Add Monad instances to `(,,) a b` and `(,,,) a b c`

7 files changed, 36 insertions, 3 deletions

diff --git a/libraries/base/Data/Foldable.hs b/libraries/base/Data/Foldable.hs
index c17de972af..82bb3d9077 100644
--- a/libraries/base/Data/Foldable.hs
+++ b/libraries/base/Data/Foldable.hs
@@ -364,6 +364,8 @@ instance Foldable ((,) a) where
     foldMap f (_, y) = f y
 
     foldr f z (_, y) = f y z
+    length _  = 1
+    null _ = False
 
 -- | @since 4.8.0.0
 instance Foldable (Array i) where
diff --git a/libraries/base/GHC/Base.hs b/libraries/base/GHC/Base.hs
index d63f5d1a86..54c6f91280 100644
--- a/libraries/base/GHC/Base.hs
+++ b/libraries/base/GHC/Base.hs
@@ -437,6 +437,32 @@ instance Monoid a => Applicative ((,) a) where
 instance Monoid a => Monad ((,) a) where
     (u, a) >>= k = case k a of (v, b) -> (u <> v, b)
 
+-- | @since 4.14.0.0
+instance Functor ((,,) a b) where
+    fmap f (a, b, c) = (a, b, f c)
+
+-- | @since 4.14.0.0
+instance (Monoid a, Monoid b) => Applicative ((,,) a b) where
+    pure x = (mempty, mempty, x)
+    (a, b, f) <*> (a', b', x) = (a <> a', b <> b', f x)
+
+-- | @since 4.14.0.0
+instance (Monoid a, Monoid b) => Monad ((,,) a b) where
+    (u, v, a) >>= k = case k a of (u', v', b) -> (u <> u', v <> v', b)
+
+-- | @since 4.14.0.0
+instance Functor ((,,,) a b c) where
+    fmap f (a, b, c, d) = (a, b, c, f d)
+
+-- | @since 4.14.0.0
+instance (Monoid a, Monoid b, Monoid c) => Applicative ((,,,) a b c) where
+    pure x = (mempty, mempty, mempty, x)
+    (a, b, c, f) <*> (a', b', c', x) = (a <> a', b <> b', c <> c', f x)
+
+-- | @since 4.14.0.0
+instance (Monoid a, Monoid b, Monoid c) => Monad ((,,,) a b c) where
+    (u, v, w, a) >>= k = case k a of (u', v', w', b) -> (u <> u', v <> v', w <> w', b)
+
 -- | @since 4.10.0.0
 instance Semigroup a => Semigroup (IO a) where
     (<>) = liftA2 (<>)
diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md
index b97ed78ced..eeed94327e 100644
--- a/libraries/base/changelog.md
+++ b/libraries/base/changelog.md
@@ -36,6 +36,9 @@
   * Add newtypes for `CSocklen` (`socklen_t`) and `CNfds` (`nfds_t`) to
     `System.Posix.Types`.
 
+  * Add `Functor`, `Applicative` and `Monad` instances to `(,,) a b`
+    and `(,,,) a b c`
+
 ## 4.13.0.0 *TBA*
   * Bundled with GHC *TBA*
 
diff --git a/testsuite/tests/ghci/scripts/T12550.stdout b/testsuite/tests/ghci/scripts/T12550.stdout
index 81be552e5c..a3117d02c2 100644
--- a/testsuite/tests/ghci/scripts/T12550.stdout
+++ b/testsuite/tests/ghci/scripts/T12550.stdout
@@ -53,6 +53,8 @@ instance Functor [] -- Defined in ‘GHC.Base’
 instance Functor Maybe -- Defined in ‘GHC.Base’
 instance Functor IO -- Defined in ‘GHC.Base’
 instance ∀ r. Functor ((->) r) -- Defined in ‘GHC.Base’
+instance ∀ a b c. Functor ((,,,) a b c) -- Defined in ‘GHC.Base’
+instance ∀ a b. Functor ((,,) a b) -- Defined in ‘GHC.Base’
 instance ∀ a. Functor ((,) a) -- Defined in ‘GHC.Base’
 datatypeName
   ∷ ∀ {d} {t ∷ ★ → (★ → ★) → ★ → ★} {f ∷ ★ → ★} {a}.
diff --git a/testsuite/tests/partial-sigs/should_compile/T10403.stderr b/testsuite/tests/partial-sigs/should_compile/T10403.stderr
index 870a72ed5a..fcc5e38e87 100644
--- a/testsuite/tests/partial-sigs/should_compile/T10403.stderr
+++ b/testsuite/tests/partial-sigs/should_compile/T10403.stderr
@@ -32,7 +32,7 @@ T10403.hs:22:15: warning: [-Wdeferred-type-errors (in -Wdefault)]
         instance Functor IO -- Defined in ‘GHC.Base’
         instance Functor (B t) -- Defined at T10403.hs:10:10
         instance Functor I -- Defined at T10403.hs:6:10
-        ...plus three others
+        ...plus five others
         ...plus two instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the second argument of ‘(.)’, namely ‘fmap (const ())’
diff --git a/testsuite/tests/typecheck/should_fail/T13292.stderr b/testsuite/tests/typecheck/should_fail/T13292.stderr
index 9f70b1df27..adb2738e69 100644
--- a/testsuite/tests/typecheck/should_fail/T13292.stderr
+++ b/testsuite/tests/typecheck/should_fail/T13292.stderr
@@ -9,7 +9,7 @@ T13292a.hs:4:12: warning: [-Wdeferred-type-errors (in -Wdefault)]
         instance Monad IO -- Defined in ‘GHC.Base’
         instance Monad Maybe -- Defined in ‘GHC.Base’
         instance Monoid a => Monad ((,) a) -- Defined in ‘GHC.Base’
-        ...plus one other
+        ...plus three others
         ...plus two instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the expression: return ()
diff --git a/testsuite/tests/typecheck/should_fail/tcfail181.stderr b/testsuite/tests/typecheck/should_fail/tcfail181.stderr
index 9cbc04b3d0..9f5252539b 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: error:
         instance Monad IO -- Defined in ‘GHC.Base’
         instance Monad Maybe -- Defined in ‘GHC.Base’
         instance Monoid a => Monad ((,) a) -- Defined in ‘GHC.Base’
-        ...plus one other
+        ...plus three others
         ...plus two instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the expression: foo