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. 14111430, 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 patternmatch 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 objectorientedlike 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 dictionarypassing 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 welltyped selector function
tag :: Counter a > a
. See Field selectors and TypeApplications
for a full description of how the types of toplevel 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 compiletime error. In other words, GHC defines a record
selector function only for fields whose type does not mention the
existentiallyquantified 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 universallyquantified type variables of the data constructor. For GADTs, the field may mention only types that appear as a simple typevariable 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
 typevariable argument in G1's result type)
6.4.6.4. Restrictions¶
There are several restrictions on the ways in which existentiallyquantified 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”, becausea
is the result off1
. One way to see why this is wrong is to ask what typef1
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
orp==q
, buta==q
is wrong because it equates the two distinct types arising from the twoBaz1
constructors.You can’t patternmatch on an existentially quantified constructor in a
let
orwhere
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 patternmatch on an existentiallyquantified constructor in a
case
expression or in the patterns of a function definition. The reason for this restriction is really an implementation one. Typechecking 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 existentiallyquantified type “escaping”. So for now, there’s a simpletostate 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 forOrd t
and a value of typet
. That contradicts the idea thatnewtype
should have no concrete representation. You can get just the same efficiency and effect by usingdata
instead ofnewtype
. If there is no overloading involved, then there is more of a case for allowing an existentiallyquantifiednewtype
, because thedata
version does carry an implementation cost, but singlefield existentially quantified constructors aren’t much use. So the simple restriction (no existential stuff onnewtype
) 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 twoMkT
constructors:instance Eq T where (MkT a) == (MkT b) = ???
But
a
andb
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!