witch/source/library/Witch/From.hs

{-# LANGUAGE DefaultSignatures #-}
{-# LANGUAGE MultiParamTypeClasses #-}

module Witch.From where

import qualified Data.Coerce as Coerce

-- | This type class is for converting values from some @source@ type into
-- some other @target@ type. The constraint @'From' source target@ means that
-- you can convert from a value of type @source@ into a value of type
-- @target@.
--
-- This type class is for conversions that always succeed. If your conversion
-- sometimes fails, consider implementing @TryFrom@ instead.
class From source target where
  -- | This method implements the conversion of a value between types. At call
  -- sites you may prefer to use @into@ instead.
  --
  -- > -- Avoid this:
  -- > from (x :: s)
  -- >
  -- > -- Prefer this (using [@TypeApplications@](https://downloads.haskell.org/ghc/9.6.1/docs/users_guide/exts/type_applications.html) language extension):
  -- > from @s x
  --
  -- The default implementation of this method simply calls 'Coerce.coerce',
  -- which works for types that have the same runtime representation. This
  -- means that for @newtype@s you do not need to implement this method at
  -- all. For example:
  --
  -- >>> newtype Name = Name String
  -- >>> instance From Name String
  -- >>> instance From String Name
  from :: source -> target
  default from :: (Coerce.Coercible source target) => source -> target
  from = Coerce.coerce