Re: [GHC] #15693: Abstracting out pattern into a pattern synonym fails with scary error

3 Oct 2018

      #15693: Abstracting out pattern into a pattern synonym fails with scary error
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 Initial thoughts.

 1.  Always start by writing a pattern signature if GADTs are involved.

 2.  I started to do this, and discovered en route that the signature
 inferred for `ASSO` is absurd
 {{{
 pattern ASSO
        :: forall kind (a :: kind) (b :: Ctx kind).
           () =>
           forall ks k (f :: k -> ks) (a1 :: k) (ctx :: Ctx ks) ks1 k1
                  (f1 :: k1 -> ks1) (a2 :: k1) (ctx1 :: Ctx ks1) a3.
           (kind ~ (k -> ks), a ~~ f, b ~~ (a1 ':&: ctx), ks ~ (k1 -> ks1),
            f a1 ~~ f1, ctx ~~ (a2 ':&: ctx1), ks1 ~ *, f1 a2 ~~ a3,
             ctx1 ~~ 'E) =>
           forall k ks a1 a2 ctx. (kind ~ (k -> ks), b ~ (a1 :&: (a2 :&:
 ctx))) =>
           a a1 a2 -> ApplyT kind a b
 }}}
   There is no need to quantify over so much, or to have all those
 redundant constraints.

 3. I manually simplifed the signature to
 {{{
 pattern ASSO
        :: forall kind (a :: kind) (b :: Ctx kind).
           () =>
           forall k ks a1 a2 ctx. (kind ~ (k -> ks), b ~ (a1 :&: (a2 :&:
 ctx))) =>
           a a1 a2 -> ApplyT kind a b
 }}}
   Is that a correct sigature?

   Anyway it crashes GHC
 {{{
 WARNING: file compiler/types/TyCoRep.hs, line 2486
   in_scope InScope {a_aWo a1_aWp a2_aWq k_aWr ks_aWs ctx_aWt a_aYo}
   tenv [aWo :-> a_aWo[sk:0], aWp :-> a1_aWp[sk:0],
         aWq :-> a2_aWq[sk:0], aWr :-> k_aWr[sk:0], aWs :-> ks_aWs[sk:0],
         aWt :-> ctx_aWt[sk:0]]
   cenv []
   tys [a_aWo[sk:1] a1_aWp[sk:1] a2_aWq[sk:1]
        -> ApplyT
             (k_aWr[sk:1] -> ks_aWs[sk:1])
             (a_aWo[sk:1] |> <k_aWr[sk:1]>_N ->_N Sym {co_aYt})
             (a1_aWp[sk:1] ':&: (a2_aWq[sk:1] ':&: ctx_aWt[sk:1]) |> (Ctx
 (<k_aWr[sk:1]>_N
 ->_N Sym {co_aYu}))_N)]
   cos []
   needInScope [aYo :-> a_aYo[sk:1], aYt :-> co_aYt, aYu :-> co_aYu]
 ghc-stage1: panic! (the 'impossible' happened)
   (GHC version 8.7.20181003 for x86_64-unknown-linux):
         ASSERT failed!
   Type-correct unfilled coercion hole {co_aYt}
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in
 ghc:Outputable
         pprPanic, called at compiler/utils/Outputable.hs:1219:5 in
 ghc:Outputable
         assertPprPanic, called at compiler/typecheck/TcHsSyn.hs:1805:99 in
 ghc:TcHsSyn

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

 simonpj@cam-05-unx:~/tmp$ ~/5builds/HEAD-4/inplace/bin/ghc-stage1 -c
 T15693.hs -ddump-types
 WARNING: file compiler/types/TyCoRep.hs, line 2486
   in_scope InScope {kind_aWo a_aWp b_aWq k_aWr ks_aWs a1_aWt a2_aWu
                     ctx_aWv k_aYw k_aYz}
   tenv [aWo :-> kind_aWo[sk:0], aWp :-> a_aWp[sk:0],
         aWq :-> b_aWq[sk:0], aWr :-> k_aWr[sk:0], aWs :-> ks_aWs[sk:0],
         aWt :-> a1_aWt[sk:0], aWu :-> a2_aWu[sk:0], aWv :-> ctx_aWv[sk:0]]
   cenv []
   tys [kind_aWo[sk:1] ~ (k_aWr[sk:2] -> ks_aWs[sk:2]),
        b_aWq[sk:1]
        ~ (a1_aWt[sk:2] ':&: (a2_aWu[sk:2] ':&: ctx_aWv[sk:2]) |> (Ctx
                                                                     (Sym
 {co_aYy}))_N)]
   cos []
   needInScope [aYw :-> k_aYw[sk:2], aYy :-> co_aYy,
                aYz :-> k_aYz[sk:2]]
 WARNING: file compiler/types/TyCoRep.hs, line 2486
   in_scope InScope {kind_aWo a_aWp b_aWq k_aWr ks_aWs a1_aWt a2_aWu
                     ctx_aWv k_aYw k_aYz}
   tenv [aWo :-> kind_aWo[sk:0], aWp :-> a_aWp[sk:0],
         aWq :-> b_aWq[sk:0], aWr :-> k_aWr[sk:0], aWs :-> ks_aWs[sk:0],
         aWt :-> a1_aWt[sk:0], aWu :-> a2_aWu[sk:0], aWv :-> ctx_aWv[sk:0]]
   cenv []
   tys [(a_aWp[sk:1] |> {co_aYy}) a1_aWt[sk:2] a2_aWu[sk:2]
        -> ApplyT kind_aWo[sk:1] a_aWp[sk:1] b_aWq[sk:1]]
   cos []
   needInScope [aYw :-> k_aYw[sk:2], aYy :-> co_aYy,
                aYz :-> k_aYz[sk:2]]
 WARNING: file compiler/types/TyCoRep.hs, line 2486
   in_scope InScope {kind_aWo a_aWp b_aWq k_aZT k_aZU k_aZV ks_aZW
                     a1_aZX a2_aZY ctx_aZZ}
   tenv [aWr :-> k_aZV[tau:4], aWs :-> ks_aZW[tau:4],
         aWt :-> a1_aZX[tau:4], aWu :-> a2_aZY[tau:4],
         aWv :-> ctx_aZZ[tau:4], aYw :-> k_aZU[tau:4], aYz :->
 k_aZT[tau:4]]
   cenv []
   tys [kind_aWo[sk:0] ~ (k_aWr[sk:0] -> ks_aWs[sk:0]),
        b_aWq[sk:0]
        ~ (a1_aWt[sk:0] ':&: (a2_aWu[sk:0] ':&: ctx_aWv[sk:0]) |> (Ctx
                                                                     (Sym
 {co_aYy}))_N)]
   cos []
   needInScope [aWo :-> kind_aWo[sk:0], aWq :-> b_aWq[sk:0],
                aYy :-> co_aYy]
 ghc-stage1: panic! (the 'impossible' happened)
   (GHC version 8.7.20181003 for x86_64-unknown-linux):
         ASSERT failed!
   Type-correct unfilled coercion hole {co_aYy}
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in
 ghc:Outputable
         pprPanic, called at compiler/utils/Outputable.hs:1219:5 in
 ghc:Outputable
         assertPprPanic, called at compiler/typecheck/TcHsSyn.hs:1805:99 in
 ghc:TcHsSyn

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 4.  I GADT-ified it thus
 {{{
 pattern ASSO :: a a1 a2 -> ApplyT (k -> ks) a (a1 :&: (a2 :&: ctx))
 }}}
   (NB: as the manual says, this isn't quite the same as the form with
 equality constraints.)  But that crashed GHC too
 {{{
 WARNING: file compiler/types/TyCoRep.hs, line 2486
   in_scope InScope {a_aWo a1_aWp a2_aWq k_aWr ks_aWs ctx_aWt a_aYo}
   tenv [aWo :-> a_aWo[sk:0], aWp :-> a1_aWp[sk:0],
         aWq :-> a2_aWq[sk:0], aWr :-> k_aWr[sk:0], aWs :-> ks_aWs[sk:0],
         aWt :-> ctx_aWt[sk:0]]
   cenv []
   tys [a_aWo[sk:1] a1_aWp[sk:1] a2_aWq[sk:1]
        -> ApplyT
             (k_aWr[sk:1] -> ks_aWs[sk:1])
             (a_aWo[sk:1] |> <k_aWr[sk:1]>_N ->_N Sym {co_aYt})
             (a1_aWp[sk:1] ':&: (a2_aWq[sk:1] ':&: ctx_aWt[sk:1]) |> (Ctx
 (<k_aWr[sk:1]>_N
 ->_N Sym {co_aYu}))_N)]
   cos []
   needInScope [aYo :-> a_aYo[sk:1], aYt :-> co_aYt, aYu :-> co_aYu]
 ghc-stage1: panic! (the 'impossible' happened)
   (GHC version 8.7.20181003 for x86_64-unknown-linux):
         ASSERT failed!
   Type-correct unfilled coercion hole {co_aYt}
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in
 ghc:Outputable
         pprPanic, called at compiler/utils/Outputable.hs:1219:5 in
 ghc:Outputable
         assertPprPanic, called at compiler/typecheck/TcHsSyn.hs:1805:99 in
 ghc:TcHsSyn

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 So we have multiple issues here

 * GHC should not crash

 * We should not infer such a redundant signature.

 What signature do you expect?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15693#comment:2>
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