November 2015 - ghc-tickets

[GHC] #9505: Bounded instance for Word (and possibly others) uses unboxed literals
by GHC 03 Dec '18

03 Dec '18

#9505: Bounded instance for Word (and possibly others) uses unboxed literals -------------------------------------+------------------------------------- Reporter: schyler | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- There's a comment above saying GHC won't optimise. I think since 7.8 they are unboxed by default, so that comment and the explicit constructors can be removed. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9505> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8043: Feature Request : Qualified module exports
by GHC 01 Dec '18

01 Dec '18

#8043: Feature Request : Qualified module exports ------------------------------------+------------------------------------- Reporter: erikd | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: Other Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- I quite like the practice of using `-XNoImplicitPrelude` and then defining my own (often per-project) Prelude doing things like: {{{ module MyProject.Prelude ( module P ) import Data.Text as P (Text) import Prelude as P hiding (head, init, tail, last) }}} As an extension to the above idea, I would like to add qualified module exports defined so that I can do this: {{{ module MyProject.Prelude ( module P , qualified module T ) import Data.Text as P (Text) import Prelude as P hiding (head, init, tail, last) import qualified Data.Text as T }}} so that any file which imports `MyProject.Prelude` will then have all the identifiers that they would normally get if they did: {{{ import qualified Data.Text as T }}} Is this something that would have to go through the Haskell Next procedure? If so, how does one get that kicked off? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8043> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8177: Roles for type families
by GHC 29 Nov '18

29 Nov '18

#8177: Roles for type families ------------------------------------+------------------------------------- Reporter: simonpj | 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: | ------------------------------------+------------------------------------- Now that we have [wiki:Roles roles], it might be helpful to be able to give a role signature for a data/type family. At the moment, data/type family constructors have all parameters conservatively assigned to be role `N`. Thus {{{ data family D a -- Parameter conservatively assumed to be N class C a where op :: D a -> a instance C Int where .... newtype N a = MkN a deriving( C ) -- Rejected }}} The generalised-newtype-deriving clause `deriving( C )` is rejected because `D` might use its parameter at role `N` thus: {{{ data instance D [b] = MkD (F b) -- F is a type function }}} It would be strictly more expressive if we could * '''Declare''' the roles of D's arguments (as we can declare their kinds). E.g. {{{ data family D a@R }}} * '''Check''' that each family instance obeys that role signature. E.g. given the preceding role signature, reject this instance: {{{ data instance D [b] = MkD (F b) -- F is a type function }}} I think there is no technical difficulty here. Just a question of doing it. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8177> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10844: CallStack should not be inlined
by GHC 26 Nov '18

26 Nov '18

#10844: CallStack should not be inlined -------------------------------------+------------------------------------- Reporter: nomeata | Owner: Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- The use of CallStack based error messages (since changeset:6740d70d95cb) has led to some code size increase, and I believe this needs to be improved. While investigating the #10788, I was looking at the core produced by that code, and I found this in the code: {{{ -- RHS size: {terms: 2, types: 0, coercions: 0} Main.main17 :: [Char] [GblId, Str=DmdType, Unf=Unf{Src=<vanilla>, TopLvl=True, Value=False, ConLike=False, WorkFree=False, Expandable=False, Guidance=IF_ARGS [] 100 0}] Main.main17 = unpackCString# "vecto_20t0fI93Jj9LUbaZg6e04I"# -- RHS size: {terms: 2, types: 0, coercions: 0} Main.main16 :: [Char] [GblId, Str=DmdType, Unf=Unf{Src=<vanilla>, TopLvl=True, Value=False, ConLike=False, WorkFree=False, Expandable=False, Guidance=IF_ARGS [] 110 0}] Main.main16 = unpackCString# "Data.Vector.Primitive.Mutable"# -- RHS size: {terms: 2, types: 0, coercions: 0} Main.main15 :: [Char] [GblId, Str=DmdType, Unf=Unf{Src=<vanilla>, TopLvl=True, Value=False, ConLike=False, WorkFree=False, Expandable=False, Guidance=IF_ARGS [] 120 0}] Main.main15 = unpackCString# "./Data/Vector/Primitive/Mutable.hs"# -- RHS size: {terms: 2, types: 0, coercions: 0} Main.main14 :: Int [GblId, Caf=NoCafRefs, Str=DmdType, Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True, WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 10 20}] Main.main14 = I# 97# -- RHS size: {terms: 2, types: 0, coercions: 0} Main.main13 :: Int [GblId, Caf=NoCafRefs, Str=DmdType, Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True, WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 10 20}] Main.main13 = I# 16# -- RHS size: {terms: 2, types: 0, coercions: 0} Main.main12 :: Int [GblId, Caf=NoCafRefs, Str=DmdType, Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True, WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 10 20}] Main.main12 = I# 21# -- RHS size: {terms: 8, types: 0, coercions: 0} Main.main11 :: SrcLoc [GblId, Str=DmdType, Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True, WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 10 80}] Main.main11 = SrcLoc Main.main17 Main.main16 Main.main15 Main.main14 Main.main13 Main.main14 Main.main12 }}} This is clearly a CallStack from a library, and there is no point in copying that into the (every?) use of wherever that came from, as it will only increase code size and slow down compilation. I don’t know how to fix it, though. Float out CallStacks more aggressively? Or is the problem that unfoldings are recorded before that can happen? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10844> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10803: New TypeSignatureSections extension
by GHC 26 Nov '18

26 Nov '18

#10803: New TypeSignatureSections extension -------------------------------------+------------------------------------- Reporter: hvr | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.12.1 Component: Compiler | Version: 7.10.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- = Preliminary specification Have - `(::` ''type'' `)` be syntax sugar for - `(\x -> x ::` ''type'' `)` This new syntax extension is enabled via a new `TypeSignatureSections` language pragma, i.e. {{{#!hs {#- LANGUAGE TypeSignatureSections #-} }}} TODO: formalise this a bit more -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10803> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8634: Code valid in GHC 7.6 is impossible to move over GHC 7.7 (because of liberal coverage condition)
by GHC 22 Nov '18

22 Nov '18

#8634: Code valid in GHC 7.6 is impossible to move over GHC 7.7 (because of liberal coverage condition) ------------------------------------+------------------------------------- Reporter: danilo2 | Owner: Type: bug | Status: new Priority: high | Milestone: Component: Compiler | Version: 7.7 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- == Abstract == Hi! I'm writing a compiler, which produces Haskell code. I've discovered it is impossible to keep currently used features / logic using GHC 7.7 instead of 7.6 Below is more detailed description of the problem: == The idea == I'm writing a [DSL][1], which compiles to Haskell. Users of this language can define own immutable data structures and associated functions. By associated function I mean a function, which belongs to a data structure. For example, user can write (in "pythonic" pseudocode): {{{#!python data Vector a: x,y,z :: a def method1(self, x): return x }}} (which is equivalent to the following code, but shows also, that associated functions beheva like type classes with open world assumption): {{{#!python data Vector a: x,y,z :: a def Vector.method1(self, x): return x }}} In this example, `method1` is a function associated with `Vector` data type, and can be used like `v.testid(5)` (where `v` is instance of `Vector` data type). I'm translating such code to Haskell code, but I'm facing a problem, which I'm trying to solve for a long time. == The problem == I'm trying to move the code from GHC 7.6 over GHC 7.7. The code works perfectly under GHC 7.6, but does not under GHC 7.7. I want to ask you how can I fix it to make it working in the new version of the compiler? == Example code == Lets see a simplified version of generated (by my compiler) Haskell code: {{{#!haskell {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE FunctionalDependencies #-} import Data.Tuple.OneTuple ------------------------------ -- data types ------------------------------ data Vector a = Vector {x :: a, y :: a, z :: a} deriving (Show) -- the Vector_testid is used as wrapper over a function "testid". newtype Vector_testid a = Vector_testid a ------------------------------ -- sample function, which is associated to data type Vector ------------------------------ testid (v :: Vector a) x = x ------------------------------ -- problematic function (described later) ------------------------------ testx x = call (method1 x) $ OneTuple "test" ------------------------------ -- type classes ------------------------------ -- type class used to access "method1" associated function class Method1 cls m func | cls -> m, cls -> func where method1 :: cls -> m func -- simplified version of type class used to "evaluate" functions based on -- their input. For example: passing empty tuple as first argument of `call` -- indicates evaluating function with default arguments (in this example -- the mechanism of getting default arguments is not available) class Call a b where call :: a -> b ------------------------------ -- type classes instances ------------------------------ instance (out ~ (t1->t1)) => Method1 (Vector a) Vector_testid out where method1 = (Vector_testid . testid) instance (base ~ (OneTuple t1 -> t2)) => Call (Vector_testid base) (OneTuple t1 -> t2) where call (Vector_testid val) = val ------------------------------ -- example usage ------------------------------ main = do let v = Vector (1::Int) (2::Int) (3::Int) -- following lines equals to a pseudocode of ` v.method1 "test" ` -- OneTuple is used to indicate, that we are passing single element. -- In case of more or less elements, ordinary tuples would be used. print $ call (method1 v) $ OneTuple "test" print $ testx v }}} The code compiles and works fine with GHC 7.6. When I'm trying to compile it with GHC 7.7, I'm getting following error: {{{ debug.hs:61:10: Illegal instance declaration for ‛Method1 (Vector a) Vector_testid out’ The liberal coverage condition fails in class ‛Method1’ for functional dependency: ‛cls -> func’ Reason: lhs type ‛Vector a’ does not determine rhs type ‛out’ In the instance declaration for ‛Method1 (Vector a) Vector_testid out’ }}} The error is caused by new rules of checking what functional dependencies can do, namely `liberal coverage condition` (as far as I know, this is `coverage condition` relaxed by using `-XUndecidableInstances`) == Some attemps to fix the problem == I was trying to overcome this problem by changing the definition of `Method1` to: {{{#!haskell class Method1 cls m func | cls -> m where method1 :: cls -> m func }}} Which resolves the problem with functional dependencies, but then the line: {{{#!haskell testx x = call (method1 x) $ OneTuple "test" }}} is not allowed anymore, causing a compile error (in both 7.6 and 7.7 versions): {{{ Could not deduce (Method1 cls m func0) arising from the ambiguity check for ‛testx’ from the context (Method1 cls m func, Call (m func) (OneTuple [Char] -> s)) bound by the inferred type for ‛testx’: (Method1 cls m func, Call (m func) (OneTuple [Char] -> s)) => cls -> s at debug.hs:50:1-44 The type variable ‛func0’ is ambiguous When checking that ‛testx’ has the inferred type ‛forall cls (m :: * -> *) func s. (Method1 cls m func, Call (m func) (OneTuple [Char] -> s)) => cls -> s’ Probable cause: the inferred type is ambiguous }}} It is also impossible to solve this issue using type families (as far as I know). If we replace `Method1` type class and instances with following code (or simmilar): {{{#!haskell class Method1 cls m | cls -> m where type Func cls method1 :: cls -> m (Func cls) instance Method1 (Vector a) Vector_testid where type Func (Vector a) = (t1->t1) method1 = (Vector_testid . testid) }}} We would get obvious error `Not in scope: type variable ‛t1’`, because type families does not allow to use types, which does not appear on LHS of type expression. == The final question == How can I make this idea work under GHC 7.7? I know the new `liberal coverage condition` allows GHC devs make some progress with type checking, but it should somehow be doable to port idea working in GHC 7.6 over never compiler version. (without forcing user of my DSL to introduce any further types - everything so far, like type class instances, I'm genarating using Template Haskell) Maybe is there a way to indroduce an extension, which will disable liberal coverage condition in such situations? There is also a StackOverflow discussion available, here: http://stackoverflow.com/questions/20778588/liberal-coverage-condition- introduced-in-ghc-7-7-breaks-code-valid-in-ghc-7-6 [1]: http://en.wikipedia.org/wiki/Domain-specific_language -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8634> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10445: Wrong stack space size when using -Ksize
by GHC 19 Nov '18

19 Nov '18

#10445: Wrong stack space size when using -Ksize -------------------------------------+------------------------------------- Reporter: asr | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.10.2 Component: Compiler | Version: 7.10.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- After compiling the first example in https://wiki.haskell.org/Performance/Accumulating_parameter: {{{ $ cat Test.hs len :: [a] -> Int len [] = 0 len (x:xs) = len xs + 1 main :: IO () main = print $ len [1..1000000] }}} {{{ $ ghc Test.hs -rtsopts }}} and running {{{ ./Test +RTS -K10K }}} GHC 7.6.3 reports {{{ Stack space overflow: current size 10240 bytes. }}} GHC 7.8.4 reports {{{ Stack space overflow: current size 33632 bytes. }}} and GHC 7.10.1 reports {{{ Stack space overflow: current size 99136 bytes. }}} GHC 7.8.3 and 7.10.1 report an incorrect stack space size. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10445> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10347: Spurious "unused constructor" warning with Coercible
by GHC 19 Nov '18

19 Nov '18

#10347: Spurious "unused constructor" warning with Coercible -------------------------------------+------------------------------------- Reporter: goldfire | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.11 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect Unknown/Multiple | warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- When I say {{{ {-# LANGUAGE RoleAnnotations #-} module Bug (N, twiddle) where import Data.Coerce newtype N a = MkN Int type role N representational twiddle :: N a -> N b twiddle = coerce }}} I get {{{ Bug.hs:7:15: warning: Defined but not used: data constructor ‘MkN’ }}} But the constructor ''is'' used, in order to do the coercion. The role annotation is to force GHC to use the constructor to do the coercion -- otherwise, it might not have. This bug exists in 7.10 and in HEAD. I will fix. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10347> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9207: Detect obvious cases of infinite recursion.
by GHC 18 Nov '18

18 Nov '18

#9207: Detect obvious cases of infinite recursion. --------------------------------------+------------------------------------ Reporter: mrugiero | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: infinite recursion | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: None/Unknown Blocked By: | Test Case: Related Tickets: | Blocking: --------------------------------------+------------------------------------ Pure functions are guaranteed to return the same value if the same arguments are used. Because of that, a compiler could be able to detect infinite recursion in the form of a function calling itself with the same arguments (case, some distracted programmer forgot to apply the function tail to a list). The way I see it, one of Haskell's main selling points is that it helps the programmer to avoid sources of bugs as much as it can, and infinite recursion is one of the few that stand unchecked. I specifically point to this example because both, I'm a newbie and made that newbie mistake, and think it shouldn't be hard to implement to the parsing stage of the compiler/interpreter. It probably should throw an error when this happens in pure functions, and maybe an opt-in warning if it happens in monads. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9207> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10675: GHC does not check the functional dependency consistency condition correctly
by GHC 16 Nov '18

16 Nov '18

#10675: GHC does not check the functional dependency consistency condition correctly -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- Consider {{{ {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances, ScopedTypeVariables #-} class C a b | a -> b where op :: a -> b instance C [x] [x] instance {-# OVERLAPS #-} C x y => C [x] [Maybe y] f x = op [x] }}} Should these two instance declarations be accepted? They are simply ''inconsistent'' in the sense of Definition 6 of the [http://research.microsoft.com/en-us/um/people/simonpj/papers/fd- chr/jfp06.pdf FDs through CHRs paper]. Sadly GHC does not currently reject these as inconsistent. As a result it'll use ''both'' instance for improvement. In the definition of `f` for example we get {{{ C [alpha] beta }}} where `x:alpha` and the result type of `f` is `beta`. By using both instances for improvement we get {{{ C [Maybe gamma] [Maybe gamma] }}} Is that what we want? The two instances don't ''contradict'' each other, but neither do they agree as all published work on FDs says they should! Examples in the testsuite that exploit this loophole are {{{ ghci/scripts ghci047 polykinds T9106 typecheck/should_compile FD4 typecheck/should_compile T7875 }}} I'm not sure what the right thing here is. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10675> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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