6.4.13. Representation polymorphism¶
In order to allow full flexibility in how kinds are used, it is necessary
to use the kind system to differentiate between boxed, lifted types
(normal, everyday types like Int
and [Bool]
) and unboxed, primitive
types (Unboxed types and primitive operations) like Int#
. We thus have socalled representation
polymorphism.
Here are the key definitions, all available from GHC.Exts
:
TYPE :: RuntimeRep > Type  highly magical, built into GHC
data Levity = Lifted  for things like `Int`
 Unlifted  for things like `Array#`
data RuntimeRep = BoxedRep Levity  for anything represented by a GCmanaged pointer
 IntRep  for `Int#`
 TupleRep [RuntimeRep]  unboxed tuples, indexed by the representations of the elements
 SumRep [RuntimeRep]  unboxed sums, indexed by the representations of the disjuncts
 ...
type LiftedRep = BoxedRep Lifted
type Type = TYPE LiftedRep  Type is just an ordinary type synonym
The idea is that we have a new fundamental type constant TYPE
, which
is parameterised by a RuntimeRep
. We thus get Int# :: TYPE IntRep
and Bool :: TYPE LiftedRep
. Anything with a type of the form
TYPE x
can appear to either side of a function arrow >
. We can
thus say that >
has type
TYPE r1 > TYPE r2 > TYPE LiftedRep
. The result is always lifted
because all functions are lifted in GHC.
6.4.13.1. Levity polymorphism¶
A special case of representation polymorphism is levity polymorphism,
where we abstract over a variable of kind Levity
, such as:
example :: forall (l :: Levity) (a :: TYPE (BoxedRep l)). (Int > a) > a
example f = f 42
With UnliftedDatatypes
, we can even declare levitypolymorphic
data types:
type PEither :: Type > Type > TYPE (BoxedRep l)
data PEither l r = PLeft l  PRight r
6.4.13.2. No representationpolymorphic variables or arguments¶
If GHC didn’t have to compile programs that run in the real world, that would be the end of the story. But representation polymorphism can cause quite a bit of trouble for GHC’s code generator. Consider
bad :: forall (r1 :: RuntimeRep) (r2 :: RuntimeRep)
(a :: TYPE r1) (b :: TYPE r2).
(a > b) > a > b
bad f x = f x
This seems like a generalisation of the standard $
operator. If we
think about compiling this to runnable code, though, problems appear.
In particular, when we call bad
, we must somehow pass x
into
bad
. How wide (that is, how many bits) is x
? Is it a pointer?
What kind of register (floatingpoint or integral) should x
go in?
It’s all impossible to say, because x
’s type, a :: TYPE r1
is
representationpolymorphic. We thus forbid such constructions, via the
following straightforward rule:
No variable may have a representationpolymorphic type.
This eliminates bad
because the variable x
would have a
representationpolymorphic type.
However, not all is lost. We can still do this:
($) :: forall r (a :: Type) (b :: TYPE r).
(a > b) > a > b
f $ x = f x
Here, only b
is representationpolymorphic. There are no variables
with a representationpolymorphic type. And the code generator has no
trouble with this. Indeed, this is the true type of GHC’s $
operator,
slightly more general than the Haskell 98 version.
Because the code generator must store and move arguments as well as variables, the logic above applies equally well to function arguments, which may not be representationpolymorphic.
6.4.13.3. Representationpolymorphic bottoms¶
We can use representation polymorphism to good effect with error
and undefined
, whose types are given here:
undefined :: forall (r :: RuntimeRep) (a :: TYPE r).
HasCallStack => a
error :: forall (r :: RuntimeRep) (a :: TYPE r).
HasCallStack => String > a
These functions do not bind a representationpolymorphic variable, and so are accepted. Their polymorphism allows users to use these to conveniently stub out functions that return unboxed types.
6.4.13.4. Printing representationpolymorphic types¶

fprintexplicitruntimereps
¶ Print
RuntimeRep
andLevity
parameters as they appear; otherwise, they are defaulted toLiftedRep
andLifted
, respectively.
Most GHC users will not need to worry about representation polymorphism
or unboxed types. For these users, seeing the representation polymorphism
in the type of $
is unhelpful. And thus, by default, it is suppressed,
by supposing all type variables of type RuntimeRep
to be LiftedRep
when printing, and printing TYPE LiftedRep
as Type
(or *
when
StarIsType
is on).
Should you wish to see representation polymorphism in your types, enable
the flag fprintexplicitruntimereps
. For example,
ghci> :t ($) ($) :: (a > b) > a > b ghci> :set fprintexplicitruntimereps ghci> :t ($) ($) :: forall (r :: GHC.Types.RuntimeRep) a (b :: TYPE r). (a > b) > a > b