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GHC.TypeLits

Safe Haskell Trustworthy
Language Haskell2010

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

Kinds

data Nat :: * Source

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

Instances

type (==) Nat a b
type (==) Nat a b

data Symbol :: * Source

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

Instances

type (==) Symbol a b
type (==) Symbol a b

Linking type and value level

class KnownNat (n :: Nat) 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 :: Symbol) 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.

Since: 4.10.0.0

Constructors

KnownNat n => SomeNat (Proxy n)

data SomeSymbol Source

This type represents unknown type-level symbols.

Constructors

KnownSymbol n => SomeSymbol (Proxy n)

Since: 4.7.0.0

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 :: Nat) <=? (n :: Nat) :: 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 :: Nat) + (n :: Nat) :: Nat infixl 6 Source

Addition of type-level naturals.

type family (m :: Nat) * (n :: Nat) :: Nat infixl 7 Source

Multiplication of type-level naturals.

type family (m :: Nat) ^ (n :: Nat) :: Nat infixr 8 Source

Exponentiation of type-level naturals.

type family (m :: Nat) - (n :: Nat) :: Nat infixl 6 Source

Subtraction of type-level naturals.

Since: 4.7.0.0

type family AppendSymbol (m :: Symbol) (n :: Symbol) :: Symbol Source

Concatenation of type-level symbols.

Since: 4.10.0.0

type family CmpNat (m :: Nat) (n :: Nat) :: Ordering Source

Comparison of type-level naturals, as a function.

Since: 4.7.0.0

type family CmpSymbol (m :: Symbol) (n :: Symbol) :: Ordering Source

Comparison of type-level symbols, as a function.

Since: 4.7.0.0

User-defined type errors

type family TypeError (a :: ErrorMessage) :: b where ... 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-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

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.

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/8.2.1/docs/html/libraries/base-4.10.0.0/GHC-TypeLits.html