September 2018 - ghc-tickets

[GHC] #15640: Add "difficulty" field to tickets
by GHC 14 Sep '18

14 Sep '18

#15640: Add "difficulty" field to tickets -------------------------------------+------------------------------------- Reporter: flip101 | Owner: hvr Type: feature | Status: new request | Priority: normal | Milestone: Component: Trac & Git | Version: Keywords: | Operating System: Unknown/Multiple Architecture: Other | Type of failure: None/Unknown Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I propose to add a new field to tickets to indicate the difficulty of fixing a bug. This for three purposes: * The priority of the bug can be affected by the difficulty, thus making the difficulty visible as a field can be handy to set/affect the priority accordingly, example: https://ghc.haskell.org/trac/ghc/ticket/15579#comment:1 * Can be welcoming to newcomers who want to contribute to GHC to start with tickets with low priority. The new field should be searchable so they can get a list of all tickets with low difficulty * Can make work on GHC more efficient by doing more high-priority / low- difficulty bugs. This plays into pareto principle - 80/20 rule As for the actual levels, "easy" or "hard" is subjective. So i think it's a good idea to have levels according to some experience level somebody should have. I can imagine some bugs can only be squashed by a few or even one person, while others can be jumped on by people who have decent haskell experience (given they want to read the background of setting up GHC dev and such). One could then argue that this field should then be named "skill level needed", but i like difficulty more :) And this also allows for situations where the bug can be fixed only by a few people who are really knowledgeable about the matter but still find a need for different gradients of difficulty level. Maybe something like: * Very easy - new to haskell * Easy - need haskell experience * Medium - need a lot of haskell experience * Hard - need experience with GHC dev * Very Hard - as before + profiling/debugging tools + system knowledge * Very Hard - as before + specific compiling techniques * Very Hard - as before + need C skills (for RTS) * .. I'm not sure how and who can change the Trac settings, but the procedure for this is described on this page: https://trac.edgewall.org/wiki/TracTicketsCustomFields -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15640> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15644: Test Suite Unexpected failure for ghci062(ghci-ext)
by GHC 13 Sep '18

13 Sep '18

#15644: Test Suite Unexpected failure for ghci062(ghci-ext) -----------------------------------------+---------------------------- Reporter: JulianLeviston | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: Test Suite | Version: Keywords: | Operating System: MacOS X Architecture: x86 | Type of failure: Other Test Case: ghci062(ghci-ext) | Blocked By: Blocking: | Related Tickets: 15643 Differential Rev(s): | Wiki Page: -----------------------------------------+---------------------------- Testing on MacOS X 10.13.6 MacBook (Retina, 12-inch, 2017) 1.4GHz i7 1 processor, 2 cores git sha is d36b1ffac9960db70043aaab43c931ce217912ba Ran `cd ghc/testsuite/tests/ghci` then `make test THREADS=2` The erroring output is as follows (snipped): {{{#!shell =====> ghci063(ghci) 99 of 271 [0, 1, 2] cd "scripts/ghci063.run" && HC="/Users/julianleviston/code/haskell/ghc/inplace/test spaces/ghc- stage2" HC_OPTS="-dcore-lint -dcmm-lint -no-user-package-db -rtsopts -fno- warn-missed-specialisations -fshow-warning-groups -fdiagnostics- color=never -fno-diagnostics-show-caret -dno-debug-output " "/Users/julianleviston/code/haskell/ghc/inplace/test spaces/ghc-stage2" --interactive -v0 -ignore-dot-ghci -fno-ghci-history +RTS -I0.1 -RTS -fghci-leak-check -dcore-lint -dcmm-lint -no-user-package-db -rtsopts -fno-warn-missed-specialisations -fshow-warning-groups -fdiagnostics- color=never -fno-diagnostics-show-caret -dno-debug-output < ghci063.script =====> ghci062(ghci-ext) 99 of 271 [0, 1, 2] cd "scripts/ghci062.run" && HC="/Users/julianleviston/code/haskell/ghc/inplace/test spaces/ghc- stage2" HC_OPTS="-dcore-lint -dcmm-lint -no-user-package-db -rtsopts -fno- warn-missed-specialisations -fshow-warning-groups -fdiagnostics- color=never -fno-diagnostics-show-caret -dno-debug-output " "/Users/julianleviston/code/haskell/ghc/inplace/test spaces/ghc-stage2" --interactive -v0 -ignore-dot-ghci -fno-ghci-history -fexternal- interpreter +RTS -I0.1 -RTS -dcore-lint -dcmm-lint -no-user-package-db -rtsopts -fno-warn-missed-specialisations -fshow-warning-groups -fdiagnostics-color=never -fno-diagnostics-show-caret -dno-debug-output < ghci062.script Actual stderr output differs from expected: diff -uw "/dev/null" "scripts/ghci063.run/ghci063.run.stderr.normalised" --- /dev/null 2018-09-12 23:13:35.000000000 +1000 +++ scripts/ghci063.run/ghci063.run.stderr.normalised 2018-09-12 23:13:39.000000000 +1000 @@ -0,0 +1,2 @@ + +B.hs:1:1: parse error on input ‘***’ *** unexpected failure for ghci063(ghci) =====> T2452(ghci) 100 of 271 [0, 2, 2] cd "scripts/T2452.run" && HC="/Users/julianleviston/code/haskell/ghc/inplace/test spaces/ghc- stage2" HC_OPTS="-dcore-lint -dcmm-lint -no-user-package-db -rtsopts -fno- warn-missed-specialisations -fshow-warning-groups -fdiagnostics- color=never -fno-diagnostics-show-caret -dno-debug-output " "/Users/julianleviston/code/haskell/ghc/inplace/test spaces/ghc-stage2" --interactive -v0 -ignore-dot-ghci -fno-ghci-history +RTS -I0.1 -RTS -fghci-leak-check -dcore-lint -dcmm-lint -no-user-package-db -rtsopts -fno-warn-missed-specialisations -fshow-warning-groups -fdiagnostics- color=never -fno-diagnostics-show-caret -dno-debug-output < T2452.script Wrong exit code for ghci062(ghci-ext)(expected 0 , actual 1 ) Stderr ( ghci062 ): ghc-iserv.bin: lookupSymbol failed in relocateSection (RELOC_GOT) /Users/julianleviston/code/haskell/ghc/libraries/integer-gmp/dist- install/build/HSinteger-gmp-1.0.2.0.o: unknown symbol `___gmp_rands' ghc-stage2: unable to load package `integer-gmp-1.0.2.0' *** unexpected failure for ghci062(ghci-ext) }}} Note, two errors occurred but I've created an error report for ghci063 in another ticket (#15643) -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15644> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15643: Test Suite Fails for GHCI
by GHC 13 Sep '18

13 Sep '18

#15643: Test Suite Fails for GHCI --------------------------------------+---------------------------- Reporter: JulianLeviston | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Test Suite | Version: Keywords: | Operating System: MacOS X Architecture: x86 | Type of failure: Other Test Case: T1914(ghci) | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: --------------------------------------+---------------------------- Git sha is ce23451c2c771bfbbac27ce63c5fdccc7ed02b3b (version 8.7, I believe - the latest as at day before yesterday) First run through of tests ever for me. Did `cd ghc/testsuite/tests/ghci && make THREADS=8` and the following error ensued: {{{#!shell =====> T1914(ghci) 102 of 271 [0, 0, 0] cd "scripts/T1914.run" && HC="/Users/julian/code/haskell/ghc/inplace/test spaces/ghc-stage2" HC_OPTS="-dcore-lint -dcmm-lint -no-user-package-db -rtsopts -fno-warn-missed-specialisations -fshow-warning-groups -fdiagnostics-color=never -fno-diagnostics-show-caret -dno-debug-output " "/Users/julian/code/haskell/ghc/inplace/test spaces/ghc-stage2" --interactive -v0 -ignore-dot-ghci -fno-ghci-history +RTS -I0.1 -RTS -fghci-leak-check -dcore-lint -dcmm-lint -no-user-package-db -rtsopts -fno-warn-missed-specialisations -fshow-warning-groups -fdiagnostics- color=never -fno-diagnostics-show-caret -dno-debug-output < T1914.script Actual stderr output differs from expected: diff -uw "/dev/null" "scripts/ghci063.run/ghci063.run.stderr.normalised" --- /dev/null 2018-09-12 23:42:18.000000000 +1000 +++ scripts/ghci063.run/ghci063.run.stderr.normalised 2018-09-12 23:42:20.000000000 +1000 @@ -0,0 +1,2 @@ + +B.hs:1:1: parse error on input ‘***’ *** unexpected failure for ghci063(ghci) }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15643> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14902: GHC HEAD cannot be booted GHC 8.4.1
by GHC 13 Sep '18

13 Sep '18

#14902: GHC HEAD cannot be booted GHC 8.4.1 -------------------------------------+------------------------------------- Reporter: osa1 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I get this when I try to boot GHC HEAD with GHC 8.4.1 (using official binaries): {{{ $ make ===--- building phase 0 make --no-print-directory -f ghc.mk phase=0 phase_0_builds utils/genprimopcode/ghc.mk:19: utils/genprimopcode/dist/package-data.mk: No such file or directory utils/deriveConstants/ghc.mk:19: utils/deriveConstants/dist/package- data.mk: No such file or directory utils/genapply/ghc.mk:23: utils/genapply/dist/package-data.mk: No such file or directory libraries/hpc/ghc.mk:3: libraries/hpc/dist-boot/package-data.mk: No such file or directory libraries/binary/ghc.mk:3: libraries/binary/dist-boot/package-data.mk: No such file or directory libraries/text/ghc.mk:3: libraries/text/dist-boot/package-data.mk: No such file or directory libraries/transformers/ghc.mk:3: libraries/transformers/dist-boot/package- data.mk: No such file or directory libraries/mtl/ghc.mk:3: libraries/mtl/dist-boot/package-data.mk: No such file or directory libraries/parsec/ghc.mk:3: libraries/parsec/dist-boot/package-data.mk: No such file or directory libraries/Cabal/Cabal/ghc.mk:3: libraries/Cabal/Cabal/dist-boot/package- data.mk: No such file or directory libraries/ghc-boot-th/ghc.mk:3: libraries/ghc-boot-th/dist-boot/package- data.mk: No such file or directory libraries/ghc-boot/ghc.mk:3: libraries/ghc-boot/dist-boot/package-data.mk: No such file or directory libraries/template-haskell/ghc.mk:3: libraries/template-haskell/dist-boot /package-data.mk: No such file or directory libraries/terminfo/ghc.mk:3: libraries/terminfo/dist-boot/package-data.mk: No such file or directory libraries/ghci/ghc.mk:3: libraries/ghci/dist-boot/package-data.mk: No such file or directory compiler/ghc.mk:446: compiler/stage1/package-data.mk: No such file or directory utils/hsc2hs/ghc.mk:21: utils/hsc2hs/dist/package-data.mk: No such file or directory utils/ghc-pkg/ghc.mk:70: utils/ghc-pkg/dist/package-data.mk: No such file or directory ghc/ghc.mk:111: ghc/stage1/package-data.mk: No such file or directory "inplace/bin/ghc-cabal" configure libraries/binary dist-boot --with- ghc="/home/omer/ghc_bins/ghc-8.4.1-bin/bin/ghc" --with-ghc- pkg="/home/omer/ghc_bins/ghc-8.4.1-bin/bin/ghc-pkg" --package- db=/home/omer/haskell/ghc/libraries/bootstrapping.conf --disable-library- for-ghci --enable-library-vanilla --enable-library-for-ghci --disable- library-profiling --disable-shared --configure-option=CFLAGS="-Wall -fno- stack-protector -Werror=unused-but-set-variable -Wno-error=inline" --configure- option=LDFLAGS=" " --configure-option=CPPFLAGS=" " --gcc-options="-Wall -fno-stack-protector -Werror=unused-but-set-variable -Wno-error=inline " --constraint "binary == 0.8.5.1" --constraint "text == 1.2.3.0" --constraint "transformers == 0.5.5.0" --constraint "mtl == 2.2.2" --constraint "parsec == 3.1.13.0" --constraint "Cabal == 2.2.0.0" --constraint "hpc == 0.6.0.3" --constraint "ghc-boot-th == 8.5" --constraint "ghc-boot == 8.5" --constraint "template-haskell == 2.13.0.0" --constraint "ghci == 8.5" --constraint "terminfo == 0.4.1.1" --with-gcc="gcc" --with-ld="ld.gold" --with-ar="ar" --with- alex="/home/omer/.local/bin/alex" --with- happy="/home/omer/.local/bin/happy" Warning: binary.cabal:19:24: unexpected unknown build-type: 'Simple' Simple ghc-cabal: dieVerbatim: user error (ghc-cabal: Failed parsing "./binary.cabal". ) libraries/binary/ghc.mk:3: recipe for target 'libraries/binary/dist-boot /package-data.mk' failed make[1]: *** [libraries/binary/dist-boot/package-data.mk] Error 1 Makefile:122: recipe for target 'all' failed make: *** [all] Error 2 }}} Switching to 8.2.1 works fine. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14902> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12420: Users guide link for hsc2hs has bitrotten
by GHC 13 Sep '18

13 Sep '18

#12420: Users guide link for hsc2hs has bitrotten -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: task | Status: new Priority: normal | Milestone: Component: hsc2hs | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- https://hackage.haskell.org/package/hsc2hs links to http://www.haskell.org/ghc/docs/latest/html/users_guide/hsc2hs.html which is dead. I think https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/utils.html #writing-haskell-interfaces-to-c-code-hsc2hs is what is desired. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12420> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14896: QuantifiedConstraints: GHC does doesn't discharge constraints if they are quantified
by GHC 13 Sep '18

13 Sep '18

#14896: QuantifiedConstraints: GHC does doesn't discharge constraints if they are quantified -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Keywords: | Operating System: Unknown/Multiple QuantifiedConstraints, wipT2893 | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This came up on a [https://www.reddit.com/r/haskell/comments/8257mz/how_quantifiedconstraints_… reddit thread], {{{#!hs {-# Language QuantifiedConstraints #-} class (forall aa. Functor (bi aa)) => Bifunctor bi where first :: (a -> a') -> (bi a b -> bi a' b) bimap :: Bifunctor bi => (a -> a') -> (b -> b') -> (bi a b -> bi a' b') bimap f g = first f . fmap g }}} This is the type we want for `bimap` even if we mix & match `Bifunctor` and `Functor`. We already know that we can `fmap @(bi xx)` for any `xx` but this is not the inferred type. Instead GHC infers a type (tidied up) with a superfluous `Functor` constraint {{{#!hs bimap :: (Bifunctor bi, Functor (bi a)) => (a -> a') -> (b -> b') -> (bi a b -> bi a' b') }}} Indeed post-composing with a superfluous `fmap @(bi a')` incurs yet another `Functor` constraint {{{#!hs bimap :: (Bifunctor bi, Functor (bi a), Functor (bi a')) => (a -> a') -> (b -> b') -> (bi a b -> bi a' b') bimap f g = fmap id . first f . fmap g }}} A terminology question, I'm not sure how to phrase what GHC isn't doing to the `Functor` constraints: ‘discharge’? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14896> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15563: Typo in the documentation for Numeric.Natural.
by GHC 13 Sep '18

13 Sep '18

#15563: Typo in the documentation for Numeric.Natural. -------------------------------------+------------------------------------- Reporter: asr | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Documentation | Version: 8.4.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The [https://downloads.haskell.org/~ghc/8.4.3/docs/html/libraries/base-4.11.1.0 /Numeric-Natural.html documentation] for `Numeric.Natural` says {{{ >>> 2^20 :: Natural 1267650600228229401496703205376 }}} Please replace the `20` by `100`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15563> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15285: "strange closure type" in T7919 with the threaded2 way
by GHC 13 Sep '18

13 Sep '18

#15285: "strange closure type" in T7919 with the threaded2 way ----------------------------------------+---------------------------------- Reporter: alpmestan | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Runtime System | Version: 8.5 Keywords: | Operating System: Linux Architecture: Unknown/Multiple | Type of failure: Runtime crash Test Case: T7919 | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: ----------------------------------------+---------------------------------- Last night's `./validate --slow` run on Circle CI revealed the following failure: {{{ Wrong exit code for T7919(threaded2)(expected 0 , actual 134 ) Stderr ( T7919 ): T7919: internal error: evacuate: strange closure type 32554624 (GHC version 8.5.20180617 for x86_64_unknown_linux) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug Aborted *** unexpected failure for T7919(threaded2) }}} The full log is available [https://circleci.com/api/v1.1/project/github/ghc/ghc/6205/output/107/0?file… here]. Among other things, you can see that it indeed only fails in the threaded2 way. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15285> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15636: Implication constraint priority breaks default class implementations
by GHC 13 Sep '18

13 Sep '18

#15636: Implication constraint priority breaks default class implementations -------------------------------------+------------------------------------- Reporter: i-am-tom | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.6.1-beta1 Keywords: | Operating System: Unknown/Multiple QuantifiedConstraints | Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Hello, Not 100% sure that this is a bug, but I've done some investigating (with a ''lot'' of help from Csongor Kiss) and thought it was, at the very least, behaviour worth clarifying. The following code... {{{#!hs {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE QuantifiedConstraints #-} {-# LANGUAGE UndecidableInstances #-} module Test where class D a where f :: a -> String g :: a -> String g = f class C a instance (forall a. C a => D a) => D x where f _ = "uh oh" }}} ... produces the error: {{{ • Could not deduce (C x) arising from a use of ‘Test.$dmg’ from the context: forall a. C a => D a bound by the instance declaration at Test.hs:19:10-38 Possible fix: add (C x) to the context of the instance declaration • In the expression: Test.$dmg @(x) In an equation for ‘g’: g = Test.$dmg @(x) In the instance declaration for ‘D x’ | 19 | instance (forall a. C a => D a) => D x where | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ }}} It appears that the problem here is with the default implementation for `g`. Namely, when `f` is called, two matching instances are found: - `forall a. C a => D a` - `(forall a. C a => D a) => D x` The issue, as far as we can tell, is that the first instance is chosen (and then the constraint check fails). I'm currently working around this by introducing a newtype into the head of the quantified constraint†, but I thought it best to check whether this is a bug or, indeed, the expected behaviour in this situation. Let me know if I've missed anything from this ticket - it's my first one! Thanks, Tom † An example of this can be found at https://github.com/i-am-tom/learn- me-a-haskell/blob/dbf2a22c5abb78ab91124dcf1e0e7ecd3d88831d/src/Bag/QuantifiedInstances.hs#L92-L94 -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15636> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15629: "No skolem info" panic (GHC 8.6 only)
by GHC 13 Sep '18

13 Sep '18

#15629: "No skolem info" panic (GHC 8.6 only) -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.6.1-beta1 (Type checker) | Keywords: TypeInType | Operating System: Unknown/Multiple Architecture: | Type of failure: Compile-time Unknown/Multiple | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following program: {{{#!hs {-# LANGUAGE RankNTypes #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeInType #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE UndecidableInstances #-} module Bug where import Data.Kind import Data.Proxy import GHC.Generics data TyFun :: Type -> Type -> Type type a ~> b = TyFun a b -> Type infixr 0 ~> type family Apply (f :: k1 ~> k2) (x :: k1) :: k2 data family Sing :: forall k. k -> Type newtype instance Sing (f :: k1 ~> k2) = SLambda { applySing :: forall t. Sing t -> Sing (Apply f t) } singFun1 :: forall f. (forall t. Sing t -> Sing (Apply f t)) -> Sing f singFun1 f = SLambda f data From1Sym0 :: forall k (f :: k -> Type) (a :: k). f a ~> Rep1 f a data To1Sym0 :: forall k (f :: k -> Type) (a :: k). Rep1 f a ~> f a type family ((f :: b ~> c) :. (g :: a ~> b)) (x :: a) :: c where (f :. g) x = Apply f (Apply g x) data (.@#@$$$) :: forall b c a. (b ~> c) -> (a ~> b) -> (a ~> c) type instance Apply (f .@#@$$$ g) x = (f :. g) x (%.) :: forall a b c (f :: b ~> c) (g :: a ~> b) (x :: a). Sing f -> Sing g -> Sing x -> Sing ((f :. g) x) (sf %. sg) sx = applySing sf (applySing sg sx) (%.$$$) :: forall a b c (f :: b ~> c) (g :: a ~> b) (x :: a). Sing f -> Sing g -> Sing (f .@#@$$$ g) sf %.$$$ sg = singFun1 (sf %. sg) f :: forall (m :: Type -> Type) x. Proxy (m x) -> () f _ = () where sFrom1Fun :: forall ab. Sing (From1Sym0 :: m ab ~> Rep1 m ab) sFrom1Fun = undefined sTo1Fun :: forall ab. Sing (To1Sym0 :: Rep1 m ab ~> m ab) sTo1Fun = undefined sFrom1To1Fun :: forall ab. Sing (((From1Sym0 :: m z ~> Rep1 m z) .@#@$$$ To1Sym0) :: Rep1 m ab ~> Rep1 m ab) sFrom1To1Fun = sFrom1Fun %.$$$ sTo1Fun }}} Panics on GHC 8.6: {{{ $ /opt/ghc/8.6.1/bin/ghc Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) Bug.hs:50:39: error: • Expected kind ‘m z ~> Rep1 m ab1’, but ‘(From1Sym0 :: m z ~> Rep1 m z)’ has kind ‘m z ~> Rep1 m z’ • In the first argument of ‘(.@#@$$$)’, namely ‘(From1Sym0 :: m z ~> Rep1 m z)’ In the first argument of ‘Sing’, namely ‘(((From1Sym0 :: m z ~> Rep1 m z) .@#@$$$ To1Sym0) :: Rep1 m ab ~> Rep1 m ab)’ In the type signature: sFrom1To1Fun :: forall ab. Sing (((From1Sym0 :: m z ~> Rep1 m z) .@#@$$$ To1Sym0) :: Rep1 m ab ~> Rep1 m ab) | 50 | sFrom1To1Fun :: forall ab. Sing (((From1Sym0 :: m z ~> Rep1 m z) .@#@$$$ To1Sym0) :: Rep1 m ab ~> Rep1 m ab) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Bug.hs:51:20: error:ghc: panic! (the 'impossible' happened) (GHC version 8.6.0.20180823 for x86_64-unknown-linux): No skolem info: [ab_a1UW[sk:1]] Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable pprPanic, called at compiler/typecheck/TcErrors.hs:2891:5 in ghc:TcErrors }}} But merely errors on GHC 8.4.3: {{{ $ /opt/ghc/8.4.3/bin/ghc Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) Bug.hs:50:39: error: • Expected kind ‘m z ~> Rep1 m ab2’, but ‘(From1Sym0 :: m z ~> Rep1 m z)’ has kind ‘m z ~> Rep1 m z’ • In the first argument of ‘(.@#@$$$)’, namely ‘(From1Sym0 :: m z ~> Rep1 m z)’ In the first argument of ‘Sing’, namely ‘(((From1Sym0 :: m z ~> Rep1 m z) .@#@$$$ To1Sym0) :: Rep1 m ab ~> Rep1 m ab)’ In the type signature: sFrom1To1Fun :: forall ab. Sing (((From1Sym0 :: m z ~> Rep1 m z) .@#@$$$ To1Sym0) :: Rep1 m ab ~> Rep1 m ab) | 50 | sFrom1To1Fun :: forall ab. Sing (((From1Sym0 :: m z ~> Rep1 m z) .@#@$$$ To1Sym0) :: Rep1 m ab ~> Rep1 m ab) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Bug.hs:51:20: error: • Couldn't match type ‘ab1’ with ‘z1’ because type variable ‘z1’ would escape its scope This (rigid, skolem) type variable is bound by the type signature for: sFrom1To1Fun :: forall z1 ab3. Sing (From1Sym0 .@#@$$$ To1Sym0) at Bug.hs:50:5-112 Expected type: Sing From1Sym0 Actual type: Sing From1Sym0 • In the first argument of ‘(%.$$$)’, namely ‘sFrom1Fun’ In the expression: sFrom1Fun %.$$$ sTo1Fun In an equation for ‘sFrom1To1Fun’: sFrom1To1Fun = sFrom1Fun %.$$$ sTo1Fun • Relevant bindings include sFrom1To1Fun :: Sing (From1Sym0 .@#@$$$ To1Sym0) (bound at Bug.hs:51:5) | 51 | sFrom1To1Fun = sFrom1Fun %.$$$ sTo1Fun | ^^^^^^^^^ Bug.hs:51:36: error: • Couldn't match type ‘ab’ with ‘z1’ because type variable ‘z1’ would escape its scope This (rigid, skolem) type variable is bound by the type signature for: sFrom1To1Fun :: forall z1 ab3. Sing (From1Sym0 .@#@$$$ To1Sym0) at Bug.hs:50:5-112 Expected type: Sing To1Sym0 Actual type: Sing To1Sym0 • In the second argument of ‘(%.$$$)’, namely ‘sTo1Fun’ In the expression: sFrom1Fun %.$$$ sTo1Fun In an equation for ‘sFrom1To1Fun’: sFrom1To1Fun = sFrom1Fun %.$$$ sTo1Fun • Relevant bindings include sFrom1To1Fun :: Sing (From1Sym0 .@#@$$$ To1Sym0) (bound at Bug.hs:51:5) | 51 | sFrom1To1Fun = sFrom1Fun %.$$$ sTo1Fun | ^^^^^^^ }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15629> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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