6.16. Unboxed types and primitive operations¶
GHC is built on a raft of primitive data types and operations; “primitive” in the sense that they cannot be defined in Haskell itself. While you really can use this stuff to write fast code, we generally find it a lot less painful, and more satisfying in the long run, to use higherlevel language features and libraries. With any luck, the code you write will be optimised to the efficient unboxed version in any case. And if it isn’t, we’d like to know about it.
All these primitive data types and operations are exported by the library GHC.Exts.
If you want to mention any of the primitive data types or operations in
your program, you must first import GHC.Exts
to bring them into
scope. Many of them have names ending in #
, and to mention such names
you need the MagicHash
extension.
The primops make extensive use of unboxed types and unboxed tuples, which we briefly summarise here.
6.16.1. Unboxed types¶
Most types in GHC are boxed, which means that values of that type are
represented by a pointer to a heap object. The representation of a
Haskell Int
, for example, is a twoword heap object. An unboxed
type, however, is represented by the value itself, no pointers or heap
allocation are involved.
Unboxed types correspond to the “raw machine” types you would use in C:
Int#
(long int), Double#
(double), Addr#
(void *), etc. The
primitive operations (PrimOps) on these types are what you might
expect; e.g., (+#)
is addition on Int#
s, and is the
machineaddition that we all know and love—usually one instruction.
Primitive (unboxed) types cannot be defined in Haskell, and are
therefore built into the language and compiler. Primitive types are
always unlifted; that is, a value of a primitive type cannot be bottom.
(Note: a “boxed” type means that a value is represented by a pointer to a heap
object; a “lifted” type means that terms of that type may be bottom. See
the next paragraph for an example.)
We use the convention (but it is only a convention) that primitive
types, values, and operations have a #
suffix (see
The magic hash). For some primitive types we have special syntax for
literals, also described in the same section.
Primitive values are often represented by a simple bitpattern, such as
Int#
, Float#
, Double#
. But this is not necessarily the case:
a primitive value might be represented by a pointer to a heapallocated
object. Examples include Array#
, the type of primitive arrays. Thus,
Array#
is an unlifted, boxed type. A
primitive array is heapallocated because it is too big a value to fit
in a register, and would be too expensive to copy around; in a sense, it
is accidental that it is represented by a pointer. If a pointer
represents a primitive value, then it really does point to that value:
no unevaluated thunks, no indirections. Nothing can be at the other end
of the pointer than the primitive value. A numericallyintensive program
using unboxed types can go a lot faster than its “standard”
counterpart—we saw a threefold speedup on one example.
6.16.2. Unboxed type kinds¶
Because unboxed types are represented without the use of pointers, we
cannot store them in a polymorphic data type.
For example, the Just
node
of Just 42#
would have to be different from the Just
node of
Just 42
; the former stores an integer directly, while the latter
stores a pointer. GHC currently does not support this variety of Just
nodes (nor for any other data type). Accordingly, the kind of an unboxed
type is different from the kind of a boxed type.
The Haskell Report describes that *
(spelled Type
and imported from
Data.Kind
in the GHC dialect of Haskell) is the kind of ordinary data types,
such as Int
. Furthermore, type constructors can have kinds with arrows; for
example, Maybe
has kind Type > Type
. Unboxed types have a kind that
specifies their runtime representation. For example, the type Int#
has kind
TYPE IntRep
and Double#
has kind TYPE DoubleRep
. These kinds say
that the runtime representation of an Int#
is a machine integer, and the
runtime representation of a Double#
is a machine doubleprecision floating
point. In contrast, the kind Type
is actually just a synonym for TYPE
LiftedRep
. More details of the TYPE
mechanisms appear in the section
on runtime representation polymorphism.
Given that Int#
’s kind is not Type
, then it follows that Maybe
Int#
is disallowed. Similarly, because type variables tend to be of kind
Type
(for example, in (.) :: (b > c) > (a > b) > a > c
, all the
type variables have kind Type
), polymorphism tends not to work over
primitive types. Stepping back, this makes some sense, because a polymorphic
function needs to manipulate the pointers to its data, and most primitive types
are unboxed.
There are some restrictions on the use of primitive types:
You cannot define a newtype whose representation type (the argument type of the data constructor) is an unboxed type. Thus, this is illegal:
newtype A = MkA Int#
However, this restriction can be relaxed by enabling
UnliftedNewtypes
. The section on unlifted newtypes details the behavior of such types.You cannot bind a variable with an unboxed type in a toplevel binding.
You cannot bind a variable with an unboxed type in a recursive binding.
You may bind unboxed variables in a (nonrecursive, nontoplevel) pattern binding, but you must make any such patternmatch strict. (Failing to do so emits a warning
Wunbangedstrictpatterns
.) For example, rather than:data Foo = Foo Int Int# f x = let (Foo a b, w) = ..rhs.. in ..body..
you must write:
data Foo = Foo Int Int# f x = let !(Foo a b, w) = ..rhs.. in ..body..
since
b
has typeInt#
.
6.16.3. Unboxed tuples¶

UnboxedTuples
¶ Implies: UnboxedSums
Since: 6.8.1
Unboxed tuples aren’t really exported by GHC.Exts
; they are a
syntactic extension (UnboxedTuples
). An
unboxed tuple looks like this:
(# e_1, ..., e_n #)
where e_1..e_n
are expressions of any type (primitive or
nonprimitive). The type of an unboxed tuple looks the same.
Note that when unboxed tuples are enabled, (#
is a single lexeme, so
for example when using operators like #
and #
you need to write
( # )
and ( # )
rather than (#)
and (#)
.
Unboxed tuples are used for functions that need to return multiple
values, but they avoid the heap allocation normally associated with
using fullyfledged tuples. When an unboxed tuple is returned, the
components are put directly into registers or on the stack; the unboxed
tuple itself does not have a composite representation. Many of the
primitive operations listed in primops.txt.pp
return unboxed tuples.
In particular, the IO
and ST
monads use unboxed tuples to avoid
unnecessary allocation during sequences of operations.
The typical use of unboxed tuples is simply to return multiple
values, binding those multiple results with a case
expression,
thus:
f x y = (# x+1, y1 #)
g x = case f x x of { (# a, b #) > a + b }
You can have an unboxed tuple in a pattern binding, thus
f x = let (# p,q #) = h x in ..body..
If the types of p
and q
are not unboxed, the resulting
binding is lazy like any other Haskell pattern binding. The above
example desugars like this:
f x = let t = case h x of { (# p,q #) > (p,q) }
p = fst t
q = snd t
in ..body..
Indeed, the bindings can even be recursive.
To refer to the unboxed tuple type constructors themselves, e.g. if you
want to attach instances to them, use (# #)
, (#,#)
, (#,,#)
, etc.
This mirrors the syntax for boxed tuples ()
, (,)
, (,,)
, etc.
6.16.4. Unboxed sums¶

UnboxedSums
¶ Implied by: UnboxedTuples
Since: 8.2.1 Enable the use of unboxed sum syntax. Implied by
UnboxedTuples
.
XUnboxedSums enables new syntax for anonymous, unboxed sum types. The syntax for an unboxed sum type with N alternatives is
(# t_1  t_2  ...  t_N #)
where t_1
… t_N
are types (which can be unlifted, including unboxed
tuples and sums).
Unboxed tuples can be used for multiarity alternatives. For example:
(# (# Int, String #)  Bool #)
The term level syntax is similar. Leading and preceding bars () indicate which alternative it is. Here are two terms of the type shown above:
(# (# 1, "foo" #)  #)  first alternative
(#  True #)  second alternative
The pattern syntax reflects the term syntax:
case x of
(# (# i, str #)  #) > ...
(#  bool #) > ...
Note that spaces are always required around bars. For example, (#  1#   #)
is valid, but (#  1#  #)
and (# 1#   #)
are both invalid.
The type constructors themselves can be written in prefix form as (#  #)
,
(#   #)
, (#    #)
, etc. Partial applications must also use prefix form,
i.e. (#  #) Int#
. Saturated applications can be written either way,
so that (#  #) Int# Float#
is equivalent to (# Int#  Float# #)
.
Unboxed sums are “unboxed” in the sense that, instead of allocating sums in the heap and representing values as pointers, unboxed sums are represented as their components, just like unboxed tuples. These “components” depend on alternatives of a sum type. Like unboxed tuples, unboxed sums are lazy in their lifted components.
The code generator tries to generate as compact layout as possible for each unboxed sum. In the best case, size of an unboxed sum is size of its biggest alternative plus one word (for a tag). The algorithm for generating the memory layout for a sum type works like this:
All types are classified as one of these classes: 32bit word, 64bit word, 32bit float, 64bit float, pointer.
For each alternative of the sum type, a layout that consists of these fields is generated. For example, if an alternative has
Int
,Float#
andString
fields, the layout will have an 32bit word, 32bit float and pointer fields.Layout fields are then overlapped so that the final layout will be as compact as possible. For example, suppose we have the unboxed sum:
(# (# Word32#, String, Float# #)  (# Float#, Float#, Maybe Int #) #)
The final layout will be something like
Int32, Float32, Float32, Word32, Pointer
The first
Int32
is for the tag. There are twoFloat32
fields because floating point types can’t overlap with other types, because of limitations of the code generator that we’re hoping to overcome in the future. The second alternative needs twoFloat32
fields: TheWord32
field is for theWord32#
in the first alternative. ThePointer
field is shared betweenString
andMaybe Int
values of the alternatives.As another example, this is the layout for the unboxed version of
Maybe a
type,(# (# #)  a #)
:Int32, Pointer
The
Pointer
field is not used when tag says that it’sNothing
. OtherwisePointer
points to the value inJust
. As mentioned above, this type is lazy in its lifted field. Therefore, the typedata Maybe' a = Maybe' (# (# #)  a #)
is precisely isomorphic to the type
Maybe a
, although its memory representation is different.In the degenerate case where all the alternatives have zero width, such as the
Bool
like(# (# #)  (# #) #)
, the unboxed sum layout only has anInt32
tag field (i.e., the whole thing is represented by an integer).
6.16.5. Unlifted Newtypes¶

UnliftedNewtypes
¶ Since: 8.10.1 Enable the use of newtypes over types with nonlifted runtime representations.
GHC implements an UnliftedNewtypes
extension as specified in
the GHC proposal #98.
UnliftedNewtypes
relaxes the restrictions around what types can appear inside
of a newtype
. For example, the type
newtype A = MkA Int#
is accepted when this extension is enabled. This creates a type
A :: TYPE IntRep
and a data constructor MkA :: Int# > A
.
Although the kind of A
is inferred by GHC, there is nothing visually
distinctive about this type that indicated that is it not of kind Type
like newtypes typically are. GADTSyntax can be used to
provide a kind signature for additional clarity
newtype A :: TYPE IntRep where
MkA :: Int# > A
The Coercible
machinery works with unlifted newtypes just like it does with
lifted types. In either of the equivalent formulations of A
given above,
users would additionally have access to a coercion between A
and Int#
.
As a consequence of the
representationpolymorphic binder restriction,
representationpolymorphic fields are disallowed in data constructors
of data types declared using data
. However, since newtype
data
constructor application is implemented as a coercion instead of as function
application, this restriction does not apply to the field inside a newtype
data constructor. Thus, the type checker accepts
newtype Identity# :: forall (r :: RuntimeRep). TYPE r > TYPE r where
MkIdentity# :: forall (r :: RuntimeRep) (a :: TYPE r). a > Identity# a
And with UnboxedSums enabled
newtype Maybe# :: forall (r :: RuntimeRep). TYPE r > TYPE (SumRep '[r, TupleRep '[]]) where
MkMaybe# :: forall (r :: RuntimeRep) (a :: TYPE r). (# a  (# #) #) > Maybe# a
This extension also relaxes some of the restrictions around data family
instances. In particular, UnliftedNewtypes
permits a
newtype instance
to be given a return kind of TYPE r
, not just
Type
. For example, the following newtype instance
declarations would be
permitted:
class Foo a where
data FooKey a :: TYPE IntRep
class Bar (r :: RuntimeRep) where
data BarType r :: TYPE r
instance Foo Bool where
newtype FooKey Bool = FooKeyBoolC Int#
instance Bar WordRep where
newtype BarType WordRep = BarTypeWordRepC Word#
It is worth noting that UnliftedNewtypes
is not required to give
the data families themselves return kinds involving TYPE
, such as the
FooKey
and BarType
examples above. The extension is
only required for newtype instance
declarations, such as FooKeyBoolC
and BarTypeWorkRepC
above.
This extension impacts the determination of whether or not a newtype has a Complete UserSpecified Kind Signature (CUSK). The exact impact is specified the section on CUSKs.
6.16.6. Unlifted Datatypes¶

UnliftedDatatypes
¶ Implies: DataKinds
,StandaloneKindSignatures
Since: 9.2.1 Enable the declaration of data types with unlifted or levitypolymorphic result kind.
GHC implements the UnliftedDatatypes
extension as specified in
the GHC proposal #265.
UnliftedDatatypes
relaxes the restrictions around what result kinds
are allowed in data declarations. For example, the type
data UList a :: UnliftedType where
UCons :: a > UList a > UList a
UNil :: UList a
defines a list type that lives in kind UnliftedType
(e.g., TYPE (BoxedRep Unlifted)
). As such, each occurrence of a term of that
type is assumed to be evaluated (and the compiler makes sure that is indeed the
case). In other words: Unlifted data types behave like data types in strict
languages such as OCaml or Idris. However unlike StrictData
,
this extension will not change whether the fields of a (perhaps unlifted)
data type are strict or lazy. For example, UCons
is lazy in its first
argument as its field has kind Type
.
The fact that unlifted types are always evaluated allows GHC to elide evaluatedness checks at runtime. See the Motivation section of the proposal for how this can improve performance for some programs.
The above data declaration in GADT syntax correctly suggests that unlifted
data types are compatible with the full GADT feature set. Somewhat conversely,
you can also declare unlifted data types in Haskell98 syntax, which requires you
to specify the result kind via StandaloneKindSignatures
:
type UList :: Type > UnliftedType
data UList a = UCons a (UList a)  UNil
You may even declare levitypolymorphic data types:
type PEither :: Type > Type > TYPE (BoxedRep l)
data PEither l r = PLeft l  PRight r
f :: PEither @Unlifted Int Bool > Bool
f (PRight b) = b
f _ = False
While f
above could reasonably be levitypolymorphic (as it evaluates its
argument either way), GHC currently disallows the more general type
PEither @l Int Bool > Bool
. This is a consequence of the
representationpolymorphic binder restriction,
Due to %s19487, it’s currently not possible to declare levitypolymorphic data types with nullary data constructors. There’s a workaround, though:
type T :: TYPE (BoxedRep l)
data T where
MkT :: forall l. (() :: Constraint) => T @l
The use of =>
makes the type of MkT
lifted.
If you want a zeroruntimecost alternative, use MkT :: Proxy# () > T @l
instead and bear with the additional proxy#
argument at construction sites.
This extension also relaxes some of the restrictions around data family
instances. In particular, UnliftedDatatypes
permits a
data instance
to be given a return kind that unifies with
TYPE (BoxedRep l)
, not just Type
. For example, the following data
instance
declarations would be permitted:
data family F a :: UnliftedType
data instance F Int = FInt
data family G a :: TYPE (BoxedRep l)
data instance G Int = GInt Int  defaults to Type
data instance G Bool :: UnliftedType where
GBool :: Bool > G Bool
data instance G Char :: Type where
GChar :: Char > G Char
data instance G Double :: forall l. TYPE (BoxedRep l) where
GDouble :: Int > G @l Double
It is worth noting that UnliftedDatatypes
is not required to give
the data families themselves return kinds involving TYPE
, such as the
G
example above. The extension is only required for data instance
declarations, such as FInt
and GBool
above.