August 2017 - ghc-tickets

[GHC] #12980: Unlifted class method rejected
by GHC 13 Aug '18

13 Aug '18

#12980: Unlifted class method rejected -------------------------------------+------------------------------------- Reporter: goldfire | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 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: -------------------------------------+------------------------------------- If I say {{{#!hs class C a where meth :: Array# a }}} I get {{{ • Expecting a lifted type, but ‘Array# a’ is unlifted • In the type signature: meth :: Array# a In the class declaration for ‘C’ }}} This definition looks OK to me. I don't see why we can't have an unlifted member of a class. (As @oerjan points out in comment:25:ticket:12708, we can't have a ''levity-polymorphic'' class member, but this is solidly levity- monomorphic.) I'm happy to lift this restriction in ongoing work in this area, but I wanted to double-check before doing so. For the record, the same error occurs even when the class has more than one member, so it's not the no-unlifted-newtypes restriction. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12980> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9214: UNPACK support for sum types
by GHC 11 Aug '18

11 Aug '18

#9214: UNPACK support for sum types ------------------------------------+------------------------------------- Reporter: mojojojo | Owner: Type: feature request | 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: | ------------------------------------+------------------------------------- Currently the pragma only supports single-constructor types. So in the following example it will simply be ignored: {{{ data A = A1 Char | A2 {-# UNPACK #-} !B data B = B1 Int | B2 Bool }}} However the problem seems to be easily solvable by denormalizing the type {{{A}}} to something like the following during unpacking: {{{ data A = A1 Char | A2_1 Int | -- from B1 A2_2 Bool -- from B2 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9214> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12065: there is a way to override the .tix path with HPCTIXFILE
by GHC 11 Aug '18

11 Aug '18

#12065: there is a way to override the .tix path with HPCTIXFILE -------------------------------------+------------------------------------- Reporter: kostmo | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Documentation | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Documentation Unknown/Multiple | bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Under the [https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/hpc.html#id… "Caveats and Shortcomings of Haskell Program Coverage" section], it says: > There is no way to change the name of the .tix file generated, apart from renaming the binary. However, specifying `HPCTIXFILE` as an environment variable when running the executable program does change the output path of the `.tix` file. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12065> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13296: stat() calls can block Haskell runtime
by GHC 10 Aug '18

10 Aug '18

#13296: stat() calls can block Haskell runtime -------------------------------------+------------------------------------- Reporter: nh2 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Runtime | Version: 8.0.2 System | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Runtime Unknown/Multiple | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- `getFileStatus` (`stat()` syscall) is marked as `unsafe`, which means that if I have e.g. `+RTS -N4`, I can't stat more than 4 files at the same time without completely stopping the Haskell world. This is an issue on network file systems, where a single `stat()` can easily take 2 milliseconds, so you typically want to do them in parallel (but due to the above you can't). The underlying problem is that there are some Linux syscalls you typically need on networked file systems that have no asynchronous equivalent; according to http://blog.libtorrent.org/2012/10/asynchronous-disk-io/ these are at least: * `stat()` * `open()` * `fallocate()` * `rename()` A quick skim through `libraries/base/System/Posix/Internals.hs` reveals the situation: * `stat()` only exists as `unsafe` * `open()` has both `safe` and `unsafe` variants (I haven't checked which one is used in practice) The remaining ones are in the `unix` package * `fallocate()` is `safe` * `rename()` is `unsafe` It seems to me that there are two issues here: 1) None of these calls should be `unsafe` because they may block for a very long time (e.g. > 0.5 ms even on the fastest LANs). 2) We need to answer the question: If we marked them as `safe`, how many of them would the RTS execute in parallel? To my current knowledge (thanks `rwbarton` and `slyfox` on #ghc), there's a pool of Haskell executing threads (the usual `-RTS -N`), and a pool of FFI threads. Are there any restrictions on the size of that latter pool? The docs https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/ffi- chap.html#foreign-imports-and-multi-threading are not specific on that topic, simply mentioning "but there may be an arbitrary number of foreign calls in progress at any one time, regardless of the +RTS -N value". Does that mean the amount of FFI threads is truly unbounded? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13296> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9334: Implement "instance chains"
by GHC 09 Aug '18

09 Aug '18

#9334: Implement "instance chains" -------------------------------------+------------------------------------- Reporter: diatchki | Owner: diatchki Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.9 checker) | Differential Revisions: Keywords: | Architecture: Operating System: Unknown/Multiple | Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------- It would be useful to implement a version of "instance chains" [1] in GHC, which would eliminate the need for OVERLAPPING_INSTANCES in most (all practcial?) programs. The idea is that programmers can explicitly group and order instances into an "instance chain". For example: {{{ instance (Monad m) => StateM (StateT s m) s where ... else (MonadTrans t, StateM m s) => StateM (t m) s where ... }}} When GHC searches for instances, the instances in a chain are considered together and in order, starting with the first one: 1. If the goal matches the current instance's head, then this instance is selected and the rest are ignored, as if they were not there; 2. If the goal does not match the current instance's head, AND it does not unify with the current instance's head, then we skip the instance and proceed to the next member of the chain; 3. If the goal does not match the current instance's head, but it does unify with it, then we cannot use this chain to solve the goal. In summary: earlier instances in a chain "hide" later instances, and later instances can be reached only if we are sure that none of the previous instance will match. [1] http://web.cecs.pdx.edu/~mpj/pubs/instancechains.pdf -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9334> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12002: Pragmas after a module declaration are ignored without warning.
by GHC 08 Aug '18

08 Aug '18

#12002: Pragmas after a module declaration are ignored without warning. -------------------------------------+------------------------------------- Reporter: seanparsons | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Other Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Normally pragmas are placed above the module declaration like so: {{{#!hs {-# LANGUAGE OverloadedStrings #-} module Main where }}} However if the above 2 lines are flipped around like this: {{{#!hs module Main where {-# LANGUAGE OverloadedStrings #-} }}} Then the behaviour seen by a user is that equivalent to the pragma not being present, because it would appear GHC ignores the multi-line comment it looks like. For a novice (I'm not one but it just tripped me up) this would be incredibly baffling (it was for me), it would be nice to see a warning for this at the least. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12002> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12043: internal error: evacuate: strange closure type
by GHC 08 Aug '18

08 Aug '18

#12043: internal error: evacuate: strange closure type -------------------------------------+------------------------------------- Reporter: mattchan | Owner: Type: bug | Status: new Priority: normal | Milestone: Research needed Component: Compiler | Version: 7.10.3 Keywords: internal | Operating System: Unknown/Multiple error, strange closuren | Architecture: ia64 | Type of failure: None/Unknown Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Scary error while building Liquid Haskell from source: {{{ $ stack install [truncated] text-1.2.2.1: copy/register Progress: 40/61 -- While building package Cabal-1.22.8.0 using: /Users/matt/.stack/setup-exe-cache/x86_64-osx/setup-Simple- Cabal-1.22.5.0-ghc-7.10.3 --builddir=.stack- work/dist/x86_64-osx/Cabal-1.22.5.0 build --ghc-options " -ddump-hi -ddump-to-file" Process exited with code: ExitFailure (-6) Logs have been written to: /Users/matt/liquid/liquidhaskell/.stack- work/logs/Cabal-1.22.8.0.log Configuring Cabal-1.22.8.0... Building Cabal-1.22.8.0... Preprocessing library Cabal-1.22.8.0... [ 1 of 81] Compiling Paths_Cabal ( .stack- work/dist/x86_64-osx/Cabal-1.22.5.0/build/autogen/Paths_Cabal.hs, .stack- work/dist/x86_64-osx/Cabal-1.22.5.0/build/Paths_Cabal.o ) [ 2 of 81] Compiling Distribution.TestSuite ( Distribution/TestSuite.hs, .stack- work/dist/x86_64-osx/Cabal-1.22.5.0/build/Distribution/TestSuite.o ) [ 3 of 81] Compiling Distribution.Simple.PreProcess.Unlit ( Distribution/Simple/PreProcess/Unlit.hs, .stack- work/dist/x86_64-osx/Cabal-1.22.5.0/build/Distribution/Simple/PreProcess/Unlit.o ) [ 4 of 81] Compiling Distribution.GetOpt ( Distribution/GetOpt.hs, .stack-work/dist/x86_64-osx/Cabal-1.22.5.0/build/Distribution/GetOpt.o ) [ 5 of 81] Compiling Distribution.PackageDescription.Utils ( Distribution/PackageDescription/Utils.hs, .stack- work/dist/x86_64-osx/Cabal-1.22.5.0/build/Distribution/PackageDescription/Utils.o ) [ 6 of 81] Compiling Distribution.Simple.CCompiler ( Distribution/Simple/CCompiler.hs, .stack- work/dist/x86_64-osx/Cabal-1.22.5.0/build/Distribution/Simple/CCompiler.o ) [ 7 of 81] Compiling Distribution.Compat.Binary ( Distribution/Compat/Binary.hs, .stack- work/dist/x86_64-osx/Cabal-1.22.5.0/build/Distribution/Compat/Binary.o ) [ 8 of 81] Compiling Distribution.Compat.ReadP ( Distribution/Compat/ReadP.hs, .stack- work/dist/x86_64-osx/Cabal-1.22.5.0/build/Distribution/Compat/ReadP.o ) [ 9 of 81] Compiling Distribution.Text ( Distribution/Text.hs, .stack- work/dist/x86_64-osx/Cabal-1.22.5.0/build/Distribution/Text.o ) [10 of 81] Compiling Distribution.Version ( Distribution/Version.hs, .stack-work/dist/x86_64-osx/Cabal-1.22.5.0/build/Distribution/Version.o ) [11 of 81] Compiling Language.Haskell.Extension ( Language/Haskell/Extension.hs, .stack- work/dist/x86_64-osx/Cabal-1.22.5.0/build/Language/Haskell/Extension.o ) [12 of 81] Compiling Distribution.Compiler ( Distribution/Compiler.hs, .stack-work/dist/x86_64-osx/Cabal-1.22.5.0/build/Distribution/Compiler.o ) [13 of 81] Compiling Distribution.Simple.Compiler ( Distribution/Simple/Compiler.hs, .stack- work/dist/x86_64-osx/Cabal-1.22.5.0/build/Distribution/Simple/Compiler.o ) [14 of 81] Compiling Distribution.Simple.GHC.ImplInfo ( Distribution/Simple/GHC/ImplInfo.hs, .stack- work/dist/x86_64-osx/Cabal-1.22.5.0/build/Distribution/Simple/GHC/ImplInfo.o ) [15 of 81] Compiling Distribution.License ( Distribution/License.hs, .stack-work/dist/x86_64-osx/Cabal-1.22.5.0/build/Distribution/License.o ) [16 of 81] Compiling Distribution.ModuleName ( Distribution/ModuleName.hs, .stack- work/dist/x86_64-osx/Cabal-1.22.5.0/build/Distribution/ModuleName.o ) [17 of 81] Compiling Distribution.Package ( Distribution/Package.hs, .stack-work/dist/x86_64-osx/Cabal-1.22.5.0/build/Distribution/Package.o ) [18 of 81] Compiling Distribution.System ( Distribution/System.hs, .stack-work/dist/x86_64-osx/Cabal-1.22.5.0/build/Distribution/System.o ) [19 of 81] Compiling Distribution.PackageDescription ( Distribution/PackageDescription.hs, .stack- work/dist/x86_64-osx/Cabal-1.22.5.0/build/Distribution/PackageDescription.o ) ghc: internal error: evacuate: strange closure type 41832616 (GHC version 7.10.3 for x86_64_apple_darwin) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} [[Image(http://i.imgur.com/T44UPg0.png)]] -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12043> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10927: IndexError: pop from empty list
by GHC 08 Aug '18

08 Aug '18

#10927: IndexError: pop from empty list -------------------------------------+------------------------------------- Reporter: schwab | Owner: hvr Type: bug | Status: new Priority: normal | Milestone: Component: Trac & Git | Version: Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | -------------------------------------+------------------------------------- ==== Zum Reproduzieren ==== Während der Ausführung von GET auf `/attachment/ticket/10926/` hat Trac einen internen Fehler gemeldet. ''(Bitte geben Sie hier weitere Details an)'' Anfrageparameter: {{{ {u'action': u'new', 'path': u'10926/', 'realm': u'ticket'} }}} User agent: `Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.21 (KHTML, like Gecko) konqueror/4.14.9 Safari/537.21` ==== Systeminformationen ==== ''Systeminformation nicht verfügbar'' ==== Aktive Plugins ==== ''Plugininformation nicht verfügbar'' ==== Python-Zurückverfolgungsinformationen ==== {{{ Traceback (most recent call last): File "/usr/local/lib/python2.7/dist- packages/Trac-1.0.9-py2.7.egg/trac/web/main.py", line 554, in _dispatch_request dispatcher.dispatch(req) File "/usr/local/lib/python2.7/dist- packages/Trac-1.0.9-py2.7.egg/trac/web/main.py", line 267, in dispatch iterable=chrome.use_chunked_encoding) File "/usr/local/lib/python2.7/dist- packages/Trac-1.0.9-py2.7.egg/trac/web/chrome.py", line 1111, in render_template encoding='utf-8') File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 184, in render return encode(generator, method=method, encoding=encoding, out=out) File "/usr/local/lib/python2.7/dist-packages/genshi/output.py", line 58, in encode for chunk in iterator: File "/usr/local/lib/python2.7/dist-packages/genshi/output.py", line 350, in __call__ for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/output.py", line 829, in __call__ for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/output.py", line 669, in __call__ for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/output.py", line 774, in __call__ for kind, data, pos in chain(stream, [(None, None, None)]): File "/usr/local/lib/python2.7/dist-packages/genshi/output.py", line 594, in __call__ for ev in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 289, in _ensure for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 289, in _ensure for event in stream: File "/usr/local/lib/python2.7/dist- packages/Trac-1.0.9-py2.7.egg/trac/web/chrome.py", line 1309, in _strip_accesskeys for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 289, in _ensure for event in stream: File "/usr/local/lib/python2.7/dist- packages/Trac-1.0.9-py2.7.egg/trac/web/chrome.py", line 1298, in _generate for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 289, in _ensure for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 289, in _ensure for event in stream: File "/usr/local/lib/python2.7/dist- packages/genshi/filters/transform.py", line 706, in _unmark for mark, event in stream: File "/usr/local/lib/python2.7/dist- packages/genshi/filters/transform.py", line 1101, in __call__ for mark, event in stream: File "/usr/local/lib/python2.7/dist- packages/genshi/filters/transform.py", line 118, in __iter__ event = self.stream.next() File "/usr/local/lib/python2.7/dist- packages/genshi/filters/transform.py", line 734, in __call__ for mark, event in stream: File "/usr/local/lib/python2.7/dist- packages/genshi/filters/transform.py", line 702, in _mark for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/template/base.py", line 618, in _include for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/template/markup.py", line 378, in _match ctxt, start=idx + 1, **vars): File "/usr/local/lib/python2.7/dist-packages/genshi/template/markup.py", line 378, in _match ctxt, start=idx + 1, **vars): File "/usr/local/lib/python2.7/dist-packages/genshi/template/markup.py", line 362, in _match content = list(content) File "/usr/local/lib/python2.7/dist-packages/genshi/template/base.py", line 618, in _include for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/template/markup.py", line 326, in _match for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/template/markup.py", line 315, in _strip event = next() File "/usr/local/lib/python2.7/dist-packages/genshi/template/base.py", line 558, in _flatten for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 289, in _ensure for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/path.py", line 588, in _generate subevent = next() File "/usr/local/lib/python2.7/dist-packages/genshi/template/base.py", line 618, in _include for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/template/markup.py", line 315, in _strip event = next() File "/usr/local/lib/python2.7/dist-packages/genshi/template/base.py", line 558, in _flatten for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 289, in _ensure for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/path.py", line 588, in _generate subevent = next() File "/usr/local/lib/python2.7/dist-packages/genshi/template/base.py", line 618, in _include for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/template/markup.py", line 315, in _strip event = next() File "/usr/local/lib/python2.7/dist-packages/genshi/template/base.py", line 558, in _flatten for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/filters/i18n.py", line 178, in _generate for event in msgbuf.translate(gettext(msgbuf.format())): File "/usr/local/lib/python2.7/dist-packages/genshi/filters/i18n.py", line 1051, in translate events = self.events[order].pop(0) IndexError: pop from empty list }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10927> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13835: ghci with ":set +t" should print type before starting evaluation
by GHC 08 Aug '18

08 Aug '18

#13835: ghci with ":set +t" should print type before starting evaluation -------------------------------------+------------------------------------- Reporter: j.waldmann | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: GHCi | 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: -------------------------------------+------------------------------------- Not an actual bug, but questionable design perhaps. I think this is backwards, as static analysis comes before evaluation, not after: {{{ Prelude> :set +t Prelude> reverse [True] [True] it :: [Bool] }}} When evaluation raises an exception, no type is printed. {{{ Prelude> :set +t Prelude> True && undefined *** Exception: Prelude.undefined CallStack (from HasCallStack): error, called at libraries/base/GHC/Err.hs:79:14 in base:GHC.Err undefined, called at <interactive>:8:9 in interactive:Ghci4 }}} For teaching (and for my own sanity) I'd much prefer output in logical succession. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13835> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12421: TestEquality and TestCoercion documentation is confusing
by GHC 08 Aug '18

08 Aug '18

#12421: TestEquality and TestCoercion documentation is confusing -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.2 Component: Core | Version: 8.0.1 Libraries | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Documentation Unknown/Multiple | bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Currently, the documentation for the `TestEquality` class indicates > This class contains types where you can learn the equality of two types from information contained in terms. Typically, only singleton types should inhabit this class. The `TestCoercion` documentation includes a similar caution about singleton types. But this is far too conservative! Length-indexed vectors can be made valid instances of `TestEquality` and `TestCoercion` in exactly one way: {{{#!hs data Nat = Z | S Nat data Vec a n where Nil :: Vec a 'Z Cons :: a -> Vec a n -> Vec a ('S n) instance TestEquality (Vec a) where testEquality Nil Nil = Just Refl testEquality (Cons _ xs) (Cons _ ys) = fmap (\Refl -> Refl) (testEquality xs ys) testEquality _ _ = Nothing instance TestCoercion (Vec a) where testCoercion xs ys = fmap (\Refl -> Coercion) (testEquality xs ys) }}} Polykinded heterogeneous lists are another nice non-singleton example for which each class has a single "most reasonable" instance: {{{#!hs data HList (f :: k -> *) (xs :: [k]) where HNil :: HList f '[] HCons :: f a -> HList f as -> HList f (a ': as) instance TestEquality f => TestEquality (HList f) where testEquality HNil HNil = Just Refl testEquality (HCons x xs) (HCons y ys) = do Refl <- testEquality x y Refl <- testEquality xs ys Just Refl testEquality _ _ = Nothing instance TestCoercion f => TestCoercion (HList f) where testCoercion HNil HNil = Just Coercion testCoercion (HCons x xs) (HCons y ys) = do Coercion <- testCoercion x y Coercion <- testCoercion xs ys Just Coercion testCoercion _ _ = Nothing }}} I don't know just how far the warning should be weakened; it may make sense to go as far as saying the type should generally be a GADT. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12421> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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