{-# LANGUAGE Trustworthy #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE ConstraintKinds #-} {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE NoImplicitPrelude #-} {-# LANGUAGE MagicHash #-} {-# LANGUAGE PolyKinds #-} {-| GHC's @DataKinds@ language extension lifts data constructors, natural numbers, and strings to the type level. This module provides the primitives needed for working with type-level numbers (the 'Nat' kind) and strings (the 'Symbol') kind. It also defines the 'TypeError' type family, a feature that makes use of type-level strings to support user defined type errors. For now, this module is the API for working with type-level literals. However, please note that it is a work in progress and is subject to change. Once the design of the @DataKinds@ feature is more stable, this will be considered only an internal GHC module, and the programmer interface for working with type-level data will 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.-) , type N.Div, type N.Mod, type N.Log2 , AppendSymbol , N.CmpNat, CmpSymbol -- * User-defined type errors , TypeError , ErrorMessage(..) ) where import GHC.Base(Eq(..), Ord(..), 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((:~:)(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 {-# NOINLINE someSymbolVal #-} -- For details see Note [NOINLINE someNatVal] in "GHC.TypeNats" -- The issue described there applies to `someSymbolVal` as well. -- | @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 ] -------------------------------------------------------------------------------- -- | 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 'Prelude.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