[commit: ghc] master: Add displayException method to Exception (#9822) (3222b7a)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/3222b7ae347be092bdd414f7b43bee18861b0e1e/ghc

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

commit 3222b7ae347be092bdd414f7b43bee18861b0e1e
Author: Michael Snoyman <michael at snoyman.com>
Date:   Fri Nov 21 23:26:09 2014 +0100

    Add displayException method to Exception (#9822)
    
    Defaults to using `show` to prevent any breakage of existing code.  Also
    provide a custom implementation for `SomeException` which uses the
    underlying exception's `displayException`.
    
    Differential Revision: https://phabricator.haskell.org/D517


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

3222b7ae347be092bdd414f7b43bee18861b0e1e
 libraries/base/GHC/Exception.hs | 9 +++++++++
 libraries/base/changelog.md     | 2 ++
 2 files changed, 11 insertions(+)

diff --git a/libraries/base/GHC/Exception.hs b/libraries/base/GHC/Exception.hs
index 7a7c8c2..e2b7149 100644
--- a/libraries/base/GHC/Exception.hs
+++ b/libraries/base/GHC/Exception.hs
@@ -140,9 +140,18 @@ class (Typeable e, Show e) => Exception e where
     toException = SomeException
     fromException (SomeException e) = cast e
 
+    -- | Render this exception value in a human-friendly manner.
+    --
+    -- Default implementation: @'show'@.
+    --
+    -- /Since: 4.8.0.0/
+    displayException :: e -> String
+    displayException = show
+
 instance Exception SomeException where
     toException se = se
     fromException = Just
+    displayException (SomeException e) = displayException e
 
 -- | Throw an exception.  Exceptions may be thrown from purely
 -- functional code, but may only be caught within the 'IO' monad.
diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md
index 3e110a7..a5ae8ea 100644
--- a/libraries/base/changelog.md
+++ b/libraries/base/changelog.md
@@ -104,6 +104,8 @@
 
   * Add `fillBytes` to `Foreign.Marshal.Utils`.
 
+  * Add new `displayException` method to `Exception` typeclass. (#9822)
+
 ## 4.7.0.1  *Jul 2014*
 
   * Bundled with GHC 7.8.3