Safe Haskell	Trustworthy
Language	Haskell2010

GHC.TypeLits

Contents

Kinds
Linking type and value level
Functions on type literals
User-defined type errors

Description

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

Synopsis

Kinds

data Nat :: TYPE Lifted Source

(Kind) This is the kind of type-level natural numbers.

Instances

type (==) Nat a b

data Symbol :: TYPE Lifted Source

(Kind) This is the kind of type-level symbols. Declared here because class IP needs it

Instances

type (==) Symbol a b

Linking type and value level

class KnownNat n Source

This class gives the integer associated with a type-level natural. There are instances of the class for every concrete literal: 0, 1, 2, etc.

Since: 4.7.0.0

Minimal complete definition

natSing

natVal :: forall n proxy. KnownNat n => proxy n -> Integer Source

Since: 4.7.0.0

natVal' :: forall n. KnownNat n => Proxy# n -> Integer Source

Since: 4.8.0.0

class KnownSymbol n Source

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

Minimal complete definition

symbolSing

symbolVal :: forall n proxy. KnownSymbol n => proxy n -> String Source

Since: 4.7.0.0

symbolVal' :: forall n. KnownSymbol n => Proxy# n -> String Source

Since: 4.8.0.0

data SomeNat Source

This type represents unknown type-level natural numbers.

Constructors

KnownNat n => SomeNat (Proxy n)

Since: 4.7.0.0

Instances

Eq SomeNat
Methods (==) :: SomeNat -> SomeNat -> Bool Source (/=) :: SomeNat -> SomeNat -> Bool Source
Ord SomeNat
Methods compare :: SomeNat -> SomeNat -> Ordering Source (<) :: SomeNat -> SomeNat -> Bool Source (<=) :: SomeNat -> SomeNat -> Bool Source (>) :: SomeNat -> SomeNat -> Bool Source (>=) :: SomeNat -> SomeNat -> Bool Source max :: SomeNat -> SomeNat -> SomeNat Source min :: SomeNat -> SomeNat -> SomeNat Source
Read SomeNat
Methods readsPrec :: Int -> ReadS SomeNat Source readList :: ReadS [SomeNat] Source readPrec :: ReadPrec SomeNat Source readListPrec :: ReadPrec [SomeNat] Source
Show SomeNat
Methods showsPrec :: Int -> SomeNat -> ShowS Source show :: SomeNat -> String Source showList :: [SomeNat] -> ShowS Source

data SomeSymbol Source

This type represents unknown type-level symbols.

Constructors

KnownSymbol n => SomeSymbol (Proxy n)

Since: 4.7.0.0

Instances

Eq SomeSymbol
Methods (==) :: SomeSymbol -> SomeSymbol -> Bool Source (/=) :: SomeSymbol -> SomeSymbol -> Bool Source
Ord SomeSymbol
Methods compare :: SomeSymbol -> SomeSymbol -> Ordering Source (<) :: SomeSymbol -> SomeSymbol -> Bool Source (<=) :: SomeSymbol -> SomeSymbol -> Bool Source (>) :: SomeSymbol -> SomeSymbol -> Bool Source (>=) :: SomeSymbol -> SomeSymbol -> Bool Source max :: SomeSymbol -> SomeSymbol -> SomeSymbol Source min :: SomeSymbol -> SomeSymbol -> SomeSymbol Source
Read SomeSymbol
Methods readsPrec :: Int -> ReadS SomeSymbol Source readList :: ReadS [SomeSymbol] Source readPrec :: ReadPrec SomeSymbol Source readListPrec :: ReadPrec [SomeSymbol] Source
Show SomeSymbol
Methods showsPrec :: Int -> SomeSymbol -> ShowS Source show :: SomeSymbol -> String Source showList :: [SomeSymbol] -> ShowS Source

someNatVal :: Integer -> Maybe SomeNat Source

Convert an integer into an unknown type-level natural.

Since: 4.7.0.0

someSymbolVal :: String -> SomeSymbol Source

Convert a string into an unknown type-level symbol.

Since: 4.7.0.0

sameNat :: (KnownNat a, KnownNat b) => Proxy a -> Proxy b -> Maybe (a :~: b) Source

We either get evidence that this function was instantiated with the same type-level numbers, or Nothing.

Since: 4.7.0.0

sameSymbol :: (KnownSymbol a, KnownSymbol b) => Proxy a -> Proxy b -> Maybe (a :~: b) Source

We either get evidence that this function was instantiated with the same type-level symbols, or Nothing.

Since: 4.7.0.0

Functions on type literals

type (<=) x y = (x <=? y) ~ True infix 4 Source

Comparison of type-level naturals, as a constraint.

type family m <=? n :: Bool infix 4 Source

Comparison of type-level naturals, as a function. NOTE: The functionality for this function should be subsumed by CmpNat, so this might go away in the future. Please let us know, if you encounter discrepancies between the two.

type family m + n :: Nat infixl 6 Source

Addition of type-level naturals.

type family m * n :: Nat infixl 7 Source

Multiplication of type-level naturals.

type family m ^ n :: Nat infixr 8 Source

Exponentiation of type-level naturals.

type family m - n :: Nat infixl 6 Source

Subtraction of type-level naturals.

Since: 4.7.0.0

type family CmpNat m n :: Ordering Source

Comparison of type-level naturals, as a function.

Since: 4.7.0.0

type family CmpSymbol m n :: Ordering Source

Comparison of type-level symbols, as a function.

Since: 4.7.0.0

User-defined type errors

type family TypeError a :: b Source

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-existant 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

data ErrorMessage Source

A description of a custom type error.

Constructors

Text Symbol	Show the text as is.
ShowType t	Pretty print the type. `ShowType :: k -> ErrorMessage`
ErrorMessage :<>: ErrorMessage infixl 6	Put two pieces of error message next to each other.
ErrorMessage :$$: ErrorMessage infixl 5	Stack two pieces of error message on top of each other.