April 2017 - ghc-tickets

Re: [GHC] #2648: Report out of date interface files robustly
by GHC 19 Apr '17

19 Apr '17

#2648: Report out of date interface files robustly -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: feature request | Status: new Priority: lowest | Milestone: Component: GHCi | Version: 7.6.3 Resolution: | Keywords: | TypeErrorMessages Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: GHCi crash | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * keywords: => TypeErrorMessages -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/2648#comment:22> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #2648: Report out of date interface files robustly
by GHC 19 Apr '17

19 Apr '17

#2648: Report out of date interface files robustly -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: feature request | Status: new Priority: lowest | Milestone: Component: GHCi | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: GHCi crash | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/2648#comment:21> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #1928: Confusing type error message
by GHC 19 Apr '17

19 Apr '17

#1928: Confusing type error message -------------------------------------+------------------------------------- Reporter: josef | Owner: (none) Type: bug | Status: new Priority: low | Milestone: ⊥ Component: Compiler (Type | Version: 6.8.1 checker) | Keywords: Resolution: | TypeErrorMessages Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * keywords: => TypeErrorMessages * failure: => None/Unknown -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/1928#comment:5> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #1330: Impredicativity bug: Church2 test gives a rather confusing error with the HEAD
by GHC 19 Apr '17

19 Apr '17

#1330: Impredicativity bug: Church2 test gives a rather confusing error with the HEAD -------------------------------------+------------------------------------- Reporter: igloo | Owner: simonpj Type: bug | Status: new Priority: lowest | Milestone: Component: Compiler (Type | Version: 6.7 checker) | Keywords: Resolution: | TypeErrorMessages Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Church2 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * keywords: => TypeErrorMessages * failure: => None/Unknown -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/1330#comment:21> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9456: Weird behavior with polymorphic function involving existential quantification and GADTs
by GHC 19 Apr '17

19 Apr '17

#9456: Weird behavior with polymorphic function involving existential quantification and GADTs -------------------------------------+------------------------------------- Reporter: haasn | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.8.2 checker) | Operating System: Keywords: | Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: GHC Difficulty: Unknown | rejects valid program Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- This program is rejected by GHC 7.8.2: {{{ {-# LANGUAGE GADTs, ExistentialQuantification #-} module Foo where data Box = forall a. Box a foo :: Monad m => m () foo = do box <- return (Box ()) let f x = let _ = Box x `asTypeOf` box in x g :: a -> a g = f return () }}} With the following type error: {{{ foo.hs:15:11: Couldn't match type ‘a0’ with ‘a’ because type variable ‘a’ would escape its scope This (rigid, skolem) type variable is bound by the type signature for g :: a -> a at foo.hs:14:12-17 Expected type: a -> a Actual type: a0 -> a0 Relevant bindings include g :: a -> a (bound at foo.hs:15:7) f :: a0 -> a0 (bound at foo.hs:10:7) In the expression: f In an equation for ‘g’: g = f }}} Perplexingly, however, you can get it to compile either by adding a type signature to f: {{{ {-# LANGUAGE GADTs, ExistentialQuantification #-} module Foo where data Box = forall a. Box a foo :: Monad m => m () foo = do box <- return (Box ()) let f :: x -> x f x = let _ = Box x `asTypeOf` box in x g :: a -> a g = f return () }}} Or by disabling the GADTs extension: {{{ {-# LANGUAGE ExistentialQuantification #-} module Foo where data Box = forall a. Box a foo :: Monad m => m () foo = do box <- return (Box ()) let f x = let _ = Box x `asTypeOf` box in x g :: a -> a g = f return () }}} Or by getting rid of the asTypeOf: {{{ {-# LANGUAGE GADTs, ExistentialQuantification #-} module Foo where data Box = forall a. Box a foo :: Monad m => m () foo = do box <- return (Box ()) let f x = let _ = Box x in x g :: a -> a g = f return () }}} Or by defining ‘box’ differently: {{{ {-# LANGUAGE GADTs, ExistentialQuantification #-} module Foo where data Box = forall a. Box a foo :: Monad m => m () foo = do let box = Box () let f x = let _ = Box x `asTypeOf` box in x g :: a -> a g = f return () }}} And perhaps some other factors that I haven't tested yet. It appears to me that all of these should be valid programs. Real world source: https://github.com/andygill/io- reactive/blob/master/Control/Concurrent/Reactive.hs It happens here due to the usage of (putMVar box ret), which makes the function requestit in line 77 not as polymorphic as it should be, unless you add a type signature. The putMVar serves the same role as the `asTypeOf` in my smaller example. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9456> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9605: Misleading error message with forgotten "do"
by GHC 19 Apr '17

19 Apr '17

#9605: Misleading error message with forgotten "do" -------------------------------------+------------------------------------- Reporter: owst | 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: -------------------------------------+------------------------------------- In the following (simplified) code, I forgot to add the do keyword, when doing some refactoring. This caused GHC to give a strange error message that said approximately "The function F is applied to two arguments, but its type T has only two"... which is odd! {{{#!hs import Control.Monad.RWS ( evalRWS, RWS ) import Control.Monad.Writer.Class ( tell ) main = print $ evalRWS (go 1) () () go :: Int -> RWS () [String] () () go i = --Oops, forgot a do here return (show i) >>= (\l -> tell [l]) -- go $ i + 1 go i }}} Strangely, replacing the "go i" line with the commented-out line above it gives a sensible error message, the "Possible cause: `\l -> tell [l]' is applied to too many arguments" message, which is correct, and would've pointed me straight to the problem. The invalid error message, which appears to be the same using GHC 7.6.3 and 7.8.3, is: {{{ NFA.hs:8:25: Couldn't match type ‘String -> Control.Monad.Trans.RWS.Lazy.RWST () [String] () Data.Functor.Identity.Identity ()’ with ‘()’ Expected type: Int -> String -> Control.Monad.Trans.RWS.Lazy.RWST () [String] () Data.Functor.Identity.Identity () Actual type: Int -> () The function ‘\ l -> tell [l]’ is applied to two arguments, but its type ‘(Int -> RWS () [String] () ()) -> Int -> ()’ has only two In the second argument of ‘(>>=)’, namely ‘(\ l -> tell [l]) go i’ In the expression: return (show i) >>= (\ l -> tell [l]) go i }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9605> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10450: RankNTypes type check on pattern match of "let" behaves different from "case"
by GHC 19 Apr '17

19 Apr '17

#10450: RankNTypes type check on pattern match of "let" behaves different from "case" -------------------------------------+------------------------------------- Reporter: sjcjoosten | Owner: Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 7.8.2 (Type checker) | Operating System: Unknown/Multiple Keywords: RankNTypes | Type of failure: GHC rejects Architecture: | valid program Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- When pattern matching, "let/where" and "case" have different behaviours. I believe this to be a bug, and my hope is that all of the following should be accepted. Currently, this is accepted (a version using 'where' instead of 'let' type-checks too): {{{#!hs {-# LANGUAGE RankNTypes, ScopedTypeVariables #-} module Main where data Phantom x = Phantom Int deriving Show foo :: forall y. (forall x. (Phantom x)) -> Phantom y foo (Phantom x) = Phantom (x+1) -- trying to map foo over a list, this is the only way I can get it to work typeAnnotatedMap :: (forall x. [Phantom x]) -> [Phantom y] typeAnnotatedMap intList = case intList of [] -> [] _ -> let (phead:ptail) = intList in foo phead : typeAnnotatedMap ptail }}} The following are not accepted: {{{#!hs typeAnnotatedMap :: (forall x. [Phantom x]) -> [Phantom y] typeAnnotatedMap intList = case intList of [] -> [] (phead:ptail) -> foo phead : typeAnnotatedMap ptail typeAnnotatedMap :: (forall x. [Phantom x]) -> [Phantom y] typeAnnotatedMap [] = [] typeAnnotatedMap (phead:ptail) = foo phead : typeAnnotatedMap ptail }}} The switches "ImpredicativeTypes" and "NoImpredicativeTypes" only change GHC's behaviour in the following example, for which I understand (but don't like) that it is not accepted, its error message is fine: {{{#!hs typeAnnotatedMap :: (forall x. [Phantom x]) -> [Phantom y] typeAnnotatedMap lst = map foo lst }}} Tested this on 7.8.2. Version 7.6.3 and 7.4.2 seem to behave the same. Sorry for not testing this on the latest version. Should this not be a bug, it would help if the type checker would somehow point me in the direction of the solution (first version) when I write any one of the 'problem cases'. The current type error is something like: {{{ Couldn't match type ‘x0’ with ‘x’ because type variable ‘x’ would escape its scope This (rigid, skolem) type variable is bound by a type expected by the context: [Phantom x] }}} More helpful would be something like: {{{ Your pattern match bound the following types to a shared skolem variable: ptail :: [Phantom x0] (bound at rankNtest.hs:11:25) phead :: Phantom x0 (bound at rankNtest.hs:11:19) You may have intended to use a "let" or "where" pattern-match instead. }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10450> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13582: Confusing error message with multiparameter type classes.
by GHC 19 Apr '17

19 Apr '17

#13582: Confusing error message with multiparameter type classes. -------------------------------------+------------------------------------- Reporter: | Owner: (none) facundo.dominguez | Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: type-checking | Operating System: Unknown/Multiple errors | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following program {{{ {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE UndecidableInstances #-} import Data.Typeable class First a b c | c -> b where first :: c -> a -> b class Second a b where second :: a -> b instance (Typeable b, First a b c) => Second a c where second = undefined main :: IO () main = print (second (9 :: Int) :: Int) }}} produces the following error message {{{ $ runghc-8.0.2 t.hs t.hs:18:15: error: • No instance for (Typeable b0) arising from a use of ‘second’ • In the first argument of ‘print’, namely ‘(second (9 :: Int) :: Int)’ In the expression: print (second (9 :: Int) :: Int) In an equation for ‘main’: main = print (second (9 :: Int) :: Int) }}} Note that the message does not explain where `b0` comes from. ghc-7.8.3 produced a better error message: {{{ $ runghc-7.8.3 t.hs t.hs:18:15: No instance for (First Int b Int) arising from a use of ‘second’ In the first argument of ‘print’, namely ‘(second (9 :: Int) :: Int)’ In the expression: print (second (9 :: Int) :: Int) In an equation for ‘main’: main = print (second (9 :: Int) :: Int) }}} Doing slight modifications changes the error message that ghc-8.0.2. e.g. {{{ - instance (Typeable b, First a b c) => Second a c where + instance (First a b c, Typeable b) => Second a c where }}} gives the same error as ghc-7.8.3. In a big program the current error is very puzzling. Is ghc picking the wrong error to show? Could it print more errors perhaps? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13582> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #6018: Injective type families
by GHC 19 Apr '17

19 Apr '17

#6018: Injective type families -------------------------------------+------------------------------------- Reporter: lunaris | Owner: jstolarek Type: feature request | Status: closed Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.4.1 Resolution: fixed | Keywords: TypeFamilies, | InjectiveFamilies Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: T6018, | T6018fail, T6018rnfail, T6018th, | T6018failclosed1, T6018failclosed2, | T6018ghci, T6018ghcifail, | T6018ghcirnfail Blocked By: | Blocking: Related Tickets: #4259, #10832, | Differential Rev(s): Phab:D202 #10833 | Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * keywords: TypeFamilies, Injective => TypeFamilies, InjectiveFamilies -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/6018#comment:120> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #4259: Relax restrictions on type family instance overlap
by GHC 19 Apr '17

19 Apr '17

#4259: Relax restrictions on type family instance overlap -------------------------------------+------------------------------------- Reporter: lilac | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 6.12.1 checker) | Keywords: TypeFamilies, Resolution: | InjectiveFamilies Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #8423 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * keywords: TypeFamilies => TypeFamilies, InjectiveFamilies -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4259#comment:38> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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