6.4.6. Existentially quantified data constructors¶

ExistentialQuantification¶

Implies:	`ExplicitForAll`
Since:	6.8.1
Status:	Included in `GHC2021`

Allow existentially quantified type variables in types.

The idea of using existential quantification in data type declarations was suggested by Perry, and implemented in Hope+ (Nigel Perry, The Implementation of Practical Functional Programming Languages, PhD Thesis, University of London, 1991). It was later formalised by Laufer and Odersky (Polymorphic type inference and abstract data types, TOPLAS, 16(5), pp. 1411-1430, 1994). It’s been in Lennart Augustsson’s hbc Haskell compiler for several years, and proved very useful. Here’s the idea. Consider the declaration:

data Foo = forall a. MkFoo a (a -> Bool)
         | Nil

The data type Foo has two constructors with types:

MkFoo :: forall a. a -> (a -> Bool) -> Foo
Nil   :: Foo

Notice that the type variable a in the type of MkFoo does not appear in the data type itself, which is plain Foo. For example, the following expression is fine:

[MkFoo 3 even, MkFoo 'c' isUpper] :: [Foo]

Here, (MkFoo 3 even) packages an integer with a function even that maps an integer to Bool; and MkFoo 'c' isUpper packages a character with a compatible function. These two things are each of type Foo and can be put in a list.

What can we do with a value of type Foo? In particular, what happens when we pattern-match on MkFoo?

f (MkFoo val fn) = ???

Since all we know about val and fn is that they are compatible, the only (useful) thing we can do with them is to apply fn to val to get a boolean. For example:

f :: Foo -> Bool
f (MkFoo val fn) = fn val

What this allows us to do is to package heterogeneous values together with a bunch of functions that manipulate them, and then treat that collection of packages in a uniform manner. You can express quite a bit of object-oriented-like programming this way.

6.4.6.1. Why existential?¶

What has this to do with existential quantification? Simply that MkFoo has the (nearly) isomorphic type

MkFoo :: (exists a . (a, a -> Bool)) -> Foo

But Haskell programmers can safely think of the ordinary universally quantified type given above, thereby avoiding adding a new existential quantification construct.

6.4.6.2. Existentials and type classes¶

An easy extension is to allow arbitrary contexts before the constructor. For example:

data Baz = forall a. Eq a => Baz1 a a
         | forall b. Show b => Baz2 b (b -> b)

The two constructors have the types you’d expect:

Baz1 :: forall a. Eq a => a -> a -> Baz
Baz2 :: forall b. Show b => b -> (b -> b) -> Baz

But when pattern matching on Baz1 the matched values can be compared for equality, and when pattern matching on Baz2 the first matched value can be converted to a string (as well as applying the function to it). So this program is legal:

f :: Baz -> String
f (Baz1 p q) | p == q    = "Yes"
             | otherwise = "No"
f (Baz2 v fn)            = show (fn v)

Operationally, in a dictionary-passing implementation, the constructors Baz1 and Baz2 must store the dictionaries for Eq and Show respectively, and extract it on pattern matching.

6.4.6.3. Record Constructors¶

GHC allows existentials to be used with records syntax as well. For example:

data Counter a = forall self. NewCounter
    { _this    :: self
    , _inc     :: self -> self
    , _display :: self -> IO ()
    , tag      :: a
    }

Here tag is a public field, with a well-typed selector function tag :: Counter a -> a. See Field selectors and TypeApplications for a full description of how the types of top-level field selectors are determined.

The self type is hidden from the outside; any attempt to apply _this, _inc or _display as functions will raise a compile-time error. In other words, GHC defines a record selector function only for fields whose type does not mention the existentially-quantified variables. (This example used an underscore in the fields for which record selectors will not be defined, but that is only programming style; GHC ignores them.)

To make use of these hidden fields, we need to create some helper functions:

inc :: Counter a -> Counter a
inc (NewCounter x i d t) = NewCounter
    { _this = i x, _inc = i, _display = d, tag = t }

display :: Counter a -> IO ()
display NewCounter{ _this = x, _display = d } = d x

Now we can define counters with different underlying implementations:

counterA :: Counter String
counterA = NewCounter
    { _this = 0, _inc = (1+), _display = print, tag = "A" }

counterB :: Counter String
counterB = NewCounter
    { _this = "", _inc = ('#':), _display = putStrLn, tag = "B" }

main = do
    display (inc counterA)         -- prints "1"
    display (inc (inc counterB))   -- prints "##"

Record update syntax is supported for existentials (and GADTs):

setTag :: Counter a -> a -> Counter a
setTag obj t = obj{ tag = t }

The rule for record update is this:

the types of the updated fields may mention only the universally-quantified type variables of the data constructor. For GADTs, the field may mention only types that appear as a simple type-variable argument in the constructor’s result type.

For example:

data T a b where { T1 { f1::a, f2::b, f3::(b,c) } :: T a b } -- c is existential
upd1 t x = t { f1=x }   -- OK:   upd1 :: T a b -> a' -> T a' b
upd2 t x = t { f3=x }   -- BAD   (f3's type mentions c, which is
                        --        existentially quantified)

data G a b where { G1 { g1::a, g2::c } :: G a [c] }
upd3 g x = g { g1=x }   -- OK:   upd3 :: G a b -> c -> G c b
upd4 g x = g { g2=x }   -- BAD (g2's type mentions c, which is not a simple
                        --      type-variable argument in G1's result type)

6.4.6.4. Restrictions¶

There are several restrictions on the ways in which existentially-quantified constructors can be used.

When pattern matching, each pattern match introduces a new, distinct, type for each existential type variable. These types cannot be unified with any other type, nor can they escape from the scope of the pattern match. For example, these fragments are incorrect:
```
f1 (MkFoo a f) = a
```
Here, the type bound by MkFoo “escapes”, because a is the result of f1. One way to see why this is wrong is to ask what type f1 has:
```
f1 :: Foo -> a             -- Weird!
```
What is this “a” in the result type? Clearly we don’t mean this:
```
f1 :: forall a. Foo -> a   -- Wrong!
```
The original program is just plain wrong. Here’s another sort of error
```
f2 (Baz1 a b) (Baz1 p q) = a==q
```
It’s ok to say a==b or p==q, but a==q is wrong because it equates the two distinct types arising from the two Baz1 constructors.
You can’t pattern-match on an existentially quantified constructor in a let or where group of bindings. So this is illegal:
```
f3 x = a==b where { Baz1 a b = x }
```
Instead, use a case expression:
```
f3 x = case x of Baz1 a b -> a==b
```
In general, you can only pattern-match on an existentially-quantified constructor in a case expression or in the patterns of a function definition. The reason for this restriction is really an implementation one. Type-checking binding groups is already a nightmare without existentials complicating the picture. Also an existential pattern binding at the top level of a module doesn’t make sense, because it’s not clear how to prevent the existentially-quantified type “escaping”. So for now, there’s a simple-to-state restriction. We’ll see how annoying it is.
You can’t use existential quantification for newtype declarations. So this is illegal:
```
newtype T = forall a. Ord a => MkT a
```
Reason: a value of type T must be represented as a pair of a dictionary for Ord t and a value of type t. That contradicts the idea that newtype should have no concrete representation. You can get just the same efficiency and effect by using data instead of newtype. If there is no overloading involved, then there is more of a case for allowing an existentially-quantified newtype, because the data version does carry an implementation cost, but single-field existentially quantified constructors aren’t much use. So the simple restriction (no existential stuff on newtype) stands, unless there are convincing reasons to change it.
You can’t use deriving to define instances of a data type with existentially quantified data constructors. Reason: in most cases it would not make sense. For example:
```
data T = forall a. MkT [a] deriving( Eq )
```
To derive Eq in the standard way we would need to have equality between the single component of two MkT constructors:
```
instance Eq T where
  (MkT a) == (MkT b) = ???
```
But a and b have distinct types, and so can’t be compared. It’s just about possible to imagine examples in which the derived instance would make sense, but it seems altogether simpler simply to prohibit such declarations. Define your own instances!