Reflectacon

This is a GHC plugin that allows for easily reflecting type level promoted data constructors to the value level. It supplants certain use cases of singletons and Known* classes. It can handle GHC's native type level literals as well as any type resulting from the promotion of a data constructor via the DataKinds compiler extension.

This is done using the reflect method of the Reflectable class, which the plugin generates instances for on the fly.

Example:

{-# OPTIONS_GHC -fplugin Reflectacon #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE DataKinds #-}

import Reflectacon (reflect)

data Foo a = MkFoo a deriving Show

foo :: Foo (Maybe Bool)
foo = reflect @_ @('MkFoo ('Just 'True))

nats :: [Integer]
nats = reflect @_ @'[1, 2, 3]

strings :: [String]
strings = reflect @_ @'["these", "are", "symbols"]

Limitations:

There's one caveat about reflecting promoted constructors that contain string and numeric literals: they must be in polymorphic fields.

Thus you can have
```
newtype Wrapper a = MkWrapper { unWrap :: a }
x = reflect @_ @(MkWrapper "hello")
```
but this won't compile
```
newtype Wrapper = MkWrapper { unWrap :: Symbol }
x = reflect @_ @(MkWrapper "hello")
```
Otherwise there'd have to be a value of type Symbol, which is not an inhabited type, instead of being able to convert it to String. As a side note, Char type literals do not have this limitation.
Currently only GHC 8.10.x, 9.0.x, and 9.2.x are supported

aaronallen8455/reflectacon

Reflectacon

Limitations: