[GHC] #15693: Abstracting out pattern into a pattern synonym fails with scary error
GHC
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Wed Oct 3 13:40:34 UTC 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 at 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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