Introduce DerivingVia

This implements the `DerivingVia` proposal put forth in
https://github.com/ghc-proposals/ghc-proposals/pull/120.

This introduces the `DerivingVia` deriving strategy. This is a
generalization of `GeneralizedNewtypeDeriving` that permits the user
to specify the type to `coerce` from.

The major change in this patch is the introduction of the
`ViaStrategy` constructor to `DerivStrategy`, which takes a type
as a field. As a result, `DerivStrategy` is no longer a simple
enumeration type, but rather something that must be renamed and
typechecked. The process by which this is done is explained more
thoroughly in section 3 of this paper
( https://www.kosmikus.org/DerivingVia/deriving-via-paper.pdf ),
although I have inlined the relevant parts into Notes where possible.

There are some knock-on changes as well. I took the opportunity to
do some refactoring of code in `TcDeriv`, especially the
`mkNewTypeEqn` function, since it was bundling all of the logic for
(1) deriving instances for newtypes and
(2) `GeneralizedNewtypeDeriving`
into one huge broth. `DerivingVia` reuses much of part (2), so that
was factored out as much as possible.

Bumps the Haddock submodule.

Test Plan: ./validate

Reviewers: simonpj, bgamari, goldfire, alanz

Subscribers: alanz, goldfire, rwbarton, thomie, mpickering, carter

GHC Trac Issues: #15178

Differential Revision: https://phabricator.haskell.org/D4684

9 files changed, 94 insertions, 0 deletions

diff --git a/testsuite/tests/deriving/should_fail/all.T b/testsuite/tests/deriving/should_fail/all.T
index f1d8261e4b..0cc85ea4aa 100644
--- a/testsuite/tests/deriving/should_fail/all.T
+++ b/testsuite/tests/deriving/should_fail/all.T
@@ -74,3 +74,7 @@ test('T14728b', normal, compile_fail, [''])
 test('T14916', normal, compile_fail, [''])
 test('T15073', [extra_files(['T15073a.hs'])], multimod_compile_fail,
                ['T15073', '-v0'])
+test('deriving-via-fail', normal, compile_fail, [''])
+test('deriving-via-fail2', normal, compile_fail, [''])
+test('deriving-via-fail3', normal, compile_fail, [''])
+test('deriving-via-fail4', normal, compile_fail, [''])
diff --git a/testsuite/tests/deriving/should_fail/deriving-via-fail.hs b/testsuite/tests/deriving/should_fail/deriving-via-fail.hs
new file mode 100644
index 0000000000..fbae1e7d13
--- /dev/null
+++ b/testsuite/tests/deriving/should_fail/deriving-via-fail.hs
@@ -0,0 +1,19 @@
+{-# LANGUAGE DerivingVia #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE StandaloneDeriving #-}
+module DerivingViaFail where
+
+import Control.Category
+import Data.Functor.Identity
+
+newtype Foo1 a = Foo1 a deriving Show via (Identity b)
+
+newtype Foo2 a b = Foo2 (a -> b)
+  deriving Category
+    via fooo
+
+data Foo3 deriving Eq via (forall a. a)
+
+newtype Foo4 a = Foo4 a
+deriving via (Identity b)
+  instance Show (Foo4 a)
diff --git a/testsuite/tests/deriving/should_fail/deriving-via-fail.stderr b/testsuite/tests/deriving/should_fail/deriving-via-fail.stderr
new file mode 100644
index 0000000000..51907e02cf
--- /dev/null
+++ b/testsuite/tests/deriving/should_fail/deriving-via-fail.stderr
@@ -0,0 +1,16 @@
+
+deriving-via-fail.hs:9:34: error:
+    Type variable ‘b’ is bound in the ‘via’ type ‘(Identity b)’
+    but is not mentioned in the derived class ‘Show’, which is illegal
+
+deriving-via-fail.hs:12:12: error:
+    Type variable ‘fooo’ is bound in the ‘via’ type ‘fooo’
+    but is not mentioned in the derived class ‘Category’, which is illegal
+
+deriving-via-fail.hs:15:20: error:
+    Type variable ‘a’ is bound in the ‘via’ type ‘(forall a. a)’
+    but is not mentioned in the derived class ‘Eq’, which is illegal
+
+deriving-via-fail.hs:19:12: error:
+    Type variable ‘b’ is bound in the ‘via’ type ‘(Identity b)’
+    but is not mentioned in the derived instance ‘Show (Foo4 a)’, which is illegal
diff --git a/testsuite/tests/deriving/should_fail/deriving-via-fail2.hs b/testsuite/tests/deriving/should_fail/deriving-via-fail2.hs
new file mode 100644
index 0000000000..e9a456d048
--- /dev/null
+++ b/testsuite/tests/deriving/should_fail/deriving-via-fail2.hs
@@ -0,0 +1,7 @@
+{-# LANGUAGE DerivingVia #-}
+{-# LANGUAGE StandaloneDeriving #-}
+module DerivingViaFail2 where
+
+class C a
+data A = A
+deriving via Maybe instance C A
diff --git a/testsuite/tests/deriving/should_fail/deriving-via-fail2.stderr b/testsuite/tests/deriving/should_fail/deriving-via-fail2.stderr
new file mode 100644
index 0000000000..d5692ad6b2
--- /dev/null
+++ b/testsuite/tests/deriving/should_fail/deriving-via-fail2.stderr
@@ -0,0 +1,6 @@
+
+deriving-via-fail2.hs:7:1: error:
+    • Cannot derive instance via ‘Maybe’
+        Class ‘C’ expects an argument of kind ‘*’,
+        but ‘Maybe’ has kind ‘* -> *’
+    • In the stand-alone deriving instance for ‘C A’
diff --git a/testsuite/tests/deriving/should_fail/deriving-via-fail3.hs b/testsuite/tests/deriving/should_fail/deriving-via-fail3.hs
new file mode 100644
index 0000000000..ad8e0be542
--- /dev/null
+++ b/testsuite/tests/deriving/should_fail/deriving-via-fail3.hs
@@ -0,0 +1,3 @@
+module DerivingViaFail3 where
+
+data F deriving Eq via F
diff --git a/testsuite/tests/deriving/should_fail/deriving-via-fail3.stderr b/testsuite/tests/deriving/should_fail/deriving-via-fail3.stderr
new file mode 100644
index 0000000000..f2af73a01f
--- /dev/null
+++ b/testsuite/tests/deriving/should_fail/deriving-via-fail3.stderr
@@ -0,0 +1,4 @@
+
+deriving-via-fail3.hs:3:1: error:
+    Illegal deriving strategy: via
+    Use DerivingVia to enable this extension
diff --git a/testsuite/tests/deriving/should_fail/deriving-via-fail4.hs b/testsuite/tests/deriving/should_fail/deriving-via-fail4.hs
new file mode 100644
index 0000000000..1436d994c0
--- /dev/null
+++ b/testsuite/tests/deriving/should_fail/deriving-via-fail4.hs
@@ -0,0 +1,17 @@
+{-# LANGUAGE DerivingVia #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+module DerivingViaFail4 where
+
+class C a b where
+  c :: a -> b -> Bool
+
+instance C a a where
+  c _ _ = True
+
+newtype F1 = F1 Int
+  deriving Eq via Char
+
+newtype F2 a = MkF2 a
+  deriving (C a) via (forall a. a)
diff --git a/testsuite/tests/deriving/should_fail/deriving-via-fail4.stderr b/testsuite/tests/deriving/should_fail/deriving-via-fail4.stderr
new file mode 100644
index 0000000000..caa2bfe93b
--- /dev/null
+++ b/testsuite/tests/deriving/should_fail/deriving-via-fail4.stderr
@@ -0,0 +1,18 @@
+
+deriving-via-fail4.hs:14:12: error:
+    • Couldn't match representation of type ‘Int’ with that of ‘Char’
+        arising from the coercion of the method ‘==’
+          from type ‘Char -> Char -> Bool’ to type ‘F1 -> F1 -> Bool’
+    • When deriving the instance for (Eq F1)
+
+deriving-via-fail4.hs:17:13: error:
+    • Couldn't match representation of type ‘a1’ with that of ‘a’
+        arising from the coercion of the method ‘c’
+          from type ‘a -> a -> Bool’ to type ‘a -> F2 a1 -> Bool’
+      ‘a1’ is a rigid type variable bound by
+        the deriving clause for ‘C a (F2 a1)’
+        at deriving-via-fail4.hs:17:13-15
+      ‘a’ is a rigid type variable bound by
+        the deriving clause for ‘C a (F2 a1)’
+        at deriving-via-fail4.hs:17:13-15
+    • When deriving the instance for (C a (F2 a1))