Currently, we support two forms of type-level literals: natural numbers, and symbol constants.
Natural number literals are a family of types, all of which belong to the kind Nat. Similarly,
symbol literals are types that belong to the kind Symbol:
Both numeric and symbol literal types are empty---they have no inhabitants. However, they may be
used as parameters to other type constructors, which makes them useful.
We use this idea to link the type-level literals to specific run-time values via singleton types.
The singleton types and some useful functions for working with them are defined in module GHC.TypeLits:
module GHC.TypeLits where
A singleton type is simply a type that has only one interesting inhabitant. We define a whole family
of singleton types, parameterized by type-level literals:
data Sing :: a -> *
For example, Sing 0, Sing 127, Sing "hello", and Sing "this also" are all
singleton types. The intuition is that the only inhabitant of Sing n is the value n. Notice
that Sing has a polymorphic kind because sometimes we apply it to numbers (which are of
kind Nat) and sometimes we apply it to symbols (which are of kind Symbol).
So, how do we make values of type Sing n? This is done with
the special overloaded constant sing:
class SingI a where sing :: Sing a-- Built-in instances for all type-literals.instance SingI 0 where sing = ... the singleton value representing 0 ...instance SingI 1 where sing = ... the singleton value representing 1 ...instance SingI "hello" where sing = ... the singleton value representing "hello" ...// ... etc.
Here are some examples on the GHCi prompt to get a feel of how sing works:
> :set -XDataKinds> sing :: Sing 1> 1> sing :: Sing "hello"> "hello"
The name SingI is a mnemonic for the different uses of the class:
It is the introduction construct for 'Sing' values,
It is an implicit singleton parameter (this is discussed in more detail bellow)
It is also useful to get the actual integer or string associated with a singleton.
We can do this with the overloaded function fromSing. Its type is quite general because
it can support various singleton families, but to start, consider the following two instances
of its type:
fromSing :: Sing (a :: Nat) -> IntegerfromSing :: Sing (a :: Symbol) -> String
Notice that the return type of the function is determined by the kind of the
singleton family, not the concrete type. For example, for any type n of
kind Nat we get an Integer, while for any type s of kind Symbol we get a
string. Based on this idea, here is a more general type for fromSing:
fromSing :: SingE (KindOf a) => Sing a -> Demote a
TODO Explain this more fully.
Notice that GHCi could display values of type Sing, so they have a Show instance. As another example, here
is the definition of the Show instance:
instance (SingE (KindOf a, Show (Demote a)) => Show (Sing a) where showsPrec p = showsPrec p . fromSing
Easy! We just convert the singleton into an ordinary value (integer or string), and use itsShow instance to display it.
Next, we show two functions which make it easier to work with singleton types:
withSing :: SingI a => (Sing a -> b) -> bwithSing f = f singsingThat :: SingRep a => (Demote a -> Bool) -> Maybe (Sing a)singThat p = withSing $ \x -> if p (fromSing x) then Just x else Nothing
The first function, withSing, is useful when we want to work with the same singleton value multiple times.
The constant sing is polymorphic, so every time we use it in a program, it may refer to a potentially
different singleton, so to ensure that two singleton values are the same we have to resort to
explicit type signatures, which just adds noise to a definition. By using, withSing we avoid this problem
because we get an explicit (monomorphic) name for a given singleton, and so we can use the name many times
without any type signatures. This technique is shown in the definition of the second function, singThat.
The function singThat is similar to the constant sing in that it defines new singleton values. However,
it allows us to specify a predicate on (the representation of) the value and it only succeeds if this predicate
holds. Here are some examples of how that works:
Now, using singThat we can show the definition of the Read instance for singletons:
instance (SingRep a, Read (Demote a), Eq (Demote a)) => Read (Sing a) where readsPrec p cs = do (x,ys) <- readsPrec p cs case singThat (== x) of Just y -> [(y,ys)] Nothing -> 
We use the Read instance of the representation for the singletons to parse a value,
and then, we use singThat to make sure that this was the value corresponding to
Implicit vs. Explicit Parameters
There are two different styles of writing functions that use singleton types.
To illustrate the two style consider a type for working with C-style arrays:
newtype ArrPtr (n :: Nat) a = ArrPtr (Ptr a)
Now consider the definition of the function memset, which sets all elements
of the array to a given fixed value.
One approach is to use an explicit singleton parameter. For example:
memset_c :: Storable a => ArrPtr n a -> a -> Sing n -> IO ()memset_c (ArrPtr p) a size = mapM_ (\i -> pokeElemOff p i a) [0 .. fromIntegral (fromSing size - 1) ]
This style is, basically, a more typed version of what is found in many standard C libraries.
Callers of this function have to pass the size of the array explicitly, and the type system checks that the
size matches that of the array. Note that in the implementation of memset_c we used fromSing
to get the concrete integer associated with the singleton type, so that we know how many elements
to set with the given value/
While the explicit Sing parameter is convenient when we define the function, it is a bit
tedious to have to provide it all the time---it is easier to let the compiler infer the value,
based on the type of the array:
memset :: (Storable a, SingI n) => ArrPtr n a -> a -> IO ()memset ptr a = withSing (memset_c ptr a)