Using coerce in Haskell

I recently worked on adding CSRF checks to a form.

module Csrf (
  Token
, ...
) where

newtype Token = MkToken String
  deriving (Eq, Show)

mkTokens :: [String] -> [Token]
mkTokens = map MkToken
-- ... rest of code

While newtype comes with no run-time costs, we do encounter some costs if we dealt with converting a list of strings to tokens. I chanced upon Safe Zero-cost Coercions for Haskell, which describes safe coercions for removing this overhead.

mkTokens :: [String] -> [Token]
mkTokens = coerce

See: Coercible