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'.