From df626f008230fd55308d8c64d3879d7ec79b44df Mon Sep 17 00:00:00 2001
From: Tobias Haslop <tobiashaslop@hotmail.de>
Date: Wed, 1 Nov 2023 01:33:14 +0100
Subject: [PATCH] Elaborate on the quantified superclass of Bifunctor
This was requested in the comment
https://github.com/haskell/core-libraries-committee/issues/93#issuecomment-1597271700
for when Traversable becomes a superclass of Bitraversable, but similarly
applies to Functor/Bifunctor, which already are in a superclass relationship.
---
libraries/base/src/Data/Bifunctor.hs | 20 ++++++++++++++++----
1 file changed, 16 insertions(+), 4 deletions(-)
diff --git a/libraries/base/src/Data/Bifunctor.hs b/libraries/base/src/Data/Bifunctor.hs
index d01917ffaa75..e7a776cd995b 100644
--- a/libraries/base/src/Data/Bifunctor.hs
+++ b/libraries/base/src/Data/Bifunctor.hs
@@ -39,9 +39,23 @@ import GHC.Generics ( K1(..) )
-- Intuitively it is a bifunctor where both the first and second
-- arguments are covariant.
--
+-- The class definition of a 'Bifunctor' @p@ uses the
+-- [QuantifiedConstraints](https://downloads.haskell.org/ghc/latest/docs/users_guide/exts/quantified_constraints.html)
+-- language extension to quantify over the first type
+-- argument @a@ in its context. The context requires that @p a@
+-- must be a 'Functor' for all @a@. In other words a partially
+-- applied 'Bifunctor' must be a 'Functor'. This makes 'Functor' a
+-- superclass of 'Bifunctor' such that a function with a
+-- 'Bifunctor' constraint may use 'fmap' in its implementation.
+-- 'Functor' has been a quantified superclass of
+-- 'Bifunctor' since base-4.18.0.0.
+--
-- You can define a 'Bifunctor' by either defining 'bimap' or by
--- defining both 'first' and 'second'. A partially applied 'Bifunctor'
--- must be a 'Functor' and the 'second' method must agree with 'fmap'.
+-- defining both 'first' and 'second'. The 'second' method must
+-- agree with 'fmap':
+--
+-- @'second' ≡ 'fmap'@
+--
-- From this it follows that:
--
-- @'second' 'id' ≡ 'id'@
@@ -69,8 +83,6 @@ import GHC.Generics ( K1(..) )
-- 'second' (f '.' g) ≡ 'second' f '.' 'second' g
-- @
--
--- Since 4.18.0.0 'Functor' is a superclass of 'Bifunctor.
---
-- @since 4.8.0.0
class (forall a. Functor (p a)) => Bifunctor p where
{-# MINIMAL bimap | first, second #-}
--
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