Support wild cards in data/type family instances

Handle anonymous wild cards in type or data family instance
declarations like
unnamed type variables. For instance (pun intented):

    type family F (a :: *) (b :: *) :: *
    type instance F Int _ = Int

Is now the same as:

    type family F (a :: *) (b :: *) :: *
    type instance F Int x = Int

Note that unlike wild cards in partial type signatures, no errors (or
warnings
with -XPartialTypeSignatures) are generated for these wild cards, as
there is
nothing interesting to report to the user, i.e. the inferred kind.

Only anonymous wild cards are supported here, named and
extra-constraints wild
card are not.

Test Plan: pass new tests

Reviewers: goldfire, austin, simonpj, bgamari

Reviewed By: simonpj, bgamari

Subscribers: goldfire, thomie

Differential Revision: https://phabricator.haskell.org/D1092

GHC Trac Issues: #3699, #10586

2 files changed, 40 insertions, 0 deletions

diff --git a/docs/users_guide/7.12.1-notes.xml b/docs/users_guide/7.12.1-notes.xml
index e00706c4de..b2cb3699ee 100644
--- a/docs/users_guide/7.12.1-notes.xml
+++ b/docs/users_guide/7.12.1-notes.xml
@@ -81,6 +81,15 @@
 		<literal>phantom</literal>, like it is in regular Haskell code.
 	      </para>
 	    </listitem>
+            <listitem>
+              <para>
+                Wildcards can be used in the type arguments of type/data
+                family instance declarations to indicate that the name of a
+                type variable doesn't matter. They will be replaced with new
+                unique type variables. See <xref
+                linkend="data-instance-declarations"/> for more details.
+              </para>
+            </listitem>
        </itemizedlist>
     </sect3>
 
diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml
index b6c01d6e93..f6a4403950 100644
--- a/docs/users_guide/glasgow_exts.xml
+++ b/docs/users_guide/glasgow_exts.xml
@@ -6099,6 +6099,24 @@ data instance GMap (Either a b) v = GMapEither (GMap a v) (GMap b v)
       can be any number.
     </para>
     <para>
+      When the name of a type argument of a data or newtype instance
+      declaration doesn't matter, it can be replaced with an underscore
+      (<literal>_</literal>). This is the same as writing a type variable with
+      a unique name.
+<programlisting>
+data family F a b :: *
+data instance F Int _ = Int
+-- Equivalent to
+data instance F Int b = Int
+</programlisting>
+      This resembles the wildcards that can be used in <xref
+      linkend="partial-type-signatures"/>. However, there are some
+      differences. Only anonymous wildcards are allowed in these instance
+      declarations, named and extra-constraints wildcards are not. No error
+      messages reporting the inferred types are generated, nor does the flag
+      <option>-XPartialTypeSignatures</option> have any effect.
+    </para>
+    <para>
       Data and newtype instance declarations are only permitted when an
       appropriate family declaration is in scope - just as a class instance declaration
       requires the class declaration to be visible.  Moreover, each instance
@@ -6251,6 +6269,13 @@ type instance Elem [e] = e
     </para>
 
     <para>
+      Type arguments can be replaced with underscores (<literal>_</literal>)
+      if the names of the arguments don't matter. This is the same as writing
+      type variables with unique names. The same rules apply as for <xref
+      linkend="data-instance-declarations"/>.
+    </para>
+
+    <para>
       Type family instance declarations are only legitimate when an
       appropriate family declaration is in scope - just like class instances
       require the class declaration to be visible. Moreover, each instance
@@ -9413,6 +9438,12 @@ foo (x :: _) = (x :: _)
 -- Inferred: forall w_. w_ -> w_
 </programlisting>
 
+<para>
+  Anonymous wildcards <emphasis>can</emphasis> occur in type or data instance
+  declarations. However, these declarations are not partial type signatures
+  and different rules apply. See <xref linkend="data-instance-declarations"/>
+  for more details.
+</para>
 
 <para>
 Partial type signatures can also be used in <xref linkend="template-haskell"/> splices.