[commit: ghc] master: Add Monad instance for `((, ) a)` (#10190) (9db005a)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/9db005a444722e31aca1956b058e069bcf3cacbd/ghc

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

commit 9db005a444722e31aca1956b058e069bcf3cacbd
Author: Fumiaki Kinoshita <fumiexcel at gmail.com>
Date:   Wed Mar 25 13:30:25 2015 +0900

    Add Monad instance for `((,) a)` (#10190)
    
    This was proposed a couple of times in the past, e.g.
    
     - https://mail.haskell.org/pipermail/libraries/2011-November/017153.html
     - https://mail.haskell.org/pipermail/libraries/2013-July/020446.html
    
    but its implementation had been blocked by the fact that `Monoid` wasn't
    in scope where the `Monad` class was defined. Since the AMP/FTP restructuring
    this is no longer the case.


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

9db005a444722e31aca1956b058e069bcf3cacbd
 libraries/base/GHC/Base.hs                             | 3 +++
 libraries/base/changelog.md                            | 2 ++
 testsuite/tests/ghci/scripts/T7627.stdout              | 1 +
 testsuite/tests/ghci/scripts/ghci011.stdout            | 1 +
 testsuite/tests/typecheck/should_fail/tcfail181.stderr | 2 +-
 5 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/libraries/base/GHC/Base.hs b/libraries/base/GHC/Base.hs
index 7e04ab4..d9192d7 100644
--- a/libraries/base/GHC/Base.hs
+++ b/libraries/base/GHC/Base.hs
@@ -308,6 +308,9 @@ instance Monoid a => Applicative ((,) a) where
     pure x = (mempty, x)
     (u, f) <*> (v, x) = (u `mappend` v, f x)
 
+instance Monoid a => Monad ((,) a) where
+    return x = (mempty, x)
+    (u, a) >>= k = case k a of (v, b) -> (u `mappend` v, b)
 
 {- | The 'Functor' class is used for types that can be mapped over.
 Instances of 'Functor' should satisfy the following laws:
diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md
index 849a7ad..f402189 100644
--- a/libraries/base/changelog.md
+++ b/libraries/base/changelog.md
@@ -17,6 +17,8 @@
   * `Dual`, `Product`, and `Sum` now have `Functor`, `Applicative`, and
     `Monad` instances
 
+  * `(,) a` now has a `Monad` instance
+
   * Redundant typeclass constraints have been removed:
      - `Data.Ratio.{denominator,numerator}` have no `Integral` constraint anymore
      - **TODO**
diff --git a/testsuite/tests/ghci/scripts/T7627.stdout b/testsuite/tests/ghci/scripts/T7627.stdout
index 2713566..158672c 100644
--- a/testsuite/tests/ghci/scripts/T7627.stdout
+++ b/testsuite/tests/ghci/scripts/T7627.stdout
@@ -15,6 +15,7 @@ data (,) a b = (,) a b 	-- Defined in ‘GHC.Tuple’
 instance (Bounded a, Bounded b) => Bounded (a, b)
   -- Defined in ‘GHC.Enum’
 instance (Eq a, Eq b) => Eq (a, b) -- Defined in ‘GHC.Classes’
+instance Monoid a => Monad ((,) a) -- Defined in ‘GHC.Base’
 instance Functor ((,) a) -- Defined in ‘GHC.Base’
 instance (Ord a, Ord b) => Ord (a, b) -- Defined in ‘GHC.Classes’
 instance (Read a, Read b) => Read (a, b) -- Defined in ‘GHC.Read’
diff --git a/testsuite/tests/ghci/scripts/ghci011.stdout b/testsuite/tests/ghci/scripts/ghci011.stdout
index a608f07..8042757 100644
--- a/testsuite/tests/ghci/scripts/ghci011.stdout
+++ b/testsuite/tests/ghci/scripts/ghci011.stdout
@@ -21,6 +21,7 @@ data (,) a b = (,) a b 	-- Defined in ‘GHC.Tuple’
 instance (Bounded a, Bounded b) => Bounded (a, b)
   -- Defined in ‘GHC.Enum’
 instance (Eq a, Eq b) => Eq (a, b) -- Defined in ‘GHC.Classes’
+instance Monoid a => Monad ((,) a) -- Defined in ‘GHC.Base’
 instance Functor ((,) a) -- Defined in ‘GHC.Base’
 instance (Ord a, Ord b) => Ord (a, b) -- Defined in ‘GHC.Classes’
 instance (Read a, Read b) => Read (a, b) -- Defined in ‘GHC.Read’
diff --git a/testsuite/tests/typecheck/should_fail/tcfail181.stderr b/testsuite/tests/typecheck/should_fail/tcfail181.stderr
index e638099..787b62e 100644
--- a/testsuite/tests/typecheck/should_fail/tcfail181.stderr
+++ b/testsuite/tests/typecheck/should_fail/tcfail181.stderr
@@ -10,7 +10,7 @@ tcfail181.hs:17:9:
       instance Monad Maybe -- Defined in ‘GHC.Base’
       instance Monad IO -- Defined in ‘GHC.Base’
       instance Monad ((->) r) -- Defined in ‘GHC.Base’
-      ...plus one other
+      ...plus two others
     In the expression: foo
     In the expression: foo {bar = return True}
     In an equation for ‘wog’: wog x = foo {bar = return True}