{-# OPTIONS_GHC -Wno-redundant-constraints #-}
module Util.TypeLits
( Symbol
, KnownSymbol
, AppendSymbol
, symbolValT
, symbolValT'
, TypeError
, ErrorMessage (..)
, TypeErrorUnless
, inTypeErrorUnless
) where
import Data.Constraint (Dict(..))
import GHC.TypeLits
import Unsafe.Coerce (unsafeCoerce)
symbolValT :: forall s. KnownSymbol s => Proxy s -> Text
symbolValT = toText . symbolVal
symbolValT' :: forall s. KnownSymbol s => Text
symbolValT' = symbolValT (Proxy @s)
type family TypeErrorUnless (cond :: Bool) (err :: ErrorMessage) :: Constraint where
TypeErrorUnless 'True _ = ()
TypeErrorUnless 'False err = TypeError err
inTypeErrorUnless
:: forall cond err a.
TypeErrorUnless cond err
=> (cond ~ 'True => a)
-> a
inTypeErrorUnless a =
case unsafeCoerce @(Dict ('True ~ 'True)) @(Dict (cond ~ 'True)) Dict of
Dict -> a