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 higher-level 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.Prim. (This documentation is generated from the file compiler/GHC/Builtin/primops.txt.pp.)

If you want to mention any of the primitive data types or operations in your program, you must first import GHC.Prim 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 two-word 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 machine-addition 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 bit-pattern, such as Int#, Float#, Double#. But this is not necessarily the case: a primitive value might be represented by a pointer to a heap-allocated object. Examples include Array#, the type of primitive arrays. Thus, Array# is an unlifted, boxed type. A primitive array is heap-allocated 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 numerically-intensive 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 double-precision 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 top-level binding.

  • You cannot bind a variable with an unboxed type in a recursive binding.

  • You may bind unboxed variables in a (non-recursive, non-top-level) pattern binding, but you must make any such pattern-match strict. (Failing to do so emits a warning -Wunbanged-strict-patterns.) 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 type Int#.

6.16.3. Unboxed tuples


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 non-primitive). 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 fully-fledged 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.

There are some restrictions on the use of unboxed tuples:

  • 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, y-1 #)
    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.

6.16.4. Unboxed sums


Enable the use of unboxed sum syntax.

-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 multi-arity 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 #) -> ...

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# and String 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 two Float32 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 two Float32 fields: The Word32 field is for the Word32# in the first alternative. The Pointer field is shared between String and Maybe 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’s Nothing. Otherwise Pointer points to the value in Just. As mentioned above, this type is lazy in its lifted field. Therefore, the type

    data 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 an Int32 tag field (i.e., the whole thing is represented by an integer).

6.16.5. Unlifted Newtypes


Enable the use of newtypes over types with non-lifted runtime representations.

GHC implements an UnliftedNewtypes extension as specified in this GHC proposal. 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 levity-polymorphic binder restriction, levity-polymorphic 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 User-Specified Kind Signature (CUSK). The exact impact is specified the section on CUSKs.

6.16.6. Unlifted Datatypes

Implies:DataKinds, StandaloneKindSignatures

Enable the declaration of data types with unlifted or levity-polymorphic result kind.

GHC implements the UnliftedDatatypes extension as specified in this GHC proposal. 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 levity-polymorphic 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 levity-polymorphic (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 levity-polymorphic binder restriction,

Due to ticket 19487 <https://gitlab.haskell.org/ghc/ghc/-/issues/19487>, it’s currently not possible to declare levity-polymorphic 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 zero-runtime-cost 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.