Commit 05710922 authored by Iavor S. Diatchki's avatar Iavor S. Diatchki

Add a section about promoted literals to the manual.

parent d402d8a6
......@@ -5371,6 +5371,52 @@ Note that this requires <option>-XTypeOperators</option>.
<sect3 id="promoted-literals">
<title>Promoted Literals</title>
Numeric and string literals are prmoted to the type level, giving convenient
access to a large number of predefined type-level constants. Numeric literals
are of kind <literal>Nat</literal>, while string literals are of kind
<literal>Symbol</literal>. These kinds are defined in the module
Here is an exampe of using type-level numeric literals to provide a safe
interface to a low-level function:
import GHC.TypeLits
import Data.Word
import Foreign
newtype ArrPtr (n :: Nat) a = ArrPtr (Ptr a)
clearPage :: ArrPtr 4096 Word8 -> IO ()
clearPage (ArrPtr p) = ...
Here is an example of using type-level string literals to simulate
simple record operations:
data Label (l :: Symbol) = Get
class Has a l b | a l -> b where
from :: a -> Label l -> b
data Point = Point Int Int deriving Show
instance Has Point "x" Int where from (Point x _) _ = x
instance Has Point "y" Int where from (Point _ y) _ = y
example = from (Point 1 2) (Get :: Label "x")
<sect2 id="kind-polymorphism-limitations">
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