot derive Functor, Foldable, or Traversable instances for all data types. Currently, GHC checks if a data type meets the following criteria:

1. The data type has at least one type parameter. (For example, data NoArg = NoArg cannot have a Functor instance.)
2. The data type's last type parameter cannot be used contravariantly. (see the section on covariant and contravariant positions.)
3. The data type's last type parameter cannot be used in the "wrong place" in any constructor's data arguments. For example, in data Right a = Right [a] (Either Int a), the type parameter a is only ever used as the last type argument in [] and Either, so both [a] and Either Int a values can be fmapped. However, in data Wrong a = Wrong (Either a a), the type variable a appears in a position other than the last, so trying to fmap an Either a a value would not typecheck.

Note that there are two exceptions to this rule: tuple and function types.

4. The data type's last type variable cannot used in a -XDatatypeContexts constraint. For example, data Ord a => O a = O a deriving Functor would be rejected.

In addition, GHC performs checks for certain classes only:

1. For derived Foldable and Traversable instances, a data type cannot use function types. This restriction does not apply to derived Functor instances, however.
2. For derived Functor and Traversable instances, the data type's last type variable must be truly universally quantified, i.e., it must not have any class or equality constraints. This means that the following is legal:

data T a b where
    T1 :: a -> b -> T a b      -- Fine! Vanilla H-98
    T2 :: b -> c -> T a b      -- Fine! Existential c, but we can still map over 'b'
    T3 :: b -> T Int b         -- Fine! Constraint 'a', but 'b' is still polymorphic

deriving instance Functor (T a)

{-
instance Functor (T a) where
    fmap f (T1 a b) = T1 a (f b)
    fmap f (T2 b c) = T2 (f b) c
    fmap f (T3 x)   = T3 (f x)
-}

but the following is not legal:

data T a b where
    T4 :: Ord b => b -> T a b  -- No!  'b' is constrained
    T5 :: b -> T b b           -- No!  'b' is constrained
    T6 :: T a (b,b)            -- No!  'b' is constrained

This restriction does not apply to derived Foldable instances. See the following section for more details.

Relaxed universality check for DeriveFoldable

DeriveFunctor and DeriveTraversable cannot be used with data types that use existential constraints, since the type signatures of fmap and traverse make this impossible. However, Foldable instances are unique in that they do not produce constraints, but only consume them. Therefore, it is permissible to derive Foldable instances for constrained data types (e.g., GADTs).

For example, consider the following GADT:

data T a where
    T1 :: Ord a => a -> T a

In the type signatures for fmap :: Functor t => (a -> b) -> t a -> t b and traverse :: (Applicative f, Traversable t) => (a -> f b) -> t a -> f (t b), the t parameter appears both in an argument and the result type, so pattern-matching on a value of t must not impose any constraints, as neither fmap nor traverse would typecheck.

Foldable, however, only mentions t in argument types:

class Foldable t where
    fold :: Monoid m => t m -> m
    foldMap :: Monoid m => (a -> m) -> t a -> m
    foldr :: (a -> b -> b) -> b -> t a -> b
    foldr' :: (a -> b -> b) -> b -> t a -> b
    foldl :: (b -> a -> b) -> b -> t a -> b
    foldl' :: (b -> a -> b) -> b -> t a -> b
    foldr1 :: (a -> a -> a) -> t a -> a
    foldl1 :: (a -> a -> a) -> t a -> a
    toList :: t a -> [a]
    null :: t a -> Bool
    length :: t a -> Int
    elem :: Eq a => a -> t a -> Bool
    maximum :: forall a. Ord a => t a -> a
    minimum :: forall a. Ord a => t a -> a
    sum :: Num a => t a -> a
    product :: Num a => t a -> a

Therefore, a derived Foldable instance for T typechecks:

instance Foldable T where
    foldr f z (T1 a) = f a z -- foldr :: Ord a => (a -> b -> b) -> b -> T a -> b
    foldMap f (T1 a) = f a   -- foldMap :: (Monoid m, Ord a) => (a -> m) -> T a -> m

Deriving Foldable instances for GADTs with equality constraints could become murky, however. Consider this GADT:

data E a where
    E1 :: (a ~ Int) => a   -> E a
    E2 ::              Int -> E Int
    E3 :: (a ~ Int) => a   -> E Int
    E4 :: (a ~ Int) => Int -> E a

All four E constructors have the same "shape" in that they all take an argument of type a (or Int, to which a is constrained to be equal). Does that mean all four constructors would have their arguments folded over? While it is possible to derive perfectly valid code which would do so:

instance Foldable E where
    foldr f z (E1 e) = f e z
    foldr f z (E2 e) = f e z
    foldr f z (E3 e) = f e z
    foldr f z (E4 e) = f e z

    foldMap f (E1 e) = f e
    foldMap f (E2 e) = f e
    foldMap f (E3 e) = f e
    foldMap f (E4 e) = f e

it is much harder to determine which arguments are equivalent to a. Also consider this case:

data UnknownConstraints a where
    UC :: Mystery a => Int -> UnknownConstraints a

For all we know, it may be that a ~ Int => Mystery a. Does this mean that the Int argument in UC should be folded over?

To avoid these thorny edge cases, we only consider constructor arguments (1) whose types are syntactically equivalent to the last type parameter and (2) in cases when the last type parameter is a truly universally polymorphic. In the above E example, only E1 fits the bill, so the derived Foldable instance is actually:

instance Foldable E where
    foldr f z (E1 e) = f e z
    foldr f z (E2 e) = z
    foldr f z (E3 e) = z
    foldr f z (E4 e) = z

    foldMap f (E1 e) = f e
    foldMap f (E2 e) = mempty
    foldMap f (E3 e) = mempty
    foldMap f (E4 e) = mempty

To expound more on the meaning of criterion (2), we want not only to avoid cases like E2 :: Int -> E Int, but also something like this:

data HigherKinded f a where
    HigherKinded :: f a -> HigherKinded f (f a)

In this example, the last type variable is instantiated with f a, which contains one type variable f applied to another type variable a. We would not fold over the argument of type f a in this case, because the last type variable should be simple, i.e., contain only a single variable without any application.

For the original discussion on this proposal, see ​#10447.