Safe Haskell	Trustworthy
Language	Haskell2010

GHC.Symbol

Contents

Symbol type
Type level
Orphan instances

Description

A (slightly unsafe) term level implementation for Symbol.

As Symbol in base doesn't have any term-level operations, it's sometimes necessary to have two copies of the same data, the first one using String on term level and second one using Symbol to be promtoed to type level.

In GHC-9.2 the similar problem was fixed for Nat and Natural, which were distinct types (kinds) before that.

As String is a list of Chars, we cannot make type Symbol = String, but we could do newtype Symbol = MkSymbol String. This module fakes that by using unsafeCoerce under the hood.

Fleshing out Symbol on term level is suggested in https://gitlab.haskell.org/ghc/ghc/-/issues/10776#note_109601 in 2015.

This implementation is slightly unsafe, as currently Symbol is defined as empty data type:

data Symbol

This means that you can write

dangerous :: Symbol -> Int
dangerous x = case x of

and because GHC sees through everything, and knows that Symbol is empty, the above compiles without a warning.

If Symbol was defined as newtype Symbol = Symbol Any, the above problem would go away, and also implementation of this module would be safer, as unsafeCoerceing from lifted type to Any and back is guaranteed to work.

Of course life would be easier if we just had

newtype Symbol = MkSymbol String

but until that is done, you may find this module useful.

Note: Symbol is not Text. Text has an invariant: it represents valid Unicode text. Symbol is just a list of characters (= Unicode codepoints), like String. E.g.

>>> "\55555" :: String
"\55555"

>>> "\55555" :: Symbol
"\55555"

but text replaces surrogate codepoints:

>>> "\55555" :: Text
"\65533"

Symbol could use some packed representation of list of characters, if also KnownSymbol would use it as well. Currently KnownSymbol dictionary carries a String, so having Symbol be a String is justified'.

Synopsis

data Symbol
symbolToString :: Symbol -> String
consSymbol :: Char -> Symbol -> Symbol
unconsSymbol :: Symbol -> Maybe (Char, Symbol)
class KnownSymbol (n :: Symbol)
symbolVal :: forall n proxy. KnownSymbol n => proxy n -> Symbol
symbolVal' :: forall n. KnownSymbol n => Proxy# n -> Symbol
type family AppendSymbol (a :: Symbol) (b :: Symbol) :: Symbol where ...
type family CmpSymbol (a :: Symbol) (b :: Symbol) :: Ordering where ...
someSymbolVal :: Symbol -> SomeSymbol
data SomeSymbol = KnownSymbol n => SomeSymbol (Proxy n)
sameSymbol :: forall (a :: Symbol) (b :: Symbol). (KnownSymbol a, KnownSymbol b) => Proxy a -> Proxy b -> Maybe (a :~: b)

Symbol type

data Symbol #

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

Instances

Instances details

IsList Symbol Source #
Instance details Defined in GHC.Symbol Associated Types type Item Symbol # Methods fromList :: [Item Symbol] -> Symbol # fromListN :: Int -> [Item Symbol] -> Symbol # toList :: Symbol -> [Item Symbol] #
Eq Symbol Source #
Instance details Defined in GHC.Symbol Methods (==) :: Symbol -> Symbol -> Bool # (/=) :: Symbol -> Symbol -> Bool #
Ord Symbol Source #
Instance details Defined in GHC.Symbol Methods compare :: Symbol -> Symbol -> Ordering # (<) :: Symbol -> Symbol -> Bool # (<=) :: Symbol -> Symbol -> Bool # (>) :: Symbol -> Symbol -> Bool # (>=) :: Symbol -> Symbol -> Bool # max :: Symbol -> Symbol -> Symbol # min :: Symbol -> Symbol -> Symbol #
Read Symbol Source #
Instance details Defined in GHC.Symbol Methods readsPrec :: Int -> ReadS Symbol # readList :: ReadS [Symbol] # readPrec :: ReadPrec Symbol # readListPrec :: ReadPrec [Symbol] #
Show Symbol Source #	`>>>` `"foo" :: Symbol`"foo"
Instance details Defined in GHC.Symbol Methods showsPrec :: Int -> Symbol -> ShowS # show :: Symbol -> String # showList :: [Symbol] -> ShowS #
IsString Symbol Source #	`>>>` `"foo" :: Symbol`"foo"
Instance details Defined in GHC.Symbol Methods fromString :: String -> Symbol #
Semigroup Symbol Source #
Instance details Defined in GHC.Symbol Methods (<>) :: Symbol -> Symbol -> Symbol # sconcat :: NonEmpty Symbol -> Symbol # stimes :: Integral b => b -> Symbol -> Symbol #
Monoid Symbol Source #
Instance details Defined in GHC.Symbol Methods mempty :: Symbol # mappend :: Symbol -> Symbol -> Symbol # mconcat :: [Symbol] -> Symbol #
PrintfArg Symbol Source #
Instance details Defined in GHC.Symbol Methods formatArg :: Symbol -> FieldFormatter # parseFormat :: Symbol -> ModifierParser #
Binary Symbol Source #
Instance details Defined in GHC.Symbol Methods put :: Symbol -> Put # get :: Get Symbol # putList :: [Symbol] -> Put #
NFData Symbol Source #
Instance details Defined in GHC.Symbol Methods rnf :: Symbol -> () #
SingKind Symbol	Since: base-4.9.0.0
Instance details Defined in GHC.Generics Associated Types type DemoteRep Symbol Methods fromSing :: forall (a :: Symbol). Sing a -> DemoteRep Symbol
Lift Symbol Source #
Instance details Defined in GHC.Symbol Methods lift :: Symbol -> Q Exp # liftTyped :: Symbol -> Q (TExp Symbol) #
KnownSymbol a => SingI (a :: Symbol)	Since: base-4.9.0.0
Instance details Defined in GHC.Generics Methods sing :: Sing a
type Item Symbol Source #
Instance details Defined in GHC.Symbol type Item Symbol = Char
type DemoteRep Symbol
Instance details Defined in GHC.Generics type DemoteRep Symbol = String
data Sing (s :: Symbol)
Instance details Defined in GHC.Generics data Sing (s :: Symbol) where SSym :: forall (s :: Symbol). KnownSymbol s => Sing s

symbolToString :: Symbol -> String Source #

consSymbol :: Char -> Symbol -> Symbol Source #

Prepend a character to a Symbol

>>> consSymbol 'a' "cute"
"acute"

unconsSymbol :: Symbol -> Maybe (Char, Symbol) Source #

Inverse of consSymbol

>>> unconsSymbol ""
Nothing

>>> unconsSymbol "mother"
Just ('m',"other")

Type level

class KnownSymbol (n :: Symbol) #

This class gives the string associated with a type-level symbol. There are instances of the class for every concrete literal: "hello", etc.

Since: base-4.7.0.0

Minimal complete definition

symbolSing

symbolVal :: forall n proxy. KnownSymbol n => proxy n -> Symbol Source #

>>> symbolVal (Proxy @"foobar")
"foobar"

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

type family AppendSymbol (a :: Symbol) (b :: Symbol) :: Symbol where ... #

Concatenation of type-level symbols.

Since: base-4.10.0.0

type family CmpSymbol (a :: Symbol) (b :: Symbol) :: Ordering where ... #

Comparison of type-level symbols, as a function.

Since: base-4.7.0.0

someSymbolVal :: Symbol -> SomeSymbol Source #

>>> someSymbolVal "foobar"
"foobar"

data SomeSymbol #

This type represents unknown type-level symbols.

Constructors

KnownSymbol n => SomeSymbol (Proxy n)

Since: base-4.7.0.0

Instances

Instances details

Eq SomeSymbol	Since: base-4.7.0.0
Instance details Defined in GHC.TypeLits Methods (==) :: SomeSymbol -> SomeSymbol -> Bool # (/=) :: SomeSymbol -> SomeSymbol -> Bool #
Ord SomeSymbol	Since: base-4.7.0.0
Instance details Defined in GHC.TypeLits Methods compare :: SomeSymbol -> SomeSymbol -> Ordering # (<) :: SomeSymbol -> SomeSymbol -> Bool # (<=) :: SomeSymbol -> SomeSymbol -> Bool # (>) :: SomeSymbol -> SomeSymbol -> Bool # (>=) :: SomeSymbol -> SomeSymbol -> Bool # max :: SomeSymbol -> SomeSymbol -> SomeSymbol # min :: SomeSymbol -> SomeSymbol -> SomeSymbol #
Read SomeSymbol	Since: base-4.7.0.0
Instance details Defined in GHC.TypeLits Methods readsPrec :: Int -> ReadS SomeSymbol # readList :: ReadS [SomeSymbol] # readPrec :: ReadPrec SomeSymbol # readListPrec :: ReadPrec [SomeSymbol] #
Show SomeSymbol	Since: base-4.7.0.0
Instance details Defined in GHC.TypeLits Methods showsPrec :: Int -> SomeSymbol -> ShowS # show :: SomeSymbol -> String # showList :: [SomeSymbol] -> ShowS #

sameSymbol :: forall (a :: Symbol) (b :: Symbol). (KnownSymbol a, KnownSymbol b) => Proxy a -> Proxy b -> Maybe (a :~: b) #

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

Since: base-4.7.0.0

Orphan instances

IsList Symbol Source #
Instance details Associated Types type Item Symbol # Methods fromList :: [Item Symbol] -> Symbol # fromListN :: Int -> [Item Symbol] -> Symbol # toList :: Symbol -> [Item Symbol] #
Eq Symbol Source #
Instance details Methods (==) :: Symbol -> Symbol -> Bool # (/=) :: Symbol -> Symbol -> Bool #
Ord Symbol Source #
Instance details Methods compare :: Symbol -> Symbol -> Ordering # (<) :: Symbol -> Symbol -> Bool # (<=) :: Symbol -> Symbol -> Bool # (>) :: Symbol -> Symbol -> Bool # (>=) :: Symbol -> Symbol -> Bool # max :: Symbol -> Symbol -> Symbol # min :: Symbol -> Symbol -> Symbol #
Read Symbol Source #
Instance details Methods readsPrec :: Int -> ReadS Symbol # readList :: ReadS [Symbol] # readPrec :: ReadPrec Symbol # readListPrec :: ReadPrec [Symbol] #
Show Symbol Source #	`>>>` `"foo" :: Symbol`"foo"
Instance details Methods showsPrec :: Int -> Symbol -> ShowS # show :: Symbol -> String # showList :: [Symbol] -> ShowS #
IsString Symbol Source #	`>>>` `"foo" :: Symbol`"foo"
Instance details Methods fromString :: String -> Symbol #
Semigroup Symbol Source #
Instance details Methods (<>) :: Symbol -> Symbol -> Symbol # sconcat :: NonEmpty Symbol -> Symbol # stimes :: Integral b => b -> Symbol -> Symbol #
Monoid Symbol Source #
Instance details Methods mempty :: Symbol # mappend :: Symbol -> Symbol -> Symbol # mconcat :: [Symbol] -> Symbol #
PrintfArg Symbol Source #
Instance details Methods formatArg :: Symbol -> FieldFormatter # parseFormat :: Symbol -> ModifierParser #
Binary Symbol Source #
Instance details Methods put :: Symbol -> Put # get :: Get Symbol # putList :: [Symbol] -> Put #
NFData Symbol Source #
Instance details Methods rnf :: Symbol -> () #
Lift Symbol Source #
Instance details Methods lift :: Symbol -> Q Exp # liftTyped :: Symbol -> Q (TExp Symbol) #