#15468: in ghci -fbreak-on-exception omits exception info -------------------------------------+------------------------------------- Reporter: George | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: GHCi | Version: 8.4.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Poor/confusing Unknown/Multiple | error message Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- In ghc 8.4.3 Without -fbreak-on-exception you get a good error message: {{{ ghci -ignore-dot-ghci GHCi, version 8.4.3: http://www.haskell.org/ghc/ :? for help Prelude> let 2 + 2 = 5 Prelude> 2 + 2 5 Prelude> 3 + 3 *** Exception: <interactive>:1:5-13: Non-exhaustive patterns in function + }}} With -fbreak-on-exception there is no info about the exception: {{{ ghci -ignore-dot-ghci -fbreak-on-exception GHCi, version 8.4.3: http://www.haskell.org/ghc/ :? for help Prelude> let 2 + 2 = 5 Prelude> 2 + 2 5 Prelude> 3 + 3 Stopped in <exception thrown>, <unknown> _exception :: e = _ [<unknown>] [<unknown>] Prelude> }}} In addition, contrary to the documentation, it doesn't seem possible to get ghci to print out the value of the exception {{{ <unknown>] [<unknown>] Prelude> :print _exception _exception = (_t1::e) [<unknown>] [<unknown>] Prelude> :info _exception _exception :: e -- Defined in ‘interactive:Ghci3’ [<unknown>] [<unknown>] Prelude> }}} Finally the fact that you get now warning about {{{ let 2 + 2 = 5 }}} or for {{{ 2 = 3 }}} makes me think I should file an ER to add the following to the default warnings for ghci {{{ ghci -ignore-dot-ghci -Wname-shadowing -Wunused-pattern-binds GHCi, version 8.4.3: http://www.haskell.org/ghc/ :? for help Prelude> let 2 + 2 = 5 <interactive>:1:7: warning: [-Wname-shadowing] This binding for ‘+’ shadows the existing binding imported from ‘Prelude’ (and originally defined in ‘GHC.Num’) Prelude> 2 = 3 <interactive>:2:1: warning: [-Wunused-pattern-binds] This pattern-binding binds no variables: 2 = 3 Prelude> }}} I noticed the second and when googling to see if it had been reported I came across the following which documented the first: https://code.likeagirl.io/2-2-5-and-why-compiler-warnings-are-good- eaadef327146 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15468 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler