base-4.10.0.0: GHC/TypeLits.hs
{-# LANGUAGE Trustworthy #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE UndecidableInstances #-} -- for compiling instances of (==)
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE MagicHash #-}
{-# LANGUAGE PolyKinds #-}
{-| This module is an internal GHC module. It declares the constants used
in the implementation of type-level natural numbers. The programmer interface
for working with type-level naturals should be defined in a separate library.
@since 4.6.0.0
-}
module GHC.TypeLits
( -- * Kinds
Nat, Symbol -- Both declared in GHC.Types in package ghc-prim
-- * Linking type and value level
, N.KnownNat, natVal, natVal'
, KnownSymbol, symbolVal, symbolVal'
, N.SomeNat(..), SomeSymbol(..)
, someNatVal, someSymbolVal
, N.sameNat, sameSymbol
-- * Functions on type literals
, type (N.<=), type (N.<=?), type (N.+), type (N.*), type (N.^), type (N.-)
, AppendSymbol
, N.CmpNat, CmpSymbol
-- * User-defined type errors
, TypeError
, ErrorMessage(..)
) where
import GHC.Base(Eq(..), Ord(..), Bool(True,False), Ordering(..), otherwise)
import GHC.Types( Nat, Symbol )
import GHC.Num(Integer, fromInteger)
import GHC.Base(String)
import GHC.Show(Show(..))
import GHC.Read(Read(..))
import GHC.Real(toInteger)
import GHC.Prim(magicDict, Proxy#)
import Data.Maybe(Maybe(..))
import Data.Proxy (Proxy(..))
import Data.Type.Equality(type (==), (:~:)(Refl))
import Unsafe.Coerce(unsafeCoerce)
import GHC.TypeNats (KnownNat)
import qualified GHC.TypeNats as N
--------------------------------------------------------------------------------
-- | This class gives the string associated with a type-level symbol.
-- There are instances of the class for every concrete literal: "hello", etc.
--
-- @since 4.7.0.0
class KnownSymbol (n :: Symbol) where
symbolSing :: SSymbol n
-- | @since 4.7.0.0
natVal :: forall n proxy. KnownNat n => proxy n -> Integer
natVal p = toInteger (N.natVal p)
-- | @since 4.7.0.0
symbolVal :: forall n proxy. KnownSymbol n => proxy n -> String
symbolVal _ = case symbolSing :: SSymbol n of
SSymbol x -> x
-- | @since 4.8.0.0
natVal' :: forall n. KnownNat n => Proxy# n -> Integer
natVal' p = toInteger (N.natVal' p)
-- | @since 4.8.0.0
symbolVal' :: forall n. KnownSymbol n => Proxy# n -> String
symbolVal' _ = case symbolSing :: SSymbol n of
SSymbol x -> x
-- | This type represents unknown type-level symbols.
data SomeSymbol = forall n. KnownSymbol n => SomeSymbol (Proxy n)
-- ^ @since 4.7.0.0
-- | Convert an integer into an unknown type-level natural.
--
-- @since 4.7.0.0
someNatVal :: Integer -> Maybe N.SomeNat
someNatVal n
| n >= 0 = Just (N.someNatVal (fromInteger n))
| otherwise = Nothing
-- | Convert a string into an unknown type-level symbol.
--
-- @since 4.7.0.0
someSymbolVal :: String -> SomeSymbol
someSymbolVal n = withSSymbol SomeSymbol (SSymbol n) Proxy
-- | @since 4.7.0.0
instance Eq SomeSymbol where
SomeSymbol x == SomeSymbol y = symbolVal x == symbolVal y
-- | @since 4.7.0.0
instance Ord SomeSymbol where
compare (SomeSymbol x) (SomeSymbol y) = compare (symbolVal x) (symbolVal y)
-- | @since 4.7.0.0
instance Show SomeSymbol where
showsPrec p (SomeSymbol x) = showsPrec p (symbolVal x)
-- | @since 4.7.0.0
instance Read SomeSymbol where
readsPrec p xs = [ (someSymbolVal a, ys) | (a,ys) <- readsPrec p xs ]
type family EqSymbol (a :: Symbol) (b :: Symbol) where
EqSymbol a a = 'True
EqSymbol a b = 'False
type instance a == b = EqSymbol a b
--------------------------------------------------------------------------------
-- | Comparison of type-level symbols, as a function.
--
-- @since 4.7.0.0
type family CmpSymbol (m :: Symbol) (n :: Symbol) :: Ordering
-- | Concatenation of type-level symbols.
--
-- @since 4.10.0.0
type family AppendSymbol (m ::Symbol) (n :: Symbol) :: Symbol
-- | A description of a custom type error.
data {-kind-} ErrorMessage = Text Symbol
-- ^ Show the text as is.
| forall t. ShowType t
-- ^ Pretty print the type.
-- @ShowType :: k -> ErrorMessage@
| ErrorMessage :<>: ErrorMessage
-- ^ Put two pieces of error message next
-- to each other.
| ErrorMessage :$$: ErrorMessage
-- ^ Stack two pieces of error message on top
-- of each other.
infixl 5 :$$:
infixl 6 :<>:
-- | The type-level equivalent of 'error'.
--
-- The polymorphic kind of this type allows it to be used in several settings.
-- For instance, it can be used as a constraint, e.g. to provide a better error
-- message for a non-existent instance,
--
-- @
-- -- in a context
-- instance TypeError (Text "Cannot 'Show' functions." :$$:
-- Text "Perhaps there is a missing argument?")
-- => Show (a -> b) where
-- showsPrec = error "unreachable"
-- @
--
-- It can also be placed on the right-hand side of a type-level function
-- to provide an error for an invalid case,
--
-- @
-- type family ByteSize x where
-- ByteSize Word16 = 2
-- ByteSize Word8 = 1
-- ByteSize a = TypeError (Text "The type " :<>: ShowType a :<>:
-- Text " is not exportable.")
-- @
--
-- @since 4.9.0.0
type family TypeError (a :: ErrorMessage) :: b where
--------------------------------------------------------------------------------
-- | We either get evidence that this function was instantiated with the
-- same type-level symbols, or 'Nothing'.
--
-- @since 4.7.0.0
sameSymbol :: (KnownSymbol a, KnownSymbol b) =>
Proxy a -> Proxy b -> Maybe (a :~: b)
sameSymbol x y
| symbolVal x == symbolVal y = Just (unsafeCoerce Refl)
| otherwise = Nothing
--------------------------------------------------------------------------------
-- PRIVATE:
newtype SSymbol (s :: Symbol) = SSymbol String
data WrapS a b = WrapS (KnownSymbol a => Proxy a -> b)
-- See Note [magicDictId magic] in "basicType/MkId.hs"
withSSymbol :: (KnownSymbol a => Proxy a -> b)
-> SSymbol a -> Proxy a -> b
withSSymbol f x y = magicDict (WrapS f) x y