Matthew Pickering pushed to branch wip/t26751 at Glasgow Haskell Compiler / GHC Commits: 7ae17f5d by Matthew Pickering at 2026-01-09T11:38:34+00:00 Evaluate backtraces for "error" exceptions at the moment they are thrown See Note [Capturing the backtrace in throw] and Note [Hiding precise exception signature in throw] which explain the implementation. This commit makes `error` and `throw` behave the same with regard to backtraces. Previously, exceptiosn raised by `error` would not contain useful IPE backtraces. I did try and implement `error` in terms of `throw` but it started to involve putting diverging functions into hs-boot files, which seemed to risky if the compiler wouldn't be able to see if applying a function would diverge. Fixes #26751 - - - - - 5 changed files: - libraries/ghc-internal/src/GHC/Internal/Err.hs - testsuite/tests/ghci.debugger/scripts/T8487.stdout - testsuite/tests/ghci.debugger/scripts/break011.stdout - testsuite/tests/ghci.debugger/scripts/break017.stdout - testsuite/tests/ghci.debugger/scripts/break025.stdout Changes: ===================================== libraries/ghc-internal/src/GHC/Internal/Err.hs ===================================== @@ -1,6 +1,7 @@ {-# LANGUAGE Trustworthy #-} {-# LANGUAGE NoImplicitPrelude, MagicHash, ImplicitParams #-} {-# LANGUAGE RankNTypes, PolyKinds, DataKinds #-} +{-# LANGUAGE BangPatterns #-} {-# OPTIONS_HADDOCK not-home #-} ----------------------------------------------------------------------------- @@ -25,6 +26,7 @@ module GHC.Internal.Err( absentErr, error, errorWithoutStackTrace, undefined ) where import GHC.Internal.Types (Char, RuntimeRep) import GHC.Internal.Stack.Types +import GHC.Internal.Magic import GHC.Internal.Prim import {-# SOURCE #-} GHC.Internal.Exception ( errorCallWithCallStackException @@ -33,7 +35,10 @@ import {-# SOURCE #-} GHC.Internal.Exception -- | 'error' stops execution and displays an error message. error :: forall (r :: RuntimeRep). forall (a :: TYPE r). HasCallStack => [Char] -> a -error s = raise# (errorCallWithCallStackException s ?callStack) +error s = + -- Evaluate SomeException before to get accurate callstacks (like throw) + let !se = noinline (errorCallWithCallStackException s ?callStack) + in raise# se -- Bleh, we should be using 'GHC.Internal.Stack.callStack' instead of -- '?callStack' here, but 'GHC.Internal.Stack.callStack' depends on -- 'GHC.Internal.Stack.popCallStack', which is partial and depends on @@ -73,7 +78,9 @@ undefined :: forall (r :: RuntimeRep). forall (a :: TYPE r). -- nor wanted (see #19886). We’d like to use withFrozenCallStack, but that -- is not available in this module yet, and making it so is hard. So let’s just -- use raise# directly. -undefined = raise# (errorCallWithCallStackException "Prelude.undefined" ?callStack) +undefined = + let !se = noinline (errorCallWithCallStackException "Prelude.undefined" ?callStack) + in raise# se -- | Used for compiler-generated error message; -- encoding saves bytes of string junk. ===================================== testsuite/tests/ghci.debugger/scripts/T8487.stdout ===================================== @@ -1,4 +1,5 @@ Breakpoint 0 activated at T8487.hs:(5,8)-(7,53) Stopped in Main.f, T8487.hs:(5,8)-(7,53) _result :: IO String = _ -ma :: Either SomeException String = Left _ +ma :: Either SomeException String = Left + (SomeException (ErrorCall ...)) ===================================== testsuite/tests/ghci.debugger/scripts/break011.stdout ===================================== @@ -4,9 +4,10 @@ HasCallStack backtrace: error, called at <interactive>:2:1 in interactive:Ghci1 Stopped in <exception thrown>, <unknown> -_exception :: e = _ +_exception :: e = GHC.Internal.Exception.Type.SomeException + (GHC.Internal.Exception.ErrorCall _) Stopped in <exception thrown>, <unknown> -_exception :: e = _ +_exception :: e = SomeException (ErrorCall _) -1 : main (Test7.hs:2:18-28) -2 : main (Test7.hs:2:8-29) <end of history> @@ -26,7 +27,7 @@ _exception :: SomeException = SomeException (ErrorCall "foo") *** Exception: foo HasCallStack backtrace: - error, called at Test7.hs:2:18 in main:Main + error, called at Test7.hs:2:18 in interactive-session:Main Stopped in <exception thrown>, <unknown> _exception :: e = _ @@ -35,5 +36,5 @@ _exception :: e = _ *** Exception: foo HasCallStack backtrace: - error, called at Test7.hs:2:18 in main:Main + error, called at Test7.hs:2:18 in interactive-session:Main ===================================== testsuite/tests/ghci.debugger/scripts/break017.stdout ===================================== @@ -1,5 +1,6 @@ "Stopped in <exception thrown>, <unknown> -_exception :: e = _ +_exception :: e = GHC.Internal.Exception.Type.SomeException + (GHC.Internal.Exception.ErrorCall _) Logged breakpoint at QSort.hs:6:32-34 _result :: Char -> Bool a :: Char ===================================== testsuite/tests/ghci.debugger/scripts/break025.stdout ===================================== @@ -1,3 +1,4 @@ Stopped in <exception thrown>, <unknown> -_exception :: e = _ +_exception :: e = GHC.Internal.Exception.Type.SomeException + (GHC.Internal.Exception.ErrorCall _) () View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/7ae17f5d13f42385b462e8e75c6d14a4... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/7ae17f5d13f42385b462e8e75c6d14a4... 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