[commit: ghc] master: Mention the category laws explicitly (f762181)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/f762181ec0f48f1af9dd6d367f2a521d13988808/ghc

>---------------------------------------------------------------

commit f762181ec0f48f1af9dd6d367f2a521d13988808
Author: David Luposchainsky <dluposchainsky at gmail.com>
Date:   Fri Aug 11 12:56:32 2017 +0200

    Mention the category laws explicitly


>---------------------------------------------------------------

f762181ec0f48f1af9dd6d367f2a521d13988808
 libraries/base/Control/Category.hs | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/libraries/base/Control/Category.hs b/libraries/base/Control/Category.hs
index ba92178..6407a6f 100644
--- a/libraries/base/Control/Category.hs
+++ b/libraries/base/Control/Category.hs
@@ -28,8 +28,13 @@ import GHC.Prim (coerce)
 infixr 9 .
 infixr 1 >>>, <<<
 
--- | A class for categories.
---   id and (.) must form a monoid.
+-- | A class for categories. Instances should satisfy the laws
+--
+-- @
+-- f '.' 'id'  =  f  -- (right identity)
+-- 'id' '.' f  =  f  -- (left identity)
+-- f '.' (g '.' h)  =  (f '.' g) '.' h  -- (associativity)
+-- @
 class Category cat where
     -- | the identity morphism
     id :: cat a a