February 2018 - ghc-tickets

[GHC] #14684: combineIdenticalAlts is only partially implemented
by GHC 09 May '18

09 May '18

#14684: combineIdenticalAlts is only partially implemented -------------------------------------+------------------------------------- Reporter: mpickering | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 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: -------------------------------------+------------------------------------- combineIdenticalAlts commons up branches of case expressions which have the same RHS. However, it is not fully implemented so opportunities to common up branches are missed. For very big case expressions in might impact compilation time but it could be something which is enabled by `-O2`. For example, the `C` and `D` case for `foo` are not commened up but the `A` and `B` case in `foo2` are. {{{ module Foo where data T = A | B | C | D foo x = case x of A -> 0 B -> 1 C -> 2 D -> 2 foo2 x = case x of A -> 2 B -> 2 C -> 0 D -> 1 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14684> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14547: Wrong warning by -Wincomplete-patterns
by GHC 08 May '18

08 May '18

#14547: Wrong warning by -Wincomplete-patterns -------------------------------------+------------------------------------- Reporter: YoshikuniJujo | Owner: (none) Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 8.2.1 Keywords: incomplete- | Operating System: Linux patterns OverloadedLists | TypeFamilies | Type of failure: Incorrect Architecture: x86 | error/warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Wrong warning occur where I use -Wincomplete-patterns with OverloadedLists and TypeFamilies. {{{#!hs {-# LANGUAGE OverloadedLists #-} {-# LANGUAGE TypeFamilies #-} {-# OPTIONS_GHC -Wall -fno-warn-tabs #-} class Foo f where type It f foo :: [It f] -> f data List a = Empty | a :! List a deriving Show instance Foo (List a) where type It (List a) = a foo [] = Empty foo (x : xs) = x :! foo xs }}} Pattern match(es) are non-exhaustive In an equation for `foo': Patterns not matched: [] -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14547> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13958: Ghc linking failure for 8.2.1 rc3 on Alpine Linux
by GHC 08 May '18

08 May '18

#13958: Ghc linking failure for 8.2.1 rc3 on Alpine Linux -------------------------------------+------------------------------------- Reporter: mitchty | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc3 (Linking) | Keywords: | Operating System: Linux Architecture: x86_64 | Type of failure: Building GHC (amd64) | failed Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Bisected this down to commit 2785ef0e31a123400da950ffafebe6cb1ce3f4eb between rc2 and rc3. rc2 builds fine. This is on Alpine Linux which uses musl libc primarily and gcc/binutils patched for it as well (mostly PIE related), if wanted I can provide a Dockerfile to reproduce the issue more easily. Note that the switch provided by that commit, --disable-ld-override allows rc3 to link stage 2 properly again. But the error encountered with no override and when specifying the ld to use via configure, in this case ld.gold is this: {{{ "inplace/bin/ghc-stage2" -hisuf dyn_hi -osuf dyn_o -hcsuf dyn_hc -fPIC -dynamic -H32m -O -Wall -hide-all-packages -i -iutils/haddock/driver -iutils/haddock/haddock-api/src -iutils/haddock/haddock- library/vendor/attoparsec-0.13.1.0 -iutils/haddock/haddock-library/sr c -iutils/haddock/dist/build -Iutils/haddock/dist/build -iutils/haddock/dist/build/haddock/autogen -Iutils/haddock/dist/build/haddock/autogen -optP-DIN_GHC_TREE -optP- include -optPutils/haddock/dist/build/haddock/autogen/cabal_macros.h -package-id base-4.10.0.0 -packag e-id filepath-1.4.1.2 -package-id directory-1.3.0.2 -package-id containers-0.5.10.2 -package-id deepseq-1.4.3.0 -package-id array-0.5.2.0 -package-id xhtml-3000.2.2 -package-id Cabal-2.0.0.0 -package-id ghc- boot-8.2.0.20170704 -package-id ghc-8.2.0.20170704 -package-id bytestring-0.10.8.2 -package-id transformers-0.5.2.0 -funbox-strict-fields -Wall -fwarn-tabs -O2 -threaded -XHaskell2010 -no-user-package-db -rtsopts -Wno-unused-imports -Wno-deprecations -Wnoncanonical-monad- instances -odir utils/haddock/dist/build -hidir utils/haddo ck/dist/build -stubdir utils/haddock/dist/build -c utils/haddock/driver/ResponseFile.hs -o utils/haddock/dist/build/ResponseFile.dyn_o "inplace/bin/ghc-stage2" -hisuf dyn_hi -osuf dyn_o -hcsuf dyn_hc -fPIC -dynamic -H32m -O -Wall -hide-all-packages -i -iutils/haddock/driver -iutils/haddock/haddock-api/src -iutils/haddock/haddock- library/vendor/attoparsec-0.13.1.0 -iutils/haddock/haddock-library/sr c -iutils/haddock/dist/build -Iutils/haddock/dist/build -iutils/haddock/dist/build/haddock/autogen -Iutils/haddock/dist/build/haddock/autogen -optP-DIN_GHC_TREE -optP- include -optPutils/haddock/dist/build/haddock/autogen/cabal_macros.h -package-id base-4.10.0.0 -packag e-id filepath-1.4.1.2 -package-id directory-1.3.0.2 -package-id containers-0.5.10.2 -package-id deepseq-1.4.3.0 -package-id array-0.5.2.0 -package-id xhtml-3000.2.2 -package-id Cabal-2.0.0.0 -package-id ghc- boot-8.2.0.20170704 -package-id ghc-8.2.0.20170704 -package-id by testring-0.10.8.2 -package-id transformers-0.5.2.0 -funbox-strict-fields -Wall -fwarn-tabs -O2 -threaded -XHaskell2010 -no-user-package-db -rtsopts -Wno-unused-imports -Wno-deprecations -Wnoncanonical-monad- instances -odir utils/haddock/dist/build -hidir utils/haddock/dist/build -stubdir utils/haddock/dist/build -c utils/haddock/haddock- api/src/Haddock/GhcUtils.hs -o utils/haddock/dist/build/Haddock/GhcUtils.dyn_o "inplace/bin/ghc-stage2" -hisuf dyn_hi -osuf dyn_o -hcsuf dyn_hc -fPIC -dynamic -H32m -O -Wall -hide-all-packages -i -iutils/haddock/driver -iutils/haddock/haddock-api/src -iutils/haddock/haddock- library/vendor/attoparsec-0.13.1.0 -iutils/haddock/haddock-library/sr c -iutils/haddock/dist/build -Iutils/haddock/dist/build -iutils/haddock/dist/build/haddock/autogen -Iutils/haddock/dist/build/haddock/autogen -optP-DIN_GHC_TREE -optP- include -optPutils/haddock/dist/build/haddock/autogen/cabal_macros.h -package-id base-4.10.0.0 -package-id filepath-1.4.1.2 -package-id directory-1.3.0.2 -package-id containers-0.5.10.2 -package-id deepseq-1.4.3.0 -package-id array-0.5.2.0 -package-id xhtml-3000.2.2 -package-id Cabal-2.0.0.0 -package-id ghc-boot-8.2.0.20170704 -package-id ghc-8.2.0.20170704 -package-id by testring-0.10.8.2 -package-id transformers-0.5.2.0 -funbox-strict-fields -Wall -fwarn-tabs -O2 -threaded -XHaskell2010 -no-user-package-db -rtsopts -Wno-unused-imports -Wno-deprecations -Wnoncanonical-monad- instances -odir utils/haddock/dist/build -hidir utils/haddo ck/dist/build -stubdir utils/haddock/dist/build -c utils/haddock /haddock-api/src/Haddock/Backends/Hyperlinker/Types.hs -o utils/haddock/dist/build/Haddock/Backends/Hyperlinker/Types.dyn_o "inplace/bin/ghc-stage2" -hisuf dyn_hi -osuf dyn_o -hcsuf dyn_hc -fPIC -dynamic -H32m -O -Wall -hide-all-packages -i -iutils/haddock/driver -iutils/haddock/haddock-api/src -iutils/haddock/haddock- library/vendor/attoparsec-0.13.1.0 -iutils/haddock/haddock-library/sr c -iutils/haddock/dist/build -Iutils/haddock/dist/build -iutils/haddock/dist/build/haddock/autogen -Iutils/haddock/dist/build/haddock/autogen -optP-DIN_GHC_TREE -optP- include -optPutils/haddock/dist/build/haddock/autogen/cabal_macros.h -package-id base-4.10.0.0 -packag e-id filepath-1.4.1.2 -package-id directory-1.3.0.2 -package-id containers-0.5.10.2 -package-id deepseq-1.4.3.0 -package-id array-0.5.2.0 -package-id xhtml-3000.2.2 -package-id Cabal-2.0.0.0 -package-id ghc- boot-8.2.0.20170704 -package-id ghc-8.2.0.20170704 -package-id by testring-0.10.8.2 -package-id transformers-0.5.2.0 -funbox-strict-fields -Wall -fwarn-tabs -O2 -threaded -XHaskell2010 -no-user-package-db -rtsopts -Wno-unused-imports -Wno-deprecations -Wnoncanonical-monad- instances -odir utils/haddock/dist/build -hidir utils/haddo ck/dist/build -stubdir utils/haddock/dist/build -c utils/haddock /haddock-library/src/Documentation/Haddock/Types.hs -o utils/haddock/dist/build/Documentation/Haddock/Types.dyn_o "inplace/bin/ghc-stage2" -hisuf dyn_hi -osuf dyn_o -hcsuf dyn_hc -fPIC -dynamic -H32m -O -Wall -hide-all-packages -i -iutils/haddock/driver -iutils/haddock/haddock-api/src -iutils/haddock/haddock- library/vendor/attoparsec-0.13.1.0 -iutils/haddock/haddock-library/sr c -iutils/haddock/dist/build -Iutils/haddock/dist/build -iutils/haddock/dist/build/haddock/autogen -Iutils/haddock/dist/build/haddock/autogen -optP-DIN_GHC_TREE -optP- include -optPutils/haddock/dist/build/haddock/autogen/cabal_macros.h -package-id base-4.10.0.0 -packag e-id filepath-1.4.1.2 -package-id directory-1.3.0.2 -package-id containers-0.5.10.2 -package-id deepseq-1.4.3.0 -package-id array-0.5.2.0 -package-id xhtml-3000.2.2 -package-id Cabal-2.0.0.0 -package-id ghc- boot-8.2.0.20170704 -package-id ghc-8.2.0.20170704 -package-id by testring-0.10.8.2 -package-id transformers-0.5.2.0 -funbox-strict-fields -Wall -fwarn-tabs -O2 -threaded -XHaskell2010 -no-user-package-db -rtsopts -Wno-unused-imports -Wno-deprecations -Wnoncanonical-monad- instances -odir utils/haddock/dist/build -hidir utils/haddo ck/dist/build -stubdir utils/haddock/dist/build -c utils/haddock/dist/build/haddock/autogen/Paths_haddock.hs -o utils/haddock/dist/build/Paths_haddock.dyn_o "inplace/bin/ghc-stage2" -hisuf dyn_hi -osuf dyn_o -hcsuf dyn_hc -fPIC -dynamic -H32m -O -Wall -hide-all-packages -i -iutils/haddock/driver -iutils/haddock/haddock-api/src -iutils/haddock/haddock- library/vendor/attoparsec-0.13.1.0 -iutils/haddock/haddock-library/sr c -iutils/haddock/dist/build -Iutils/haddock/dist/build -iutils/haddock/dist/build/haddock/autogen -Iutils/haddock/dist/build/haddock/autogen -optP-DIN_GHC_TREE -optP- include -optPutils/haddock/dist/build/haddock/autogen/cabal_macros.h -package-id base-4.10.0.0 -packag e-id filepath-1.4.1.2 -package-id directory-1.3.0.2 -package-id containers-0.5.10.2 -package-id deepseq-1.4.3.0 -package-id array-0.5.2.0 -package-id xhtml-3000.2.2 -package-id Cabal-2.0.0.0 -package-id ghc- boot-8.2.0.20170704 -package-id ghc-8.2.0.20170704 -package-id by testring-0.10.8.2 -package-id transformers-0.5.2.0 -funbox-strict-fields -Wall -fwarn-tabs -O2 -threaded -XHaskell2010 -no-user-package-db -rtsopts -Wno-unused-imports -Wno-deprecations -Wnoncanonical-monad- instances -odir utils/haddock/dist/build -hidir utils/haddo ck/dist/build -stubdir utils/haddock/dist/build -c utils/haddock /haddock-library/src/Documentation/Haddock/Utf8.hs -o utils/haddock/dist/build/Documentation/Haddock/Utf8.dyn_o Error relocating /home/build/aports/testing/ghc/src/ghc-8.2.0.20170704/rts/dist/build /libHSrts_thr-ghc8.2.0.20170704.soError relocating /home/build/aports/testing/ghc/src/ghc-8.2.0.20170704/rts/dist/build /libHSrts_thr-ghc8.2.0.20170704.soError relocating /home/build/aport s/testing/ghc/src/ghc-8.2.0.20170704/rts/dist/build/libHSrts_thr- ghc8.2.0.20170704.soError relocating /home/build/aports/testing/ghc/src/ghc-8.2.0.20170704/rts/dist/build /libHSrts_thr-ghc8.2.0.20170704.so: my_task: symbol not found: my_task: symbol not found : my_task: symbol not found Error relocating /home/build/aports/testing/ghc/src/ghc-8.2.0.20170704/rts/dist/build /libHSrts_thr-ghc8.2.0.20170704.so: my_task: symbol not found Error relocating /home/build/aports/testing/ghc/src/ghc-8.2.0.20170704/rts/dist/build /libHSrts_thr-ghc8.2.0.20170704.so: my_task: symbol not found: my_task: symbol not found Error relocating /home/build/aports/testing/ghc/src/ghc-8.2.0.20170704/rts/dist/build /libHSrts_thr-ghc8.2.0.20170704.so: my_task: symbol not foundError relocating /home/build/aports/testing/ghc/src/ghc-8.2.0.20170704/rts/dist/build /libHSrts_thr-ghc8.2.0.20170704.so : my_task: symbol not found Error relocating /home/build/aports/testing/ghc/src/ghc-8.2.0.20170704/rts/dist/build /libHSrts_thr-ghc8.2.0.20170704.so: my_task: symbol not found Error relocating /home/build/aports/testing/ghc/src/ghc-8.2.0.20170704/rts/dist/build /libHSrts_thr-ghc8.2.0.20170704.so: my_task: symbol not found Error relocating /home/build/aports/testing/ghc/src/ghc-8.2.0.20170704/rts/dist/build /libHSrts_thr-ghc8.2.0.20170704.so: my_task: symbol not found Error relocating /home/build/aports/testing/ghc/src/ghc-8.2.0.20170704/rts/dist/build /libHSrts_thr-ghc8.2.0.20170704.so: my_task: symbol not found make[1]: *** [utils/haddock/ghc.mk:21: utils/haddock/dist/build/Haddock/GhcUtils.dyn_o] Error 127 make[1]: *** Waiting for unfinished jobs.... make[1]: *** [utils/haddock/ghc.mk:21: utils/haddock/dist/build/ResponseFile.dyn_o] Error 127 make[1]: *** [utils/haddock/ghc.mk:21: utils/haddock/dist/build/Haddock/Backends/Hyperlinker/Types.dyn_o] Error 127 make[1]: *** [utils/haddock/ghc.mk:21: utils/haddock/dist/build/Documentation/Haddock/Utf8.dyn_o] Error 127 make[1]: *** [utils/haddock/ghc.mk:21: utils/haddock/dist/build/Documentation/Haddock/Types.dyn_o] Error 127 make[1]: *** [utils/haddock/ghc.mk:21: utils/haddock/dist/build/Paths_haddock.dyn_o] Error 127 make: *** [Makefile:127: all] Error 2 >>> ERROR: ghc: all failed >>> ghc: Uninstalling dependencies... }}} I can attach the log file that comes from or just give a Dockerfile one can use to replicate things and debug directly. But there is a workaround so probably not a super critical bug from my perspective. I'm not sure how well this relates to the issues listed in that commit: #13541, #13810, #13883 so I'm just leaving those out of this issue for now. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13958> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10832: Generalize injective type families
by GHC 07 May '18

07 May '18

#10832: Generalize injective type families -------------------------------------+------------------------------------- Reporter: jstolarek | Owner: jstolarek Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 7.11 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- With injective type families we can now declare that type family result determines the arguments. But ITFs are not yet as expressive as functional dependencies. For example with FDs I can say: {{{#!hs data Nat = Zero | Succ a class Add a b r | a b -> r, r a -> b instance Add Zero b b instance (Add a b r) => Add (Succ a) b (Succ r) }}} Here we declare that the result and the first argument taken together uniquely determine the second argument. This is not currently possible with ITFs: {{{#!hs type family AddTF a b = r | r a -> b where AddTF Zero b = b AddTF (Succ a) b = Succ (AddTF a b) }}} But we want to be able to say that. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10832> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11080: Open data kinds
by GHC 07 May '18

07 May '18

#11080: Open data kinds -------------------------------------+------------------------------------- Reporter: dmcclean | Owner: Type: feature | Status: new request | Priority: low | Milestone: Component: Compiler | Version: 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: -------------------------------------+------------------------------------- When we need type level symbols, we used to only have one option, `EmptyDataDecls`. The symbolic type parameters would be given kind `*`, but the convention would be to use types that are (mostly) uninhabited and that were made for the purpose of serving as symbols. For an example of this see http://hackage.haskell.org/package/servant-0.4.4.5/docs/Servant- API-ContentTypes.html. Now we also have the `Symbol` kind which can serve a similar purpose. Neither of these options is fully satisfactory for examples like servant's type-level encoding of content types, or the `units` libraries type-level encoding of physical dimensions, for the following reasons: We would prefer to give these types a kind other than `*` both because they are uninhabited and because we wish to provide documentation of what types are acceptable in these parameter positions. But the only facility we have for creating new kinds is `DataKinds`, and the kinds it creates are all closed. This is problematic because we wish to allow client modules to declare new content types or new physical dimensions. We could provide "newkind" wrappers around `Symbol` using `DataKinds`, but that runs in to name collision issues. The `EmptyDataDecls` solution is superior here because types with the same name declared in different modules remain distinct. Instead I propose the addition of a new feature where data kinds can be open. In a discussion at https://github.com/bjornbm/dimensional-dk/issues/96 in which @goldfire participated, we arrived at the following proposal for concrete syntax, but certainly there are many possible variations one could imagine which might have equal or greater merit: {{{#!hs -- declares a new kind which initially has no inhabitants data open ContentType -- declares a new type of kind ContentType, which is distinct -- from any others that may have been declared elsewhere, -- even if they have the same non-module-qualified name data constructor JSON :: ContentType }}} (@goldfire raised the possibility of one day interpreting the same `data constructor ...` syntax at the type level to declare inhabited types for which some constructors are defined in one module but which are open to extension with new constructors by other modules, but that is a distinct issue in all other ways apart from possibly wishing to share syntax.) -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11080> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11953: Export Word32#, Word64# on all architectures
by GHC 06 May '18

06 May '18

#11953: Export Word32#, Word64# on all architectures -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.10.3 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: -------------------------------------+------------------------------------- Currently we have no always-available fixed-width primitive types. For instance, if you want a 64-bit unsigned integer you need to use `Word#` on 64-bit targets and `Word32#` otherwise. This adds a fair bit of complexity in places which really should be simple (e.g. `Data.Word`). -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11953> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8995: When generalising, use levels rather than global tyvars
by GHC 05 May '18

05 May '18

#8995: When generalising, use levels rather than global tyvars ------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- A long time ago Didier Remy described how to use 'ranks' or 'levels' to make the test "is this type variable free in the environment" when generalising a type. Oleg gives a [http://okmij.org/ftp/ML/generalization.html great explanation here]. GHC still uses the old "find the free type variables in the environment" method. But in fact GHC ''does'' maintain levels, just as Didier explains. The level is called `Untouchables` and is used to make sure we don't infer non-principal types for things involving GADTs. The same level stuff is used to prevent skolem-escape. Details with `Note [Untouchable type variables]` and `Note [Skolem escape prevention]` in `TcType`. So it ought to be straightforward to use the same levels for generalisation. I had a go, and came across the following tricky points: * We need to bump the level (`pushUntouchables`) when starting to typecheck a let RHS. Currently we don't. That will be a bit less efficient, because we'll end up deferring unification constraints that currently get solved immediately. * However the current set up is actually wrong. In particular, we generate a constraint where the implications do not have monotonically increasing level numbers (invariant `(ImplicInv)` in `TcType`). Try this program with `-ddump-tc-trace`. {{{ {-# LANGUAGE GADTs #-} module Bar where data T where MkT :: a -> (a->Int) -> T f x z = case x of MkT _ _ -> let g y = (y, [z,Nothing]) in (g True, g 'c') }}} I don't know how to make this bug cause an actual problem, but it's very unsettling. Bumping the level before doing the RHS would solve this. * Once we have bumped the level, we then have to do some more type- variable promotion, especially of type variables free in the finally- chosen type of the binder. This came out rather awkwardly in the code. * Because we now get more deferred equality constraints, there's a real risk that we'll end up quantifying over them. If we get, say `(gbl ~ Maybe lcl)`, where `gbl` is a type var free in the envt, we '''do not''' want to quantify over it! (It would be sound, but would lead to horrible types.) A long time ago Didier Remy described how to use 'ranks' or 'levels' to make the test "is this type variable free in the environment" when generalising a type. Oleg gives a [http://okmij.org/ftp/ML/generalization.html great explanation here]. GHC still uses the old "find the free type variables in the environment" method. But in fact GHC ''does'' maintain levels, just as Didier explains. The level is called `Untouchables` and is used to make sure we don't infer non-principal types for things involving GADTs. The same level stuff is used to prevent skolem-escape. Details with `Note [Untouchable type variables]` and `Note [Skolem escape prevention]` in `TcType`. So it ought to be straightforward to use the same levels for generalisation. I had a go, and came across the following tricky points: * We need to bump the level (`pushUntouchables`) when starting to typecheck a let RHS. Currently we don't. That will be a bit less efficient, because we'll end up deferring unification constraints that currently get solved immediately. * However the current set up is actually wrong. In particular, we generate a constraint where the implications do not have monotonically increasing level numbers (invariant `(ImplicInv)` in `TcType`). Try this program with `-ddump-tc-trace`. {{{ {-# LANGUAGE ExistentialQuantification #-} module Bar where data T = forall a. MkT a f x z = case x of MkT y -> let g y = [y,Nothing] in g True }}} Look for the `reportUnsolved` trace and you'll nested implications, both with level 1. I don't know how to make this bug cause an actual problem, but it's very unsettling. Bumping the level before doing the RHS would solve this. * Once we have bumped the level, we then have to do some more type- variable promotion, especially of type variables free in the finally- chosen type of the binder. This came out rather awkwardly in the code. * Because we now get more deferred equality constraints, there's a real risk that we'll end up quantifying over them. If we get, say `(gbl ~ Maybe lcl)`, where `gbl` is a type var free in the envt, we '''do not''' want to quantify over it! (It would be sound, but would lead to horrible types.) Rather we want to promote `lcl`, and not quantify. Currently we don't get many deferred equality constraints, becuase they all get done "on the fly". But if we do get one, we do quantify over it, which yields a rather stupid type. Try this: {{{ {-# LANGUAGE ExistentialQuantification #-} module Bar where data T = forall a. MkT a f x z = case x of MkT _ -> let g y = [z,Nothing] in (g True, g 'c') }}} We get this `g :: forall t a2. Maybe a ~ Maybe a2 => t -> [Maybe a2]`, where `x::a` is in the envt. Not very clever! This only shows up with existentials and In general I think we want to find all the type variables reachable by equality constraints from the environment, and not quantify over them. * We also use the type environment `getGlobalTyVars` in ''kind'' generalisation, and kind inference currently does not use the untoucahble story. Adapting it to use levels might be tricky because we currently assume we solve all kind equality constraints on-the-fly, and don't gather any deferreed constraints. Maybe it's easy, but needs thought. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8995> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10572: Template Haskell does not implicitly quantify types
by GHC 04 May '18

04 May '18

#10572: Template Haskell does not implicitly quantify types -------------------------------------+------------------------------------- Reporter: spinda | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Template | Version: 7.10.1 Haskell | Operating System: Unknown/Multiple Keywords: | Type of failure: None/Unknown Architecture: | Blocked By: Unknown/Multiple | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Consider the simple quasiquoter producing a type variable: {{{ tv :: QuasiQuoter tv = QuasiQuoter { quoteType = varT . mkName } }}} If a type variable is introduced elsewhere in a type signature, it is implicitly quantified as normal and the TH name resolves properly: {{{ id' :: a -> [tv|a|] id' x = x }}} But if the type variable is not mentioned elsewhere in the type, an error results: {{{ const' :: a -> [tv|b|] -> a const' x _ = x }}} {{{ Not in scope: type variable ‘b’ }}} One would expect the type variable introduced by the TH splice to be included in the implicit quantification of the type signature. In fact, there is no way to introduce a new type variable in this context, apart from using {{{Rank(2/N)Types}}} with an explicit {{{forall}}}. But this prevents the type variable from being referred to elsewhere in the type, eg: {{{ const'' :: [tv|a|] -> b -> [tv|a|] const'' x _ = x }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10572> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9113: Template Haskell should warn about non-exhaustive pattern matches
by GHC 03 May '18

03 May '18

#9113: Template Haskell should warn about non-exhaustive pattern matches -------------------------+------------------------------------------------- Reporter: tvh | Owner: Type: bug | Status: new Priority: | Milestone: normal | Version: 7.8.2 Component: | Operating System: Unknown/Multiple Template Haskell | Type of failure: Incorrect warning at Keywords: | compile-time Architecture: | Test Case: Unknown/Multiple | Blocking: Difficulty: | Unknown | Blocked By: | Related Tickets: | -------------------------+------------------------------------------------- When using Typed Template Haskell, There are no warings for non-exhaustive patterns inside the quote. {{{ {-# LANGUAGE TemplateHaskell #-} module Foo where import Language.Haskell.TH.Lib foo :: Maybe Int -> TExpQ Bool foo x = [||case x of Nothing -> False||] }}} Instead the warnings appear where the code is spliced. {{{ {-# LANGUAGE TemplateHaskell #-} module Bar where import Foo x = $$(foo (Just 1)) }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9113> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8970: Non-exhaustive pattern match warning with DataKinds and TypeFamilies
by GHC 03 May '18

03 May '18

#8970: Non-exhaustive pattern match warning with DataKinds and TypeFamilies ------------------------------------+------------------------------------- Reporter: tensor5 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- The code is: {{{ data K = Foo | Bar data D1 :: K -> * where F1 :: D1 Foo B1 :: D1 Bar class C (a :: K -> *) where data D2 a :: K -> * foo :: a k -> D2 a k -> Bool instance C D1 where data D2 D1 k where F2 :: D2 D1 Foo B2 :: D2 D1 Bar foo F1 F2 = True foo B1 B2 = True }}} With -Wall it gives the warning: {{{ Test.hs:23:5: Warning: Pattern match(es) are non-exhaustive In an equation for `foo': Patterns not matched: F1 B2 B1 F2 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8970> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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