August 2016 - ghc-tickets

[GHC] #12463: SPECIALIZABLE pragma?
by GHC 14 May '18

14 May '18

#12463: SPECIALIZABLE pragma? -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: feature | Status: new request | Priority: low | Milestone: Component: Compiler | Version: 8.0.1 Keywords: Inlining | 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 it is common practice for library authors to use the `INLINEABLE` pragma to make it more likely that a polymorphic function should get an unfolding in the module's interface file to ensure that GHC is able to specialize. While in practice this works reasonably well, it's not really saying what we often mean: we don't want to inline, we really just want GHC to behave like each use-site's module has a `SPECIALISE` pragma for each concrete type that the function is used at. For instance, consider, {{{#!hs module ALibrary where aLibraryFunction :: AClass a => a -> a aLibraryFunction = {- some large expression involving methods of AClass -} module SomeUser where import ALibrary aUser :: Int -> Int aUser = {- some large expression involving aLibraryFunction -} }}} Ideally, we would want GHC to take and produce one specialized version of `aLibraryFunction` for every concrete type which it is used at. However, without an `INLINEABLE` function, GHC won't even consider producing an unfolding for `aLibraryFunction` due to its size. If we include an `INLINEABLE` pragma (as most performance-aware authors would do) then we can convince GHC to produce an unfolding, but only at the expense of lowering its inlining cost as well. This is unfortunate since we never wanted GHC to inline; merely to specialize. This is issue especially prevalent in code using MTL-style effects, where we have ubiquitous overloading of very frequently-used functions (e.g. bind). Really what we want in this case is a way of indicating to GHC that a function shouldn't be inlined (use-sites replaced with the body of the function), but rather that GHC should try hard to specialize away particular type variables. This might look like, {{{#!hs aLibraryFunction :: AClass a => a -> a aLibraryFunction = {- some large expression involving methods of AClass -} {-# SPECIALIZE forall a. LibraryFunction a #-} }}} This would request that GHC would keep an inlining around and produce a specialized version of `aLibraryFunction` every time it saw a concrete instantiation of `a`. Moreover, it would try to minim -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12463> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10762: On Windows, out-of-codepage characters can cause GHC build to fail
by GHC 13 May '18

13 May '18

#10762: On Windows, out-of-codepage characters can cause GHC build to fail -----------------------------------------+--------------------------------- Reporter: snoyberg | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: | Operating System: Windows Architecture: x86_64 (amd64) | Type of failure: None/Unknown Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -----------------------------------------+--------------------------------- You can see where this hit us recently on stack with issues [https://github.com/commercialhaskell/stack/issues/738 738] and [https://github.com/commercialhaskell/stack/issues/734 734]. To demonstrate, I'm attaching a UTF-8 encoded Haskell program with some Hebrew characters, and some warnings. The contents of that file are: {{{#!hs module Main ( main , שלום ) where main :: IO () main = putStrLn שלום שלום = "shalom" }}} If I first set my codepage to 65001 (UTF-8), everything works as expected: {{{ C:\Users\Michael\Desktop>chcp 65001 Active code page: 65001 C:\Users\Michael\Desktop>ghc -fforce-recomp -Wall -ddump-hi -ddump-to-file shalom.hs [1 of 1] Compiling Main ( shalom.hs, shalom.o ) shalom.hs:9:1: Warning: Top-level binding with no type signature: שלום :: [Char] Linking shalom.exe ... }}} However, if I set my codepage to 437 (US), both the warnings sent to the console, and the .hi dump file, cause GHC to exit prematurely: {{{ C:\Users\Michael\Desktop>chcp 437 Active code page: 437 C:\Users\Michael\Desktop>ghc -fforce-recomp -Wall shalom.hs [1 of 1] Compiling Main ( shalom.hs, shalom.o ) shalom.hs:9:1: Warning: Top-level binding with no type signature: <stderr>: commitBuffer: invalid argument (invalid character) }}} {{{ C:\Users\Michael\Desktop>chcp 437 Active code page: 437 C:\Users\Michael\Desktop>ghc -fforce-recomp -ddump-hi -ddump-to-file shalom.hs [1 of 1] Compiling Main ( shalom.hs, shalom.o ) shalom.dump-hi: commitBuffer: invalid argument (invalid character) }}} At the very least, I would argue that -ddump-to-file should always dump to the output files as UTF-8, as this is the most useful for tooling. Beyond that, there are a few options here: * Have all output- including to the console- go out as UTF-8. This may not play terribly nicely with consoles without setting the output codepage. * Provide a command line option or environment variable to specify "output as UTF-8." * More radical: change the default way that all Handles work so that UTF-8 is the default, instead of paying attention to code pages and environment variables. Honestly, this is my preference, but it's a bigger discussion than this one bug. The workaround we've implemented in stack for now is setting the codepage to 65001 for the console while running stack. This is not ideal, since this is essentially a global setting for the entire console. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10762> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11389: Print a message when loading a .ghci file
by GHC 13 May '18

13 May '18

#11389: Print a message when loading a .ghci file -------------------------------------+------------------------------------- Reporter: rwbarton | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: GHCi | Version: 7.8.4 Keywords: newcomer | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- It'd be nice to have a notice displayed when ghci loads a .ghci file, something like {{{ rwbarton@morphism:~$ ghci-7.10.1 GHCi, version 7.10.1: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rwbarton/.ghci Prelude> }}} I want this mostly so that we can tell whether people filing ghci bugs might have anything relevant in their `.ghci` files. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11389> 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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