[GHC] #9046: Panic in GHCi when using :print
#9046: Panic in GHCi when using :print ------------------------------------+------------------------------------- Reporter: quchen | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.8.3 Component: GHCi | Version: 7.8.2 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: GHCi crash Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- When using names generated by :print, GHCi panics sometimes. According to #ghc, this is reproducible on ''x86_64-apple-darwin'' as well, but is not an issue in ''7.6.3 x86_64 linux''. {{{
let a = [1]
:print a a = (_t1::Num t => [t])
:t _t1 ghc: panic! (the 'impossible' happened) (GHC version 7.8.2 for x86_64-unknown-linux): tcTyVarDetails t{tv avD} [tv]
_t1 <same panic> }}}
From playing around with it a bit, this seems to have something to do with polymorphic values. {{{ -- No panic
let a = [1 :: Int] let a = 1 :: Int let a = "hello"
-- Panic
let a = [1] let a = 1 let a = "hello" -- With -XOverloadedStrings }}}
-- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9046 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#9046: Panic in GHCi when using :print -------------------------------------+------------------------------------ Reporter: quchen | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.8.3 Component: GHCi | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: GHCi crash | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by chris.parks): I'm seeing the same thing on x86_64-apple-darwin in GHC HEAD: {{{
:t _t1 ghc-stage2: panic! (the 'impossible' happened) (GHC version 7.9.20140520 for x86_64-apple-darwin): ASSERT failed! file compiler/typecheck/TcType.lhs line 640 t }}}
That ASSERT is in `isMetaTyVar`: {{{ isMetaTyVar tv = ASSERT2( isTcTyVar tv, ppr tv ) case tcTyVarDetails tv of MetaTv {} -> True _ -> False }}} We're expecting that `tv` was constructed by the `TcTyVar` constructor of the `Var` datatype. However, what little printf-debugging I've been able to do indicates that we're getting a plain `TyVar` instead. Maybe someone with more debugging experience can shed some light on this? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9046#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#9046: Panic in GHCi when using :print -------------------------------------+------------------------------------ Reporter: quchen | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.8.4 Component: GHCi | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: GHCi crash | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by archblob): For `:info _t1` we get: {{{ WARNING: file compiler/main/PprTyThing.hs, line 153 _t1 _t1 :: Num t => [t] -- Defined at <no location info> }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9046#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#9046: Panic in GHCi when using :print -------------------------------------+------------------------------------ Reporter: quchen | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.8.4 Component: GHCi | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: GHCi crash | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by bravit): Another strange result made visible using `-fprint-explicit-foralls`: {{{ $ ghci -fprint-explicit-foralls GHCi, version 7.8.2: http://www.haskell.org/ghc/ :? for help
let a = [1]
:print a a = (_t1::Num t => [t])
:show bindings a :: forall t. Num t => [t] = _ _t1 :: Num t => [t] = _ }}}
It looks like free type variable `t` in the signature of `_t1` causes GHC panic while typechecking it. But I can't figure out whether it is a bug of rtti type reconstruction in `:print` or incorrect behaviour during typechecking. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9046#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#9046: Panic in GHCi when using :print -------------------------------------+------------------------------------ Reporter: quchen | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.8.4 Component: GHCi | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: GHCi crash | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by bravit): No problem for unbounded polymorphic type variable: {{{ $ ghci -fprint-explicit-foralls GHCi, version 7.8.2: http://www.haskell.org/ghc/ :? for help
:t length length :: forall a. [a] -> Int
:print length length = (_t1::[a] -> Int)
:t _t1 _t1 :: [a] -> Int }}} But: {{{ :t read read :: forall a. Read a => String -> a
:print read read = (_t2::Read a1 => String -> a1)
:t _t2 ghc: panic! (the 'impossible' happened) (GHC version 7.8.2 for x86_64-unknown-linux): tcTyVarDetails a1{tv asM} [tv] }}}
-- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9046#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#9046: Panic in GHCi when using :print -------------------------------------+------------------------------------ Reporter: quchen | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.8.4 Component: GHCi | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: GHCi crash | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by bravit): We had different behaviour for `:print` back in 7.6.3: {{{ $ ghci -fprint-explicit-foralls GHCi, version 7.6.3: http://www.haskell.org/ghc/ :? for help
:print read read = (_t1::forall a. Read a => String -> a) :t _t1 _t1 :: forall a. Read a => String -> a
:print length length = (_t2::forall a. [a] -> Int) :t _t2 _t2 :: forall a. [a] -> Int }}}
-- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9046#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#9046: Panic in GHCi when using :print -------------------------------------+------------------------------------ Reporter: quchen | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.8.4 Component: GHCi | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: GHCi crash | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by bravit): All right, removing of enclosing foralls during `:print` was intentional according to this commit:
In a8ac471d435214dbdc1fa70f938c63128993a1db/ghc: Fix the deugger (fixing Trac #8557)
The runtime debugger (which has not received any love from anyone for many years) looks wrong to me; it was failing to instantiate the outer foralls of a variable when called from :force, which calls cvObtainTermFromId, which calls cvObtainTerm
I simplified the code too. But I'm flaky on how this debugger stuff is really supposed to work, so I'm partly guessing. Tests pass though.
SPJ particularly wrote in `compiler/ghci/RtClosureInspect.hs`: {{{ +-- Generalize the type: find all free and forall'd tyvars +-- and return them, together with the type inside, which +-- should not be a forall type. }}} So now it looks like typechecking issue with free bounded polymorphic tyvars: free type variable is built by `TyVar` constructor of `Var` and it stays the same during typechecking. In the presence of constraints (if tyvar is bounded) we try to solve them and then get this GHC panic: we need `TcTyVar` constructor there. I suppose solution could be to replace free tyvar with `TcTyVar` before solving constraints though I need some guidance to implement it. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9046#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#9046: Panic in GHCi when using :print -------------------------------------+------------------------------------ Reporter: quchen | Owner: Type: bug | Status: patch Priority: normal | Milestone: 7.8.4 Component: GHCi | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: GHCi crash | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Changes (by bravit): * status: new => patch -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9046#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#9046: Panic in GHCi when using :print
-------------------------------------+------------------------------------
Reporter: quchen | Owner:
Type: bug | Status: patch
Priority: normal | Milestone: 7.8.4
Component: GHCi | Version: 7.8.2
Resolution: | Keywords:
Operating System: Unknown/Multiple | Architecture: Unknown/Multiple
Type of failure: GHCi crash | Difficulty: Unknown
Test Case: | Blocked By:
Blocking: | Related Tickets:
-------------------------------------+------------------------------------
Comment (by Austin Seipp
#9046: Panic in GHCi when using :print -------------------------------------+------------------------------------- Reporter: quchen | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.8.4 Component: GHCi | Version: 7.8.2 Resolution: | Keywords: Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: GHCi crash | Blocked By: Test Case: | Related Tickets: tests/ghci.debugger/scripts/print036| Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Changes (by thoughtpolice): * status: patch => new * testcase: => tests/ghci.debugger/scripts/print036 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9046#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#9046: Panic in GHCi when using :print -------------------------------------+------------------------------------- Reporter: quchen | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.8.4 Component: GHCi | Version: 7.8.2 Resolution: | Keywords: Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: GHCi crash | Blocked By: Test Case: | Related Tickets: tests/ghci.debugger/scripts/print036| Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by francesquini): Same problem, different steps: {{{ GHCi, version 7.8.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> let x = 3 + 4 Prelude> :print x x = (_t1::Num a => a) Prelude> :force x x = _ Prelude> print _t1 ghc: panic! (the 'impossible' happened) (GHC version 7.8.3 for x86_64-unknown-linux): tcTyVarDetails a{tv atm} [tv] }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9046#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#9046: Panic in GHCi when using :print -------------------------------------+------------------------------------- Reporter: quchen | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.8.4 Component: GHCi | Version: 7.8.2 Resolution: | Keywords: Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: GHCi crash | Blocked By: Test Case: | Related Tickets: tests/ghci.debugger/scripts/print036| Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by simonpj): This is clearly a bug. (All panics are, really.) And hence embarrassing to have open for so long. But I didn't write the code for the GHCi debugger, and I only partly understand it. It needs love and care from someone. At the moment this ticket is sitting in my "look at this when I have time" pile, but I never have time. It would be great if someone was willing to take the time to dig into how the debugger works, and maybe "own" it for the future. The good thing is that it's only one or two modules, and there are few non-local ramifications, so it's a well-contained collection of issues. Also, this ticket a show-stopper for anyone? Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9046#comment:12 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#9046: Panic in GHCi when using :print -------------------------------------+------------------------------------- Reporter: quchen | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: GHCi crash | Test Case: | tests/ghci.debugger/scripts/print036 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by monoidal): Also reported at #12449 and #15299. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9046#comment:17 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#9046: Panic in GHCi when using :print -------------------------------------+------------------------------------- Reporter: quchen | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: GHCi crash | Test Case: | tests/ghci.debugger/scripts/print036 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by anarchist666): Probably #9046 not present in 8.4.3, but there are #12449 and #15299. And #15299 is a duplicate #12449. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9046#comment:18 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#9046: Panic in GHCi when using :print -------------------------------------+------------------------------------- Reporter: quchen | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: GHCi | Version: 7.8.2 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: GHCi crash | Test Case: | tests/ghci.debugger/scripts/print036 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by anarchist666): * status: new => closed * resolution: => fixed -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9046#comment:19 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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