Eq.hs

module Darcs.Patch.Witnesses.Eq
    ( EqCheck(..)
    , Eq2(..)
    , isIsEq
    ) where

import Darcs.Prelude

import Darcs.Patch.Witnesses.Unsafe ( unsafeCoerceP )

-- |'EqCheck' is used to pass around evidence (or lack thereof) of
-- two witness types being equal.
data EqCheck wA wB where
    IsEq :: EqCheck wA wA
    NotEq :: EqCheck wA wB

instance Eq (EqCheck wA wB) where
    IsEq == IsEq = True
    NotEq == NotEq = True
    _ == _ = False

instance Show (EqCheck wA wB) where
    show IsEq = "IsEq"
    show NotEq = "NotEq"

-- |An witness aware equality class.
-- A minimal definition defines any one of 'unsafeCompare', '=\/=' and '=/\='.
class Eq2 p where
    -- |It is unsafe to define a class instance via this method, because
    -- if it returns True then the default implementations of '=\/=' and '=/\='
    -- will coerce the equality of two witnesses.
    --
    -- Calling this method is safe, although '=\/=' or '=/\=' would be better
    -- choices as it is not usually meaningul to compare two patches that
    -- don't share either a starting or an ending context
    unsafeCompare :: p wA wB -> p wC wD -> Bool
    unsafeCompare a b = IsEq == (a =/\= unsafeCoerceP b)

    -- |Compare two things with the same starting witness. If the things
    -- compare equal, evidence of the ending witnesses being equal will
    -- be returned.
    (=\/=) :: p wA wB -> p wA wC -> EqCheck wB wC
    a =\/= b | unsafeCompare a b = unsafeCoerceP IsEq
             | otherwise = NotEq

    -- |Compare two things with the same ending witness. If the things
    -- compare equal, evidence of the starting witnesses being equal will
    -- be returned.
    (=/\=) :: p wA wC -> p wB wC -> EqCheck wA wB
    a =/\= b | IsEq == (a =\/= unsafeCoerceP b) = unsafeCoerceP IsEq
             | otherwise = NotEq

    {-# MINIMAL unsafeCompare | (=\/=) | (=/\=) #-}

infix 4 =\/=, =/\=

isIsEq :: EqCheck wA wB -> Bool
isIsEq IsEq = True
isIsEq NotEq = False