July 2017 - ghc-tickets

[GHC] #12034: Template Haskell + hs-boot = Not in scope during type checking, but it passed the renamer
by GHC 26 Dec '18

26 Dec '18

#12034: Template Haskell + hs-boot = Not in scope during type checking, but it passed the renamer -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 8.0.1-rc2 (Type checker) | Keywords: backpack | 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 don't think this is a very harmful bug but it definitely is a bug. Consider: {{{ -- A.hs-boot module A where data T -- B.hs module B (module A) where import {-# SOURCE #-} A -- A.hs {-# LANGUAGE TemplateHaskell #-} module A where import qualified B import Language.Haskell.TH f :: B.T -> B.T f x = x $( return [] ) data T = T B.T }}} The point of the splice is to convince GHC to typecheck `f :: B.T -> B.T` before it typechecks the type declaration. But this is not going to work, because `tcLookupGlobal` is going to bail if (1) T is not in the `tcg_type_env` and (2) T comes from this module. It would be a simple matter to improve the error message but from a user's perspective, there is no good reason for this to not typecheck. On the implementation side, I am sympathetic to not letting this typecheck: if it does typecheck, then some `TyCon`s will incorrectly refer to the definition from the hs-boot file, rather than our local definition. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12034> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11627: Segmentation fault for space_leak_001 with profiling (-hc)
by GHC 25 Dec '18

25 Dec '18

#11627: Segmentation fault for space_leak_001 with profiling (-hc) -------------------------------------+------------------------------------- Reporter: thomie | Owner: Type: bug | Status: new Priority: high | Milestone: Component: Profiling | Version: 7.10.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: perf/space_leaks/space_leak_001 | Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- `WAY=profasm` is omitted by default for this test, but the code looks like this: Test.hs: {{{ import Data.List main = print $ length $ show (foldl' (*) 1 [1..100000] :: Integer) }}} {{{ $ ghc Test.hs -prof -O $ ./Test +RTS -hc Segmentation fault (core dumped) }}} Reproducible with at least 7.10.3 and HEAD, also without `-O`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11627> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9627: Type error with functional dependencies
by GHC 24 Dec '18

24 Dec '18

#9627: Type error with functional dependencies -------------------------------------+------------------------------------- Reporter: augustss | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- The attached file should compile. The types T1, T2, and T3 are all equivalent types (T1 and T2 normalize to T3), but with foo::T1 and bar::T2 there is a type error. Rewriting the example using type families instead of FD makes it work. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9627> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13322: Pattern synonyms in hs-boot files
by GHC 24 Dec '18

24 Dec '18

#13322: Pattern synonyms in hs-boot files -------------------------------------+------------------------------------- Reporter: ezyang | Owner: (none) Type: feature | Status: new request | Priority: low | Milestone: Component: Compiler | Version: 8.1 (Type checker) | Keywords: hs-boot | Operating System: Unknown/Multiple backpack | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Pattern synonyms are not permitted in hs-boot files: {{{ ezyang@sabre:~/bun$ ghc-head -c A.hs-boot A.hs-boot:3:5: error: Misplaced pattern synonym signature: pattern X :: Bool | 3 | pattern X :: Bool | ^^^^^^^^^^^^^^^^^ A.hs-boot:3:13: error: The pattern synonym signature for ‘X’ lacks an accompanying binding | 3 | pattern X :: Bool | }}} There isn't really any reason why they shouldn't be supported. One thing to check closely: if we bundle a pattern synonym declared in an hs-boot file with another type, in an hs-boot file, what happens? In particular, does an hs file which doesn't bundle the pattern synonym and the type valid? If we add support for them, make sure that Backpack signatures handle it properly. In particular, suppose we have: {{{ signature A(T(X)) where data T pattern X :: T }}} Then, when we instantiate A, we must be careful to NOT blindly reuse the implementing module of T for X (as we do today in `uAvailInfo` in `NameShape`), since T and X may be defined in different modules. I'm not likely to implement this unless someone asks. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13322> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11534: Allow class associated types to reference functional dependencies
by GHC 22 Dec '18

22 Dec '18

#11534: Allow class associated types to reference functional dependencies -------------------------------------+------------------------------------- Reporter: ekmett | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 (Type checker) | Keywords: TypeFamilies, | Operating System: Unknown/Multiple FunctionalDependencies | Architecture: | Type of failure: Other Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider: {{{#!hs class Foo i j | i -> j where type Bar i :: * type Bar i = j }}} This is rejected as `j` is not an argument to `Bar`, but when we realize any instance `j` will be a known type, determined by `i`, as told to us by the | i -> j functional dependency on the class. This would likely require some sort of extra pass over the instances with class associated types of this form to elaborate them, or a slight modification of the default signature implementation for class associated types. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11534> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8949: switch -msse2 to be on by default
by GHC 21 Dec '18

21 Dec '18

#8949: switch -msse2 to be on by default ----------------------------------------------+---------------------------- Reporter: errge | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (CodeGen) | Version: 7.9 Keywords: | Operating System: Architecture: x86 | Unknown/Multiple Difficulty: Moderate (less than a day) | Type of failure: Other Blocked By: | Test Case: Related Tickets: | Blocking: ----------------------------------------------+---------------------------- I propose msse2 to be on by default. I guess the default was chosen way back, when Pentium III support was still relevant. Nowadays we don't really win on the CPU support, because e.g. https://github.com/tibbe/hashable/blob/master/hashable.cabal is built by default with sse2 on the injected C code level. And hashable has a lot of reverse depends, therefore on an end user system (RedHat or Debian) the user is most probably unlucky with a Pentium III CPU anyways. Flipping this default would also fix the excess precision problem for most users of GHC on the i686 platform. GHC should provide a -mno-sse2 flag for the cases when this needs to be disabled. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8949> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8655: Evaluate know-to-terminate-soon thunks
by GHC 19 Dec '18

19 Dec '18

#8655: Evaluate know-to-terminate-soon thunks ------------------------------------+------------------------------------- Reporter: nomeata | Owner: nomeata Type: task | 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: | ------------------------------------+------------------------------------- I guess I’ll better put my interior monologue in a ticket than on ghc- dev... In order to implement nested CPR (#1600), I had to enrich the CPR type to keep track of whether something, when entered, will converge for sure. My code does not solve the halting problem, but does only simple stuff, so if something is known to converge, is is very likely to be cheap. Simon suggested to measure the effect of evaluating a let-bound thunk with the known-to-terminate property, as if the body was using it strictly. This tickets tracks this suggestion, and reports progress. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8655> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8598: IO hack in demand analyzer gets in the way of CPR
by GHC 19 Dec '18

19 Dec '18

#8598: IO hack in demand analyzer gets in the way of CPR ------------------------------------+------------------------------------- Reporter: nomeata | Owner: Type: task | 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: | ------------------------------------+------------------------------------- After a lot of staring at code and comparing unexpected nofib results I found the following: The IO hack in the demand analyzer (see `dmdAnalAlt` in `StrAnal.lhs` and #1592 for a good discussion) prevents CPR in any function that uses a C call. This is a small example, reduced from the `scaleFloat` method for doubles: {{{ module Float(fun) where import GHC.Float (Double(..)) import GHC.Integer (decodeDoubleInteger, encodeDoubleInteger) fun :: Double -> Double fun x | isFix = x | otherwise = case x of (D# x#) -> case decodeDoubleInteger x# of (# i, j #) -> D# (encodeDoubleInteger i j) where isFix = isDoubleFinite x == 0 foreign import ccall unsafe "isDoubleFinite" isDoubleFinite :: Double -> Int }}} Here, `fun` does current not get the CPR property, and the work gets type `GHC.Prim.Double# -> GHC.Types.Double`. Why? Because in core, there will be a {{{ case {__pkg_ccall main isDoubleFinite GHC.Prim.Double# -> GHC.Prim.State# GHC.Prim.RealWorld -> (# GHC.Prim.State# GHC.Prim.RealWorld, GHC.Prim.Int# #)}_dQD ds_dQA GHC.Prim.realWorld# of _ [Occ=Dead, Dmd=<L,A>] { (# ds_dQC [Dmd=<L,A>, OS=OneShot], ds_dQB [Dmd=<S,1*U>] #) -> ... }}} where the case body has `DmdType m {dQz-><L,1*U(U)> dQA-><L,U>}`, but `dmdAnalAlt` detects that this is evaluating a possibly exiting function and throws aways the information using `alt_ty `lubDmdType` topDmdType`. Would it be ok to `lub` only the demand on the free variables, but keep the CPR information? In nofib (if I managed to compare the right results) this does nothing for almost all benchmarks, `-9.2%` of allocations for `mandel` and `+4.9%` for reverse-complement (but these numbers are not obtained very cleanly, and all is happening on top of the better-ho-cardinality branch. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8598> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10905: Incorrect number of parameters in "role" errors
by GHC 19 Dec '18

19 Dec '18

#10905: Incorrect number of parameters in "role" errors -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: roles | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect Unknown/Multiple | warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- The following code produces the error: {{{ import Data.Coerce import Data.Functor.Trans.Tagged toTT :: Tagged t [a] -> TaggedT t [] a toTT = coerce }}} {{{ Couldn't match representation of type ‘[a]’ with that of ‘Data.Functor.Identity.Identity [a]’ arising from trying to show that the representations of ‘Tagged t [a]’ and ‘TaggedT t [] a’ are the same Relevant role signatures: type role [] representational type role Data.Functor.Identity.Identity representational type role TaggedT nominal nominal phantom representational nominal The data constructor ‘Data.Functor.Identity.Identity’ of newtype ‘Data.Functor.Identity.Identity’ is not in scope Relevant bindings include toTT :: Tagged t [a] -> TaggedT t [] a (bound at Main.hs:9:1) In the expression: coerce In an equation for ‘toTT’: toTT = coerce }}} First, kudos for correctly identifying the problem: I failed to import the `Identity` constructor. However, it seems that the report for the roles of the `TaggedT` type has too many parameters. It is defined in tagged-transformer [https://hackage.haskell.org/package/tagged-transformer-0.8/docs/Data- Functor-Trans-Tagged.html] as `newtype TaggedT s m b = TagT { untagT :: m b }`, but is listed with 5 roles. I did try to move the definition of `TaggedT/Tagged` into my module so I didn't have to import the library, but when I did the error didn't show up: TaggedT was listed with the expected roles `phantom representational nominal`. As a user, it wasn't clear to me why there are 5 roles listed for a type with 3 parameters, and I didn't know which roles I should be looking at to help me debug the `coerce` error. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10905> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8959: GHCi should honour UnicodeSyntax
by GHC 19 Dec '18

19 Dec '18

#8959: GHCi should honour UnicodeSyntax -------------------------------------------+------------------------------- Reporter: Fuuzetsu | Owner: Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 7.6.3 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Easy (less than 1 hour) | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: -------------------------------------------+------------------------------- When we load a module into GHCi with -XUnicodeSyntax, GHCi should default to printing information with the relevant unicode symbols. {{{ *Communication> let a ∷ Int; a = 7 *Communication> :t a a :: Int }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8959> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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