Strings can be encoded as 'Enc "r-ASCII"@ only if they contain only ASCII characters (first 128 characters of the Unicode character set).

This is sometimes referred to as ASCII-7 and future versions of type-encoding may change "r-ASCII" symbol annotation to reflect this.

B8.all ((< 128) . ord) . getPayload @ '["r-ASCII"] @() @B.ByteString

>>> :set -XOverloadedStrings -XMultiParamTypeClasses -XDataKinds
>>> encodeFAll . toEncoding () $ "Hello World" :: Either EncodeEx (Enc '["r-ASCII"] () T.Text)
Right (UnsafeMkEnc Proxy () "Hello World")

>>> encodeFAll . toEncoding () $ "\194\160" :: Either EncodeEx (Enc '["r-ASCII"] () T.Text)
Left (EncodeEx "r-ASCII" (NonAsciiChar '\194'))

Since: 0.1.0.0

>>> :set -XDataKinds -XTypeApplications
>>> import qualified Data.Text as T
>>> import qualified Data.ByteString as B
>>> import qualified Data.ByteString.Char8 as B8
>>> import Test.QuickCheck
>>> import Test.QuickCheck.Instances.ByteString()
>>> import Data.TypedEncoding
>>> :{
instance Arbitrary (Enc '["r-ASCII"] () B.ByteString) where 
     arbitrary =  fmap (unsafeSetPayload ()) 
                  . flip suchThat (B8.all isAscii) 
                       $ arbitrary 
:}