Main.hs

{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE DataKinds #-}
import GHC.TypeLits

-- λ> :kind! F "Foo_" "Foo_Bar"
-- F "Foo_" "Foo_Bar" :: Symbol
-- = "bar"

type family F a b
  where
    F prefix value =
      ToLowerCaseString (StripPrefix prefix value)

type family StripPrefix
  (prefix :: Symbol)
  (value :: Symbol)
    where
      StripPrefix prefix value =
        Strip (UnconsSymbol prefix) (UnconsSymbol value)

type family Strip
  (a :: Maybe (Char, Symbol))
  (b :: Maybe (Char, Symbol)) :: Symbol
    where
      Strip Nothing (Just '(y, ys)) = ConsSymbol y ys
      Strip (Just '(x, xs)) (Just '(x, ys)) = StripPrefix xs ys
      Strip (Just '(x, xs)) (Just '(y, ys)) = ConsSymbol y ys

type family ToLower
  (c :: Char) :: Char
    where
      ToLower c = NatToChar (CharToNat c + 32)

type family ToLowerCaseString
  (c :: Symbol) :: Symbol
    where
      ToLowerCaseString xs =
        Lower (UnconsSymbol xs)

type family Lower
  (a :: Maybe (Char, Symbol)) :: Symbol
    where
      Lower (Just '(x, xs)) = ToLower x `ConsSymbol` xs

-- type family Drop
--   (n :: Nat)
--   (x :: Symbol)
--     where
--       Drop 0 x = x
--       Drop n x = Drop (n - 1) (Tail (UnconsSymbol x))

-- type family Tail
--   (x :: Maybe (Char, Symbol)) :: Symbol
--     where
--       Tail (Just '(_, xs)) = xs

Here's StripPrefix implemented using the symbols package (which also provides ToLower and friends): kcsongor/symbols#3 ... it works on GHC versions <9.2.