From ae96c751c869813ab95e712f8daac8516bb4795f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=C3=96mer=20Sinan=20A=C4=9Facan?= <omeragacan@gmail.com>
Date: Fri, 17 Jul 2015 00:02:09 +0200
Subject: Implement -fprint-expanded-synonyms

Add a flag to print type-synonyms-expanded versions of types in type
error messages (in addition to old error messages with synonyms)

 * Mailing list discussion: https://mail.haskell.org/pipermail/ghc-devs/2015-June/009247.html
 * Wiki page: https://wiki.haskell.org/Expanding_type_synonyms_in_error_messages_proposal
 * Trac: https://ghc.haskell.org/trac/ghc/ticket/10547

Test Plan:
 * I'll find some examples and add tests.

Reviewers: austin, simonpj, goldfire, bgamari

Reviewed By: austin, simonpj, goldfire, bgamari

Subscribers: rodlogic, thomie, bgamari

Differential Revision: https://phabricator.haskell.org/D1016

GHC Trac Issues: #10547
---
 docs/users_guide/using.xml | 42 +++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 41 insertions(+), 1 deletion(-)

(limited to 'docs/users_guide/using.xml')

diff --git a/docs/users_guide/using.xml b/docs/users_guide/using.xml
index 58008a2ade..772e8b9eaa 100644
--- a/docs/users_guide/using.xml
+++ b/docs/users_guide/using.xml
@@ -896,7 +896,7 @@ ghc -c Foo.hs
 
 
       <varlistentry>
-        <term><option>--fprint-explicit-foralls, -fprint-explicit-kinds, -fprint-unicode-syntax</option>
+        <term><option>-fprint-explicit-foralls, -fprint-explicit-kinds, -fprint-unicode-syntax</option>
           <indexterm><primary><option>-fprint-explicit-foralls</option></primary></indexterm>
           <indexterm><primary><option>-fprint-explicit-kinds</option></primary></indexterm>
           <indexterm><primary><option>-fprint-unicode-syntax</option></primary></indexterm>
@@ -959,6 +959,46 @@ ghci> :t (>>)
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term>
+          <option>-fprint-expanded-synonyms</option>
+          <indexterm><primary><option>-fprint-expanded-synonyms</option></primary></indexterm>
+        </term>
+        <listitem>
+          <para>
+            When enabled, GHC also prints type-synonym-expanded types in type
+            errors.
+
+            For example, with this type synonyms:
+
+<screen>
+type Foo = Int
+type Bar = Bool
+type MyBarST s = ST s Bar
+</screen>
+
+          This error message:
+
+<screen>
+Couldn't match type 'Int' with 'Bool'
+Expected type: ST s Foo
+  Actual type: MyBarST s
+</screen>
+
+          Becomes this:
+
+<screen>
+Couldn't match type 'Int' with 'Bool'
+Expected type: ST s Foo
+  Actual type: MyBarST s
+Type synonyms expanded:
+Expected type: ST s Int
+  Actual type: ST s Bool
+</screen>
+          </para>
+        </listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><option>-ferror-spans</option>
           <indexterm><primary><option>-ferror-spans</option></primary>
-- 
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