[Git][ghc/ghc][wip/romes/exceptions-layout] Display type and callstack of exception on handler

[Rodrigo Mesquita (@alt-romes)]

Rodrigo Mesquita pushed to branch wip/romes/exceptions-layout at Glasgow Haskell Compiler / GHC


Commits:
a971620c by Rodrigo Mesquita at 2024-09-24T11:28:50+01:00
Display type and callstack of exception on handler

This commit changes the Exception instance of SomeException to *simply*
display the underlying exception in `displayException`. The augmented
exception message that included the type and backtrace of the exception
are now only printed on a call to `displayExceptionWithInfo`.

At a surface level, existing programs should behave the same since the
`uncaughtExceptionHandler`, which is responsible for printing out uncaught
exceptions to the user, will use `displayExceptionWithInfo` by default.

However, unlike the instance's `displayException` method, the
`uncaughtExceptionHandler` can be overriden with
`setUncaughtExceptionHandler`. This makes the extra information opt-in
without fixing it the instance, which can be valuable if your program
wants to display uncaught exceptions to users in a user-facing way
(ie without backtraces).

This is what was originally agreed for CLC#231 or CLC#261 with regard to
the type of the exception information.

The call stack also becoming part of the default handler rather than the
Exception instance is an ammendment to CLC#164.

Discussion of the ammendment is part of CLC#285.

- - - - -


4 changed files:

- libraries/base/tests/T24807.stderr
- libraries/ghc-internal/src/GHC/Internal/Conc/Sync.hs
- libraries/ghc-internal/src/GHC/Internal/Exception.hs
- libraries/ghc-internal/src/GHC/Internal/Exception/Type.hs


Changes:

=====================================
libraries/base/tests/T24807.stderr
=====================================
@@ -1,11 +1,11 @@
-T24807: Exception:
+T24807: Uncaught exception ghc-internal:GHC.Internal.Exception.ErrorCall:
+
 hi
 CallStack (from HasCallStack):
   error, called at T24807.hs:1:8 in main:Main
-Package: ghc-internal
-Module: GHC.Internal.Exception
-Type: ErrorCall
+
 HasCallStack backtrace:
-  collectBacktraces, called at libraries/ghc-internal/src/GHC/Internal/Exception.hs:92:13 in ghc-internal:GHC.Internal.Exception
-  toExceptionWithBacktrace, called at libraries/ghc-internal/src/GHC/Internal/Exception.hs:127:5 in ghc-internal:GHC.Internal.Exception
+  collectBacktraces, called at libraries/ghc-internal/src/GHC/Internal/Exception.hs:93:13 in ghc-internal:GHC.Internal.Exception
+  toExceptionWithBacktrace, called at libraries/ghc-internal/src/GHC/Internal/Exception.hs:128:5 in ghc-internal:GHC.Internal.Exception
   error, called at T24807.hs:1:8 in main:Main
+


=====================================
libraries/ghc-internal/src/GHC/Internal/Conc/Sync.hs
=====================================
@@ -952,7 +952,7 @@ uncaughtExceptionHandler = unsafePerformIO (newIORef defaultHandler)
       defaultHandler se = do
          (hFlush stdout) `catchAny` (\ _ -> return ())
 
-         let exMsg = displayException se
+         let exMsg = displayExceptionWithInfo se
              msg = "Uncaught exception " ++ exMsg
          withCString "%s" $ \cfmt ->
           withCString msg $ \cmsg ->


=====================================
libraries/ghc-internal/src/GHC/Internal/Exception.hs
=====================================
@@ -35,6 +35,7 @@ module GHC.Internal.Exception
 
       -- * 'SomeException'
     , SomeException(..)
+    , displayExceptionWithInfo
 
       -- * Exception context
     , someExceptionContext


=====================================
libraries/ghc-internal/src/GHC/Internal/Exception/Type.hs
=====================================
@@ -29,6 +29,7 @@
 module GHC.Internal.Exception.Type
        ( Exception(..)    -- Class
        , SomeException(..)
+       , displayExceptionWithInfo
        , someExceptionContext
        , addExceptionContext
        , mapExceptionContext
@@ -213,22 +214,35 @@ instance Exception SomeException where
         in SomeException e
     fromException = Just
     backtraceDesired (SomeException e) = backtraceDesired e
-    displayException (SomeException e) =
-        case displayContext ?exceptionContext of
-          "" -> msg
-          dc -> msg ++ "\n\n" ++ dc
-        where
-            msg =
-              displayExceptionInfo (Typeable.typeOf e)
-              ++ "\n\n"
-              ++ displayException e
-
-            displayExceptionInfo :: TypeRep -> String
-            displayExceptionInfo rep =
-                tyMsg ++ ":"
-              where
-                tyMsg = Typeable.tyConPackage tyCon ++ ":" ++ Typeable.tyConModule tyCon ++ "." ++ Typeable.tyConName tyCon
-                tyCon = Typeable.typeRepTyCon rep
+    displayException (SomeException e) = displayException e
+
+-- | Displays a 'SomeException' with additional information:
+--
+--    * The type of the underlying exception
+--    * The exception context
+--
+-- By default, 'uncaughtExceptionHandler' uses 'displayExceptionWithInfo' to print uncaught exceptions.
+-- This default can be overriden with 'setUncaughtExceptionHandler', for
+-- instance, to present custom error messages on exceptions to the user.
+--
+-- @since base-4.21
+displayExceptionWithInfo :: SomeException -> String
+displayExceptionWithInfo (SomeException e) =
+    case displayContext ?exceptionContext of
+      "" -> msg
+      dc -> msg ++ "\n\n" ++ dc
+    where
+        msg =
+          displayExceptionType (Typeable.typeOf e)
+          ++ "\n\n"
+          ++ displayException e
+
+        displayExceptionType :: TypeRep -> String
+        displayExceptionType rep =
+            tyMsg ++ ":"
+          where
+            tyMsg = Typeable.tyConPackage tyCon ++ ":" ++ Typeable.tyConModule tyCon ++ "." ++ Typeable.tyConName tyCon
+            tyCon = Typeable.typeRepTyCon rep
 
 displayContext :: ExceptionContext -> String
 displayContext (ExceptionContext anns0) = mconcat $ intersperse "\n" $ map go anns0



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/a971620c7f026a0f97d883eafe9feae7be7246d6

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/a971620c7f026a0f97d883eafe9feae7be7246d6
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