January 2017 - ghc-tickets

[GHC] #12428: Allow pattern synonyms to optionally carry coerceability
by GHC 20 Feb '17

20 Feb '17

#12428: Allow pattern synonyms to optionally carry coerceability -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: feature | Status: new request | Priority: normal | Milestone: 8.2.1 Component: Compiler | 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: -------------------------------------+------------------------------------- Aside from their more interesting abilities, pattern synonyms can be used for name management. For example, if a module exports a type with a data constructor named `A`, and also re-exports a type with a data constructor named `A`, it can use a pattern synonym to rename the imported constructor. If the type is a `newtype`, this pattern synonym will not carry coerceability. Sometimes, this is exactly what one would desire (an "unsafe" module breaking through a newtype abstraction may wish to allow importers to confine coerceability). Other times, it's not (pattern synonym just for name control). When defining a pattern synonym for a type in a module in which that type is coerceable, it would be nice to be able to tell GHC that the pattern synonym should carry coerceability. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12428> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11594: closed empty type families fully applied get reduced lazily when in a constraint tuple and fully applied
by GHC 20 Feb '17

20 Feb '17

#11594: closed empty type families fully applied get reduced lazily when in a constraint tuple and fully applied -------------------------------------+------------------------------------- Reporter: carter | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.10.2 (Type checker) | 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: -------------------------------------+------------------------------------- in an effort to simulate the TypeError close type family in userland, I was doing a bit of experimentation with type families in ghc 7.10 and 8.0 RC2, and i was surprised to find that i can't get fully applied closed type families to reduce "strictly" when they are fully applied to concrete arguments within a Constraint Tuple. Roughly, I want to be able to write "Assert x" style predicates in a type signature, and have those be checked/ reduced at the definition site rather than the use site. It seems like what I would have to do is do a slight indirection the form {{{ publicFunName :: SimpleType c1 c2 --- c1 c2 are two concrete types publicFunName ... = guardedFun where guardedFun :: (MyClosedTypeFamAssert c1 c2)=> SimpleType c1 c2 guardedFun ... = ... -- myClosedTypeFamAssert (x :: *) (y :: *) :: Constraint }}} I guess this SORTOF makes sense that it doesn't get reduced strictly at the definition site (though its a sort of normalization i feel like). I think the sibling computation where i have `myClosedTypeFamAssert (x :: *) (y :: *) :: Bool` and have the guard be ('True ~ myClosedTypeFamAssert c1 c2 ), but I was hoping i could have things look a teeny bit more "Assertion" style for aesthetical reasons enclosed below is a self contained module that exhibits a more worked out example {{{ {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE TypeFamilies, TypeOperators #-} {-# LANGUAGE DataKinds, GADTs #-} {-# LANGUAGE TypeInType, ConstraintKinds #-} module Main where import GHC.Types (Constraint,TYPE,Levity(..)) main = print (7 :: Int)-- this works -- main = print (sevenBad :: Int ) -- this does error, but its at the use site rather than definition site! -- using this one ""works"", but results in a VERY confusing type error involving -- type family WorstClosedStuckError (x :: a) :: Constraint where WorstClosedStuckError a = (False ~ True) --- empty closed type families are ONLY allowed as of ghc 8.0 type family ClosedStuckSilly (x :: a) :: b where type family ClosedStuckSillyConstr (x :: a) :: Constraint where --- this is analogous to a version of the TypeError family in GHC.TypeLits in 8.0 onwards --- but kind polymorphic closed empty type families dont seem to trigger a type error in the definition site --- but rather in the USE site type family ClosedStuckTypeConstr (x :: TYPE Lifted) :: Constraint where type family ClosedStuckType (x :: TYPE Lifted) :: b where --- these two seem to work OK -- these two only only report an error once I resolve the constraint on a to something like Int etc sevenBad :: (ClosedStuckSilly 'True , Num a) => a sevenBad = 7 sevenOtherBad :: (ClosedStuckSillyConstr 'False, Num a) => a sevenOtherBad = 7 -- this one fails to type check at the definition site, but has a SUPER confusing -- error about allow ambiguous types sevenBadWorst :: (WorstClosedStuckError 'False, Num a) => a sevenBadWorst = 7 sevenOKButNotUseful :: (ClosedStuckType Bool, Num a) => a sevenOKButNotUseful = 7 -- i have to do an indirection to force an "assertion" / "reduction", the following triggers the error as expected sevenIndirect :: Num a => a sevenIndirect = sevenBad }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11594> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13072: Move large tuples to a separate module in base
by GHC 20 Feb '17

20 Feb '17

#13072: Move large tuples to a separate module in base -------------------------------------+------------------------------------- Reporter: rwbarton | Owner: Type: task | Status: new Priority: normal | Milestone: 8.2.1 Component: | Version: 8.1 libraries/base | 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: -------------------------------------+------------------------------------- Quoting from my message to ghc-devs@: > It occurred to me that rather than moving just these instances to a new module, we could move the large tuples themselves to a new module Data.LargeTuple and put the instances there. The Prelude would reexport the large tuples, so there would be no user-visible change. According to my experiments, GHC should never have to read the Data.LargeTuple interface file unless a program actually mentions a large tuple type, which is presumably rare. We could then also extend the existing instances for Eq, Show, etc., which are currently only provided through 15-tuples. > > A nontrivial aspect of this change is that tuples are wired-in types, and they currently all live in the ghc-prim package. I'm actually not sure why they need to be wired-in rather than ordinary types with a funny- looking name. In any case I need to look into this further, but the difficulties here don't seem to be insurmountable. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13072> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12409: Unboxed tuples have no type representations
by GHC 18 Feb '17

18 Feb '17

#12409: Unboxed tuples have no type representations -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.2 Component: Compiler | Version: 8.0.1 Keywords: typeable | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This ticket continues with the long tradition of types missing type representations (see #12082, #12132, #11120). This time the culprit is unboxed tuples, {{{ $ ghci GHCi, version 8.0.1: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/ben/.ghci λ> import Data.Typeable λ> :set -XUnboxedTuples λ> typeRep (Proxy :: Proxy (#Int, Int#)) ghc: panic! (the 'impossible' happened) (GHC version 8.0.1 for x86_64-unknown-linux): tyConRep (#,#) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} The problem, as usual, is that the tycons are missing from `knownKeyNames`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12409> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13132: Compilation fails with a panic: get_op runContT
by GHC 17 Feb '17

17 Feb '17

#13132: Compilation fails with a panic: get_op runContT -------------------------------------+------------------------------------- Reporter: PoroCYon | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | 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: -------------------------------------+------------------------------------- Compiling 'tersmu' from Hackage fails with the following: {{{ [13 of 16] Compiling ParseM ( ParseM.hs, ParseM.o ) ghc: panic! (the 'impossible' happened) (GHC version 8.0.1 for x86_64-unknown-linux): get_op runContT Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} The code can be found [https://gitlab.com/zugz/tersmu here]. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13132> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12243: RebindableSyntax and OverloadedLabels
by GHC 17 Feb '17

17 Feb '17

#12243: RebindableSyntax and OverloadedLabels -------------------------------------+------------------------------------- Reporter: htebalaka | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | 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: -------------------------------------+------------------------------------- Should these extensions work together? I wasn't able to come up with a good motivating example; I was playing around with lenses, and thought a more restrictive class definition might aid type inference, though it turned out in that particular case it didn't. In the general case I think it might still be useful. I didn't see any discussion on this, and it seemed like maybe it's an oversight. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12243> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9432: IncoherentInstances are too restricted
by GHC 15 Feb '17

15 Feb '17

#9432: IncoherentInstances are too restricted -------------------------------------+------------------------------------- Reporter: danilo2 | Owner: Type: feature request | Status: new Priority: high | Milestone: Component: Compiler (Type | Version: 7.8.2 checker) | Operating System: Keywords: | Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: Difficulty: Unknown | None/Unknown Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Hello! The reason behind using `IncoherentInstances`is that we want GHC to commit to a specific instance even if after instantiation of some variables, other instance could be more specific (http://www.haskell.org/ghc/docs/7.0.4/html/users_guide/type-class- extensions.html). There is one problem though. Lets consider following example (which could not make much sense, but shows the problem well): {{{#!haskell {-# OPTIONS_GHC -fno-warn-missing-methods #-} {-# LANGUAGE NoMonomorphismRestriction #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE DeriveDataTypeable #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE IncoherentInstances #-} {-# LANGUAGE ScopedTypeVariables #-} module Main where import Data.Typeable -------------------------------------------------------------------------------- -- Type classes -------------------------------------------------------------------------------- class InferType a where inferType :: Proxy a -> Proxy a inferType = undefined -------------------------------------------------------------------------------- -- Proxy datatypes -------------------------------------------------------------------------------- data Id0 = Id0 deriving (Show, Typeable) data Id1 t1 = Id1 deriving (Show, Typeable) data Id2 t1 t2 = Id2 deriving (Show, Typeable) data Id3 t1 t2 t3 = Id3 deriving (Show, Typeable) data Id4 t1 t2 t3 t4 = Id4 deriving (Show, Typeable) data Id5 t1 t2 t3 t4 t5 = Id5 deriving (Show, Typeable) -------------------------------------------------------------------------------- -- Instances -------------------------------------------------------------------------------- instance InferType Int instance (InferType m, InferType a) => InferType (m a) instance (a~Id0) => InferType (a :: *) instance (a~Id1) => InferType (a :: * -> *) instance (a~Id2) => InferType (a :: * -> * -> *) -------------------------------------------------------------------------------- -- Tests -------------------------------------------------------------------------------- toProxy :: a -> Proxy a toProxy _ = Proxy --inferTypeBase :: a -> Proxy a inferTypeBase a = inferType $ toProxy a instance InferType Foo1 instance InferType Foo2 tm _ = 5 data Foo1 a = Foo1 a deriving (Show, Typeable) data Foo2 a b = Foo2 a b deriving (Show, Typeable) instance Monad Id1 -- dummy instances just for tests instance Num Id0 main = do print $ typeOf $ inferType $ toProxy $ (return (5)) print $ typeOf $ inferType $ toProxy $ (5) print $ typeOf $ inferType $ toProxy $ (Foo1 (return 5)) print $ typeOf $ inferType $ toProxy $ (Foo2 (return 5) (return 5)) print "hello" ---- OUTPUT: -- Proxy (Id1 Id0) -- Proxy Id0 -- Proxy (Foo1 (Id1 Id0)) -- Proxy (Foo2 (Id1 Id0) (Id1 Id0)) -- "hello" }}} The idea is very simple - replace all unknown type variables with known ones. It works great, but the problem appears with the function `inferTypeBase`. It should have type of `a -> Proxy a`, but GHC claims it has got `Id0 -> Proxy Id0`. It is of course caused by enabled `-XIncoherentInstances` flag, but I think GHC should be more liberal here. If it really cannot pick any instance (causing an error using only `OverlappingInstances`), the `IncoherentInstances` should allow it to pick matching one even if more specific could be available after instantianization. But If no error would occur while using `OverlappingInstances`, `IncoherentInstances` should not affect the way it resolves the instances. I think this would make `IncoherentInstances` much more useful than now. Maybe it would be good to just add some options to the flag? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9432> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13059: High memory usage during compilation
by GHC 15 Feb '17

15 Feb '17

#13059: High memory usage during compilation -------------------------------------+------------------------------------- Reporter: domenkozar | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2-rc2 Keywords: | Operating System: Unknown/Multiple Architecture: x86_64 | Type of failure: Compile-time (amd64) | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I'm building stack using GHC 8.0.2-rc2 and noticed store uses 5.17GB of (RSS) memory during https://github.com/fpco/store/blob/master/src/Data/Store/Internal.hs compilation step. Using GHC 8.0.1 memory goes up to only 1.6GB. I haven't dug deeper, but something suspicious is going on. You should be able to reproduce using installed Nix and executing nix- build -A haskell.packages.ghc802.store '<nixpkgs>' using recent channel. Building haddock for store also goes up to 5.8GB. It'd be great if someone is able to reproduce this using plan GHC 8.0.2-rc2 See also https://github.com/fpco/store/issues/91 for upstream bug report. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13059> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11031: Record Pattern Synonym Cleanup
by GHC 14 Feb '17

14 Feb '17

#11031: Record Pattern Synonym Cleanup -------------------------------------+------------------------------------- Reporter: mpickering | Owner: Type: task | Status: new Priority: low | Milestone: Component: Compiler | Version: 7.10.2 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: -------------------------------------+------------------------------------- Ben comments that there are some loose ends with record pattern synonyms. * It's still not clear to me that the free variables produced in rnPatSynBind are correct * Perhaps in the future we want to refactor tc_single to use a function which extends the typechecking environment instead of using setGblEnv * @mpickering's suggested refactoring of RecSelId to RecSelId (Either PatSyn TyCon) Bool described in the comment in TcExpr.hs * There is a TODO in tc_patsyn_finish which should either be clarified so it can be considered actionable, removed, or just fixed The comment from TcExpr was: > I tried to refactor RecSelId to RecSelId [DataCon] Bool but this causes some pain when it comes to the interface files. Next try it is something like RecSelId (Either PatSyn TyCon) Bool with the aim for something better in the future. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11031> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12923: MultiParamTypeClasses + ExtendedDefaultRules
by GHC 14 Feb '17

14 Feb '17

#12923: MultiParamTypeClasses + ExtendedDefaultRules -------------------------------------+------------------------------------- Reporter: amindfv | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | 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: -------------------------------------+------------------------------------- -XExtendedDefaultRules allows us to avoid ambiguity for unannotated numeric literals: {{{#!hs {-# LANGUAGE ExtendedDefaultRules #-} data A = A deriving Show class ToA a where toA :: a -> A instance ToA Double where toA _ = A main = print (toA 5 :: A) }}} But if we have a multi-param typeclass, -XExtendedDefaultRules doesn't help us: {{{#!hs {-# LANGUAGE ExtendedDefaultRules #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleInstances #-} data A x = A deriving Show class ToA a x where toA :: a -> A x instance ToA Double x where toA _ = A main = print (toA 5 :: A Bool) }}} It would be really nice for an EDSL I'm working on to be able to use extended defaults here! -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12923> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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