[GHC] #14547: Wrong warning by -Wincomplete-patterns
#14547: Wrong warning by -Wincomplete-patterns -------------------------------------+------------------------------------- Reporter: YoshikuniJujo | Owner: (none) Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 8.2.1 Keywords: incomplete- | Operating System: Linux patterns OverloadedLists | TypeFamilies | Type of failure: Incorrect Architecture: x86 | error/warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Wrong warning occur where I use -Wincomplete-patterns with OverloadedLists and TypeFamilies. {{{#!hs {-# LANGUAGE OverloadedLists #-} {-# LANGUAGE TypeFamilies #-} {-# OPTIONS_GHC -Wall -fno-warn-tabs #-} class Foo f where type It f foo :: [It f] -> f data List a = Empty | a :! List a deriving Show instance Foo (List a) where type It (List a) = a foo [] = Empty foo (x : xs) = x :! foo xs }}} Pattern match(es) are non-exhaustive In an equation for `foo': Patterns not matched: [] -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14547 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14547: Wrong warning by -Wincomplete-patterns -------------------------------------+------------------------------------- Reporter: YoshikuniJujo | Owner: (none) Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: incomplete- | patterns OverloadedLists, | PatternMatchWarnings TypeFamilies Operating System: Linux | Architecture: x86 Type of failure: Incorrect | Test Case: error/warning at compile-time | Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * keywords: incomplete-patterns OverloadedLists TypeFamilies => incomplete-patterns OverloadedLists, PatternMatchWarnings TypeFamilies -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14547#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14547: Wrong warning by -Wincomplete-patterns -------------------------------------+------------------------------------- Reporter: YoshikuniJujo | Owner: (none) Type: bug | Status: patch Priority: low | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: incomplete- | patterns OverloadedLists, | PatternMatchWarnings TypeFamilies Operating System: Linux | Architecture: x86 Type of failure: Incorrect | Test Case: error/warning at compile-time | deSugar/should_compile/T14547 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4624 Wiki Page: | -------------------------------------+------------------------------------- Changes (by sighingnow): * testcase: => deSugar/should_compile/T14547 * status: new => patch * differential: => Phab:D4624 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14547#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14547: Wrong warning by -Wincomplete-patterns -------------------------------------+------------------------------------- Reporter: YoshikuniJujo | Owner: (none) Type: bug | Status: patch Priority: low | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: incomplete- | patterns OverloadedLists, | PatternMatchWarnings TypeFamilies Operating System: Linux | Architecture: x86 Type of failure: Incorrect | Test Case: error/warning at compile-time | deSugar/should_compile/T14547 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4624 Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): I'm a bit lost with this example. I think that, because of `OverloadedLists` it desugars as {{{ foo (toList -> []) = Empty foo (toList -> (x : xs)) = x :! foo xs }}} But then we actually need the `[]` (list) instance of `toList` which is the identity function? Then what? How does the bogus error message arise? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14547#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14547: Wrong warning by -Wincomplete-patterns -------------------------------------+------------------------------------- Reporter: YoshikuniJujo | Owner: (none) Type: bug | Status: patch Priority: low | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: incomplete- | patterns OverloadedLists, | PatternMatchWarnings TypeFamilies Operating System: Linux | Architecture: x86 Type of failure: Incorrect | Test Case: error/warning at compile-time | deSugar/should_compile/T14547 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4624 Wiki Page: | -------------------------------------+------------------------------------- Comment (by sighingnow): During the exhaustiveness checking, we already know the type of `[]`, we use `splitListTyConApp_maybe` to check whether it's type is `[a]` (list) or not and extract its element type. {{{#!hs splitListTyConApp_maybe :: Type -> Maybe Type splitListTyConApp_maybe ty = case splitTyConApp_maybe ty of Just (tc,[e]) | tc == listTyCon -> Just e _other -> Nothing }}} Indeed we don't need to know the `toList` of `[]` instance is identity function, we know the type of the pattern is `[]` then the `toList` is omitted and it is enough to know `[]` is the nil pattern. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14547#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14547: Wrong warning by -Wincomplete-patterns -------------------------------------+------------------------------------- Reporter: YoshikuniJujo | Owner: (none) Type: bug | Status: patch Priority: low | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: incomplete- | patterns OverloadedLists, | PatternMatchWarnings TypeFamilies Operating System: Linux | Architecture: x86 Type of failure: Incorrect | Test Case: error/warning at compile-time | deSugar/should_compile/T14547 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4624 Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj):
Indeed we don't need to know the toList of [] instance is identity function,
Good point. So this ticket is * not about overloaded strings * not about type functions It's only about view patterns. Could you re-formulate the example to eliminate the unnecessary clutter, and focus just on view patterns? And then (sorry to be slow) take us more slowly through what the problem is? It'll be something to do with the way that the pattern match checker deals with view patterns, I guess. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14547#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14547: Wrong warning by -Wincomplete-patterns -------------------------------------+------------------------------------- Reporter: YoshikuniJujo | Owner: (none) Type: bug | Status: patch Priority: low | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: incomplete- | patterns OverloadedLists, | PatternMatchWarnings TypeFamilies Operating System: Linux | Architecture: x86 Type of failure: Incorrect | Test Case: error/warning at compile-time | deSugar/should_compile/T14547 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4624 Wiki Page: | -------------------------------------+------------------------------------- Comment (by sighingnow): My previous comment addresses on `toList`, leading to some confusion. The ticket **IS** about the type functions. Look the `ListPat` case in `translatePat` (after the patch Phab:D4624): {{{#!hs -- overloaded list ListPat x lpats elem_ty (Just (pat_ty, _to_list)) | Just e_ty <- splitListTyConApp_maybe pat_ty , (_, norm_e_ty) <- normaliseType fam_insts Nominal e_ty -- e_ty can be a type family instance, like -- `It (List a)`, but we prefer `a`, see Trac #14547 , (_, norm_elem_ty) <- normaliseType fam_insts Nominal elem_ty -- elem_ty is frequently something like -- `Item [Int]`, but we prefer `Int` , norm_elem_ty `eqType` norm_e_ty -> -- We have to ensure that the element types are exactly the same. -- Otherwise, one may give an instance IsList [Int] (more specific than -- the default IsList [a]) with a different implementation for `toList' translatePat fam_insts (ListPat x lpats e_ty Nothing) -- See Note [Guards and Approximation] | otherwise -> mkCanFailPmPat pat_ty }}} For pattern `[]` in `foo [] = Empty`, we have ||pat_ty || `[It (List a_a2mv)]` || ||e_ty || `It (List a_a2mv)` || ||norm_e_ty || `a_a2mv` || ||elem_ty || `GHC.Exts.Item [a_a2mv]` || ||norm_elem_ty || `a_a2mv` || Before Phab:4624, we compared `norm_elem_ty` with `e_ty`, rather than `norm_e_ty`, then we failed to know the `[]` is list's nil pattern. We need also normalize the `e_ty` and this ticket is only about the type function. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14547#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14547: Wrong warning by -Wincomplete-patterns -------------------------------------+------------------------------------- Reporter: YoshikuniJujo | Owner: (none) Type: bug | Status: patch Priority: low | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: incomplete- | patterns OverloadedLists, | PatternMatchWarnings TypeFamilies Operating System: Linux | Architecture: x86 Type of failure: Incorrect | Test Case: error/warning at compile-time | deSugar/should_compile/T14547 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4624 Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Ah, hmm, I think I see. Given something like {{{ f (toList -> []) = e1 f (toList -> (a:as)) = e2 }}} we'd like to declare this function exhaustive. (Clearly it is!) The `ListPat` case of `Check.translatePat` deals with a very special sub- case. * Using `OverloadedLists`, we generate a special kind of view pattern, encoded in the `ListPat` constructor of `Pat`: {{{ | ListPat (XListPat p) [LPat p] (PostTc p Type) -- The type of the elements (Maybe (PostTc p Type, SyntaxExpr p)) -- For rebindable syntax -- For OverloadedLists a Just (ty,fn) gives -- overall type of the pattern, and the toList -- function to convert the scrutinee to a list value }}} * You are checking that the function actually has type `[t1] -> [t2]`, where `normalise t1 = normalise t2`. * In that case, you discard the `toList` view thing altogether, and pretend (for the purposes of pattern-match checking) that you wrote {{{ f [] = e1 f (a:as) = e2 }}} This would not be valid if you'd had {{{ f (toList -> []) = e1 f (weirdToList -> (a:as)) = e2 }}} with two different `toList` functions. But in this special case the view function was injected by the type-driven overloading machinery, so we can be confident (I think) that both functions are the same. So ''at least'' all this should be explained in a Note. But it also seems like a special case that is easily defeated. For example, what about this: {{{ f :: T -> blah f (toList -> []) = ... f (toList -> (x:xs) = ... }}} This too is exhaustive, but it won't be recorded as such, even if you use `OverloadedLists`. And of course if you use explicit view patterns, things are even worse. At least the Note should point out these deficiencies -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14547#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14547: Wrong warning by -Wincomplete-patterns -------------------------------------+------------------------------------- Reporter: YoshikuniJujo | Owner: (none) Type: bug | Status: patch Priority: low | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: incomplete- | patterns OverloadedLists, | PatternMatchWarnings TypeFamilies Operating System: Linux | Architecture: x86 Type of failure: Incorrect | Test Case: error/warning at compile-time | deSugar/should_compile/T14547 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4624 Wiki Page: | -------------------------------------+------------------------------------- Changes (by dfeuer): * cc: dfeuer (added) Comment: In Simon's example, {{{#!hs f (toList -> []) = e1 f (toList -> (a:as)) = e2 }}} I don't see how it's at all relevant that `toList` is from an `IsList` instance. I imagine we ''always'' want to common up view patterns using the same function for coverage checking. For example, given {{{#!hs f (g -> P1) = e1 f P2 = e2 f (g -> P3) = e3 }}} I imagine we want to transform that to something like {{{#!hs f x | P1 <- gx = e1 | P2 <- x = e2 | P3 <- gx = e3 where gx = g x }}} when performing coverage checks. I don't know how the details work out for the pattern checker. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14547#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14547: Wrong warning by -Wincomplete-patterns -------------------------------------+------------------------------------- Reporter: YoshikuniJujo | Owner: (none) Type: bug | Status: patch Priority: low | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: incomplete- | patterns OverloadedLists, | PatternMatchWarnings TypeFamilies Operating System: Linux | Architecture: x86 Type of failure: Incorrect | Test Case: error/warning at compile-time | deSugar/should_compile/T14547 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4624 Wiki Page: | -------------------------------------+------------------------------------- Comment (by sighingnow): Replying to [comment:7 simonpj]: Thanks for your explanation and analysis, Simon. I agree to add a note to explain this condition. Now I have some other thought with the `ListPat` case of `translatePat`.
You are checking that the function actually has type [t1] -> [t2], where normalise t1 = normalise t2.
I think in fact we don't need to do the check. For `ListPat x lpats elem_ty (Just (pat_ty, _to_list))`, we know the `_to_list` have type: {{{ toList :: l -> [Item l] }}} If we know the `pat_ty` (the `l` in toList's signature) is `[a]` (the List type), then we can conclude that `Item l = a`, in other words, `norm_elem_ty = norm_e_ty`. Because in `instance IsList [a]`, we have `type (Item [a]) = a`. Although we can give an instance IsList [Int] (more specific than the default IsList [a]), we can't overwrite the type family instance. The original comment says:
{{{ -- We have to ensure that the element types are exactly the same. -- Otherwise, one may give an instance IsList [Int] (more specific than -- the default IsList [a]) with a different implementation for `toList' }}}
However we can't achieve this goal without check the type of `_to_list`. In practice, we almost never define more specific IsList instance like `instance IsList [Int]`, we don't need to concern that case. So we can simplify the code as {{{#!hs ListPat x lpats elem_ty (Just (pat_ty, _to_list)) | Just e_ty <- splitListTyConApp_maybe pat_ty -> translatePat fam_insts (ListPat x lpats e_ty Nothing) | otherwise -> mkCanFailPmPat pat_ty }}} Or should be check whether the type of `_to_list` is `[a] -> [a]` (the most general type) ? I don't think it's necessary for practical Haskell code. **I'm not sure if I understand this correctly.** Another silly question is that I still can't see why we need to concern about the view pattern for exhaustiveness checking of overloaded list. I think the `toList` and the `[a]` instance of `IsList` are different with ordinary view functions in view pattern. I will mention this in the note. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14547#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
Another silly question is that I still can't see why we need to concern about the view pattern for exhaustiveness checking of overloaded list. I
#14547: Wrong warning by -Wincomplete-patterns -------------------------------------+------------------------------------- Reporter: YoshikuniJujo | Owner: (none) Type: bug | Status: patch Priority: low | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: incomplete- | patterns OverloadedLists, | PatternMatchWarnings TypeFamilies Operating System: Linux | Architecture: x86 Type of failure: Incorrect | Test Case: error/warning at compile-time | deSugar/should_compile/T14547 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4624 Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Alas, with `RebindableSyntax` we don't know ''anything'' about `toList`. (Without `RebindableSyntax` I think your reasoning is correct.) But now I am worried about {{{ f (x:xs) = .. f [p,q] = ... }}} The `(x:xs)` pattern will translate as an ordinary cons pattern. The `[p,q]` pattern will be subject to overloaded lists. So if `RebindableSyntax` is on and `toList` does something strange, simply dropping the `toList` altogether, and desugaring to {{{ f (x:xs) = .. f (p:q:[]) = ... }}} may not be right. So in the presence of `RebindableSyntax` I think we should treat `ListPat` like any other view pattern; i.e. coverage/exhaustiveness does not help much. In the absence of `RebindableSyntax`, but with `-XOverloadedStrings`, can we be sure that `toList` is the identity function if the pattern type is a list type? I think so. In which case your simplified version is fine. Bottom line: check for `NoRebindableSyntax`! think the toList and the [a] instance of IsList are different with ordinary view functions in view pattern. I could not parse the question. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14547#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14547: Wrong warning by -Wincomplete-patterns -------------------------------------+------------------------------------- Reporter: YoshikuniJujo | Owner: (none) Type: bug | Status: patch Priority: low | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: incomplete- | patterns OverloadedLists, | PatternMatchWarnings TypeFamilies Operating System: Linux | Architecture: x86 Type of failure: Incorrect | Test Case: error/warning at compile-time | deSugar/should_compile/T14547 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4624 Wiki Page: | -------------------------------------+------------------------------------- Comment (by sighingnow): Replying to [comment:10 simonpj]: I misread your previous comment, my bad.
Bottom line: check for `NoRebindableSyntax`!
I have revised the Phab:D4624. Now when `RebindableSyntax` is enabled we treat the overloaded list pattern as ordinary view pattern. I have added some comments about this problem. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14547#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14547: Wrong warning by -Wincomplete-patterns
-------------------------------------+-------------------------------------
Reporter: YoshikuniJujo | Owner: (none)
Type: bug | Status: patch
Priority: low | Milestone:
Component: Compiler | Version: 8.2.1
Resolution: | Keywords: incomplete-
| patterns OverloadedLists,
| PatternMatchWarnings TypeFamilies
Operating System: Linux | Architecture: x86
Type of failure: Incorrect | Test Case:
error/warning at compile-time | deSugar/should_compile/T14547
Blocked By: | Blocking:
Related Tickets: | Differential Rev(s): Phab:D4624
Wiki Page: |
-------------------------------------+-------------------------------------
Comment (by Ben Gamari
#14547: Wrong warning by -Wincomplete-patterns -------------------------------------+------------------------------------- Reporter: YoshikuniJujo | Owner: (none) Type: bug | Status: closed Priority: low | Milestone: 8.6.1 Component: Compiler | Version: 8.2.1 Resolution: fixed | Keywords: incomplete- | patterns OverloadedLists, | PatternMatchWarnings TypeFamilies Operating System: Linux | Architecture: x86 Type of failure: Incorrect | Test Case: error/warning at compile-time | deSugar/should_compile/T14547 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4624 Wiki Page: | -------------------------------------+------------------------------------- Changes (by bgamari): * status: patch => closed * resolution: => fixed * milestone: => 8.6.1 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14547#comment:13 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14547: Wrong warning by -Wincomplete-patterns
-------------------------------------+-------------------------------------
Reporter: YoshikuniJujo | Owner: (none)
Type: bug | Status: closed
Priority: low | Milestone: 8.6.1
Component: Compiler | Version: 8.2.1
Resolution: fixed | Keywords: incomplete-
| patterns OverloadedLists,
| PatternMatchWarnings TypeFamilies
Operating System: Linux | Architecture: x86
Type of failure: Incorrect | Test Case:
error/warning at compile-time | deSugar/should_compile/T14547
Blocked By: | Blocking:
Related Tickets: | Differential Rev(s): Phab:D4624
Wiki Page: |
-------------------------------------+-------------------------------------
Comment (by Ben Gamari
#14547: Wrong warning by -Wincomplete-patterns
-------------------------------------+-------------------------------------
Reporter: YoshikuniJujo | Owner: (none)
Type: bug | Status: closed
Priority: low | Milestone: 8.6.1
Component: Compiler | Version: 8.2.1
Resolution: fixed | Keywords: incomplete-
| patterns OverloadedLists,
| PatternMatchWarnings TypeFamilies
Operating System: Linux | Architecture: x86
Type of failure: Incorrect | Test Case:
error/warning at compile-time | deSugar/should_compile/T14547
Blocked By: | Blocking:
Related Tickets: | Differential Rev(s): Phab:D4624
Wiki Page: |
-------------------------------------+-------------------------------------
Comment (by Ben Gamari
#14547: Wrong warning by -Wincomplete-patterns
-------------------------------------+-------------------------------------
Reporter: YoshikuniJujo | Owner: (none)
Type: bug | Status: closed
Priority: low | Milestone: 8.6.1
Component: Compiler | Version: 8.2.1
Resolution: fixed | Keywords: incomplete-
| patterns OverloadedLists,
| PatternMatchWarnings TypeFamilies
Operating System: Linux | Architecture: x86
Type of failure: Incorrect | Test Case:
error/warning at compile-time | deSugar/should_compile/T14547
Blocked By: | Blocking:
Related Tickets: | Differential Rev(s): Phab:D4624
Wiki Page: |
-------------------------------------+-------------------------------------
Comment (by Ben Gamari
#14547: Wrong warning by -Wincomplete-patterns
-------------------------------------+-------------------------------------
Reporter: YoshikuniJujo | Owner: (none)
Type: bug | Status: closed
Priority: low | Milestone: 8.6.1
Component: Compiler | Version: 8.2.1
Resolution: fixed | Keywords: incomplete-
| patterns OverloadedLists,
| PatternMatchWarnings TypeFamilies
Operating System: Linux | Architecture: x86
Type of failure: Incorrect | Test Case:
error/warning at compile-time | deSugar/should_compile/T14547
Blocked By: | Blocking:
Related Tickets: | Differential Rev(s): Phab:D4624
Wiki Page: |
-------------------------------------+-------------------------------------
Comment (by Ben Gamari
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