January 2015 - ghc-tickets

[GHC] #9701: GADTs not specialized properly
by GHC 21 Mar '17

21 Mar '17

#9701: GADTs not specialized properly -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.9 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Runtime Blocked By: | performance bug Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- This has probably been raised before, but I can't find a ticket. If I write {{{#!hs data Silly a where Silly :: Ord a => a -> Silly a isItSilly :: a -> Silly a -> Bool isItSilly a (Silly x) = a < x isItSillyInt :: Int -> Silly Int -> Bool isItSillyInt = isItSilly }}} then I get {{{ isItSilly isItSilly = \ @ a_aBq eta_B2 eta1_B1 -> case eta1_B1 of _ { Silly $dOrd_aBs x_aAy -> < $dOrd_aBs eta_B2 x_aAy } isItSillyInt isItSillyInt = isItSilly }}} Although GHC knows that `eta_B2` is an `Int`, and `x_aAy` therefore must be one as well, it looks up the `<` method in the `eta1_B1` dictionary instead of just using `ltInt`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9701> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8751: Show parenthesised output of expressions in ghci
by GHC 20 Mar '17

20 Mar '17

#8751: Show parenthesised output of expressions in ghci ------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Iceland_jack Type: feature request | Status: new Priority: normal | Milestone: Component: GHCi | 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: | ------------------------------------+------------------------------------- Operator fixity can be a source of bugs (even for experienced users!),[#point1 (1)] precedence levels may vary between languages and then there are user-defined operators. Attached is a patch for a ghci command allowing users to quickly parenthesise expressions as a sanity check (and as an alternative to using :info and parsing the result yourself). The command itself is activated with `:paren`: {{{ ghci> :paren 2 + 10 * 4 / 3 2 + ((10 * 4) / 3) ghci> :paren \a b → a == b || b && a == (b == a) || b \ a b -> (a == b) || ((b && (a == (b == a))) || b) ghci> :paren 5 + 1 `mod` 2 5 + (1 `mod` 2) ghci> :paren 5 * 1 `mod` 2 (5 * 1) `mod` 2 }}} A friend of mine also pointed out that this could eventually be integrated into `haskell-mode` where the user could temporarily replace an expression with a parenthesised version or have different colours indicate different levels of logical nesting. [=#point1 (1)] [http://www.knosof.co.uk/cbook/accu06.html Developer beliefs about binary operator precedence] -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8751> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8255: GC Less Operation
by GHC 07 Mar '17

07 Mar '17

#8255: GC Less Operation ------------------------------+-------------------------------------------- Reporter: sirinath | Owner: Type: feature | Status: new request | Milestone: Priority: highest | Version: 7.6.3 Component: Compiler | Operating System: Unknown/Multiple Keywords: | Type of failure: Runtime performance bug Architecture: | Test Case: Unknown/Multiple | Blocking: Difficulty: Unknown | Blocked By: | Related Tickets: | ------------------------------+-------------------------------------------- Hi, Is it possible to eliminated GC through Automatic Reference Counting (ARC) like in Objective C / Clang. S -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8255> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9729: GHCi accepts invalid programs when recompiling
by GHC 01 Mar '17

01 Mar '17

#9729: GHCi accepts invalid programs when recompiling -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 7.8.3 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: GHC Blocked By: | accepts invalid program Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Here's what's happening: Module A imports module B. Module B contains a function with constraints that are '''required''' to compile module B. If I load module A in GHCi, I can remove some (required) constraints on the function in module B and GHCi will successfully reload module B. My example uses the [https://hackage.haskell.org/package/syntactic syntactic] library. I attempted to recreate the situation I just described without syntactic, but I was unsuccessful. Module A.hs: {{{ #!haskell module A where import B import Data.Syntactic.Sugar.BindingT () main = print "hello" }}} Module B.hs: {{{ #!haskell {-# LANGUAGE GADTs, TypeOperators, FlexibleContexts #-} module B where import Data.Syntactic data Let x where Let :: Let (a :-> (a -> b) :-> Full b) share :: (Let :<: sup, sup ~ Domain b, sup ~ Domain a, Internal (a -> b) ~ (Internal a -> Internal b), -- remove me Syntactic a, Syntactic b, Syntactic (a -> b), SyntacticN (a -> (a -> b) -> b) (ASTF sup (Internal a) -> ASTF sup (Internal (a -> b)) -> ASTF sup (Internal b))) => a -> (a -> b) -> b share = sugarSym Let }}} Here's a terminal transcript: {{{ $ ghci A [1 of 2] Compiling B ( testsuite/B.hs, interpreted ) [2 of 2] Compiling A ( testsuite/A.hs, interpreted ) Ok, modules loaded: A, B. > (Now remove the constraint from B and save. This *should* break module B) > :r [1 of 2] Compiling B ( testsuite/B.hs, interpreted ) Ok, modules loaded: A, B. > :q $ ghci B [1 of 2] Compiling B ( testsuite/B.hs, interpreted ) testsuite/B.hs:21:9: Could not deduce (Internal (a -> b) ~ (Internal a -> Internal b)) }}} If I had to guess what's going on, it's that GHCi is remembering the instance that A imports from BindingT: {{{ instance (...) => Syntactic (a -> b) where type Internal (a -> b) = Internal a -> Internal b }}} This instance implies the constraint that module B needs, however it should never be visible to module B. GHCi seems to be ignoring that and using the instance to recompile module B. When compiling from scratch, module B is compiled first, so of course the instance (and therefore the constraint) are not visible. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9729> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9131: Experiment with a dedicated solver for Coercible
by GHC 01 Mar '17

01 Mar '17

#9131: Experiment with a dedicated solver for Coercible ------------------------------------+------------------------------------- Reporter: nomeata | Owner: Type: task | Status: new Priority: low | 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: | ------------------------------------+------------------------------------- This is something that I might want to do some time; it should not distract us from working and improving the current design and implementation. But I think it should be explored. Currently `Coercible` appears as a type class, we have instances to explain it, and use the existing solver to solve them. The last part sometimes causes problems: * the order the instances are tried matters, there might be dead ends (#9117) * recursive newtypes are not handled as good as they could be. It would be worth exploring if a dedicated solver for `Coercible` constraints could solve these issues. It could replace the existing `getCoercibleInst`. Given a constraint `Coercible s t` it would solve it completely, do nothing or (and this is not planned out yet) return some sufficient and in some sense primitive constraint that should appear in inferred type signatures etc. Ideally we could still explain `Coercible` in terms of the instances written in the ICFP’14 paper, and simply state „GHC will find a solution using these instances, if there are any.“ instead of „The usual solver is employed, it might run into dead ends (or not, hard to tell).“ When tackling this task, I should do something that I keep postponing (because I don’t like to admit my lack of knowlege about that) but I think is really important: What are the theoretical properties of solving `Coercible` constraints? Is it even decidable? Semi decidable? Are there existing standard algorithms? How does this relate to SMT solving? And why don’t I know these things... -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9131> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9117: Coercible constraint solver misses one
by GHC 01 Mar '17

01 Mar '17

#9117: Coercible constraint solver misses one ------------------------------------+------------------------------------- Reporter: goldfire | 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: | ------------------------------------+------------------------------------- When I say {{{ import Data.Type.Coercion import Data.Coerce eta :: Coercible f g => Coercion (f a) (g a) eta = Coercion }}} I get {{{ Could not coerce from ‘f a’ to ‘g a’ because ‘f a’ and ‘g a’ are different types. arising from a use of ‘Coercion’ from the context (Coercible f g) bound by the type signature for eta :: Coercible f g => Coercion (f a) (g a) at /Users/rae/temp/Roles.hs:6:8-44 In the expression: Coercion In an equation for ‘eta’: eta = Coercion }}} But, this coercion is easily expressible in Core. If `(co :: f ~R# g)` then `(co <a> :: f a ~R# g a)`, where `<a>` is the notation for a reflexivity coercion for the type `a`. The constraint solver should be able to do this. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9117> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8779: Exhaustiveness checks for pattern synonyms
by GHC 28 Feb '17

28 Feb '17

#8779: Exhaustiveness checks for pattern synonyms -------------------------------------------+------------------------------- Reporter: nomeata | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type checker) | Version: 7.6.3 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: -------------------------------------------+------------------------------- Pattern synonyms are great, as they decouple interface from implementation. Especially if #8581 is also implemented, it should be possible to change a type completely while retaining the existing interface. Exciting! Another missing piece is exhaustiveness checks. Given this pattern {{{ initLast [] = Nothing initLast xs = Just (init xs, last xs) pattern xs ::: x <- (initLast -> Just (xs,x)) }}} we want the compiler to tell the programmer that {{{ f [] = ... f (xs ::: x) = ... }}} is complete, while {{{ g (xs ::: x) = ... }}} is not. With view pattern directly, this is impossible. But the programmer did not write view patterns! So here is what I think might work well, inspired by the new `MINIMAL` pragma: We add a new pragma `COMPLETE_PATTERNS` (any ideas for a shorter name). The syntax is essentially the same as for `MINIMAL`, i.e. a boolean formula, with constructors and pattern synonyms as atoms. In this case. {{{ {-# COMPLETE_PATTERNS [] && (:::) #-} }}} Multiple pragmas are obviously combined with `||`, and there is an implicit `{-# COMPLETE_PATTERNS [] && (:) #-}` listing all real data constructors. When checking for exhaustiveness, this would be done before unfolding view patterns, and for `g` above we get a warning that `[]` is not matched. Again, the implementation is very much analogous to `MINIMAL`. Clearly, a library author can mess up and give wrong `COMPLETE_PATTERNS` pragmas. I think that is ok (like with `MINIMAL`), and generally an improvement. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8779> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9533: Signed/unsigned integer difference between compiled and interpreted code
by GHC 26 Feb '17

26 Feb '17

#9533: Signed/unsigned integer difference between compiled and interpreted code -------------------------------------+------------------------------------- Reporter: MichaelBurge | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Keywords: | Operating System: Windows Architecture: x86_64 (amd64) | Type of failure: Incorrect Difficulty: Unknown | result at runtime Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- The following code outputs "A". {{{#!hs import Data.Word x :: Word x = 10 y :: Word y = 11 test = case x - y of -1 -> "A" _ -> "B" main = putStrLn $ show test }}} However, adding a new case that isn't matched causes the output to change: {{{#!hs import Data.Word x :: Word x = 10 y :: Word y = 11 test = case x - y of 5 -> "C" -1 -> "A" _ -> "B" main = putStrLn $ show test }}} With the extra '5 -> "C"' line in the case, the output is "B". It gets weirder - interpreted code actually continues to match the -1 case. With the second example in a file 'T.hs', here is a GHCi session: {{{ Prelude Main> :l *T.hs [1 of 1] Compiling Main ( T.hs, interpreted ) Ok, modules loaded: Main. *Main> main "A" *Main> :l T.hs Ok, modules loaded: Main. Prelude Main> main "B" Prelude Main> }}} These examples suggest to me at least 3 improvements: * Issue a warning when a negative number is used in a case statement in unsigned context * Interpreted code should give the same result as compiled code * Adding a case that isn't matched shouldn't change which other patterns get matched -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9533> 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] #9562: Type families + hs-boot files = unsafeCoerce
by GHC 13 Feb '17

13 Feb '17

#9562: Type families + hs-boot files = unsafeCoerce -------------------------------------+------------------------------------- Reporter: goldfire | 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: GHC Blocked By: | accepts invalid program Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Consider the following bundle of modules: A.hs: {{{#!hs {-# LANGUAGE TypeFamilies #-} module A where type family F a b }}} B.hs-boot: {{{#!hs module B where import A oops :: F a b -> a -> b }}} B.hs: {{{#!hs {-# LANGUAGE TypeFamilies #-} module B where import A import C type instance F a b = b oops :: F a b -> a -> b oops = const }}} C.hs: {{{#!hs module C (oops) where import {-# SOURCE #-} B }}} D.hs: {{{#!hs {-# LANGUAGE TypeFamilies #-} module D where import A import C type instance F a b = a unsafeCoerce :: a -> b unsafeCoerce x = oops x x }}} Main.hs: {{{#!hs module Main where import D ( unsafeCoerce ) main = print $ (unsafeCoerce True :: Int) }}} When loading these into GHCi, we quite reasonably get a type family instance overlap error. But, separate compilation leads to disaster: {{{ rae:01:49:47 ~/temp/bug> ghc --version The Glorious Glasgow Haskell Compilation System, version 7.8.3 rae:01:49:49 ~/temp/bug> ghc -c A.hs rae:01:49:53 ~/temp/bug> ghc -c B.hs-boot rae:01:49:58 ~/temp/bug> ghc -c C.hs rae:01:50:09 ~/temp/bug> ghc -c B.hs rae:01:50:13 ~/temp/bug> ghc -c D.hs rae:01:50:17 ~/temp/bug> ghc Main.hs -o Unsafe [6 of 6] Compiling Main ( Main.hs, Main.o ) Linking Unsafe ... rae:01:50:23 ~/temp/bug> ./Unsafe 2882303761534249061 }}} Yikes! Proposed (terrible) solution: hs-boot files '''must''' list all type instance declarations in the corresponding modules. It may also be a good idea to require all normal instance declarations in the hs-boot file as well, because this same trick can be used to introduce incoherence (I think -- haven't tested). This bug persists even if `Main` declares that it is `Safe`. I've attached a tarball of the files for ease of testing. (Credit to Edward Yang and Geoff Mainland, whose discussion provoked the line of inquiry that led to this discovery.) -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9562> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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