Re: [GHC] #2357: Implement the Haskell' proposal for polymorphic pattern bindings

[GHC]

#2357: Implement the Haskell' proposal for polymorphic pattern bindings
-+--
  Reporter:  simonmar|  Owner:  
  Type:  task| Status:  closed  
  Priority:  high|  Milestone:  7.4.1   
 Component:  Compiler (Type checker) |Version:  6.8.2   
Resolution:  fixed   |   Keywords:  
  Testcase:  typecheck/should_compile/T2357  |  Blockedby:  
Difficulty:  Unknown | Os:  Unknown/Multiple
  Blocking:  |   Architecture:  Unknown/Multiple
   Failure:  None/Unknown|  
-+--

Comment(by MartijnVanSteenbergen):

 Awesome! Will do.

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Re: [GHC] #2357: Implement the Haskell' proposal for polymorphic pattern bindings

[GHC]

#2357: Implement the Haskell' proposal for polymorphic pattern bindings
-+--
  Reporter:  simonmar|  Owner:  
  Type:  task| Status:  closed  
  Priority:  high|  Milestone:  7.4.1   
 Component:  Compiler (Type checker) |Version:  6.8.2   
Resolution:  fixed   |   Keywords:  
  Testcase:  typecheck/should_compile/T2357  |  Blockedby:  
Difficulty:  Unknown | Os:  Unknown/Multiple
  Blocking:  |   Architecture:  Unknown/Multiple
   Failure:  None/Unknown|  
-+--
Changes (by simonpj):

  * testcase:  typecheck/should_compile/T1897a,
   indexed_types/should_compile/T1897b =>
   typecheck/should_compile/T2357


Comment:

 Works in HEAD!

 Incidentally, always use `-dcore-lint` when you get weird crashes.  It
 often picks up the error earlier.

 If you make other test cases we can add them too.

 Simon

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Re: [GHC] #2357: Implement the Haskell' proposal for polymorphic pattern bindings

[GHC]

#2357: Implement the Haskell' proposal for polymorphic pattern bindings
---+
  Reporter:  simonmar   
   |  Owner:  
  Type:  task   
   | Status:  closed  
  Priority:  high   
   |  Milestone:  7.4.1   
 Component:  Compiler (Type checker)
   |Version:  6.8.2   
Resolution:  fixed  
   |   Keywords:  
  Testcase:  typecheck/should_compile/T1897a, 
indexed_types/should_compile/T1897b  |  Blockedby:  
Difficulty:  Unknown
   | Os:  Unknown/Multiple
  Blocking: 
   |   Architecture:  Unknown/Multiple
   Failure:  None/Unknown   
   |  
---+

Comment(by MartijnVanSteenbergen):

 It also panicks in 7.2.1. Here are the error messages:

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.0.3 for i386-apple-darwin):
 applyTypeToArgs
 f{v ait} [lid] $dShow{v aiz} [lid]
 a{tv aiy} [sk]
 }}}

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.2.1 for x86_64-apple-darwin):
 applyTypeToArgs
 f{v ahK} [lid] $dShow{v ahQ} [lid]
 a{tv ahP} [sk]
 }}}

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Re: [GHC] #2357: Implement the Haskell' proposal for polymorphic pattern bindings

[GHC]

#2357: Implement the Haskell' proposal for polymorphic pattern bindings
---+
  Reporter:  simonmar   
   |  Owner:  
  Type:  task   
   | Status:  closed  
  Priority:  high   
   |  Milestone:  7.4.1   
 Component:  Compiler (Type checker)
   |Version:  6.8.2   
Resolution:  fixed  
   |   Keywords:  
  Testcase:  typecheck/should_compile/T1897a, 
indexed_types/should_compile/T1897b  |  Blockedby:  
Difficulty:  Unknown
   | Os:  Unknown/Multiple
  Blocking: 
   |   Architecture:  Unknown/Multiple
   Failure:  None/Unknown   
   |  
---+

Comment(by MartijnVanSteenbergen):

 Thank you, that's great news!

 Just yesterday I wrote three programs which used -XNoMonoPatBinds and
 respectively caused the type-checker to hang, to panic and compiled fine
 but (seemed to) have wrong behaviour, using GHC 7.0.3.

 I've made the panic program as small as I could:

 {{{
 {-# LANGUAGE NoMonoPatBinds #-}

 f :: Show a => a
 (f, _) = undefined

 g :: Show a => a
 g = f
 }}}

 Perhaps you can use it as a test case for the new algorithm.

 If I manage to make the other programs smaller too, I'll post them here.

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Re: [GHC] #2357: Implement the Haskell' proposal for polymorphic pattern bindings

[GHC]

#2357: Implement the Haskell' proposal for polymorphic pattern bindings
---+
  Reporter:  simonmar   
   |  Owner:  
  Type:  task   
   | Status:  closed  
  Priority:  high   
   |  Milestone:  7.4.1   
 Component:  Compiler (Type checker)
   |Version:  6.8.2   
Resolution:  fixed  
   |   Keywords:  
  Testcase:  typecheck/should_compile/T1897a, 
indexed_types/should_compile/T1897b  |  Blockedby:  
Difficulty:  Unknown
   | Os:  Unknown/Multiple
  Blocking: 
   |   Architecture:  Unknown/Multiple
   Failure:  None/Unknown   
   |  
---+

Comment(by simonmar):

 Great, thanks Simon!

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Re: [GHC] #2357: Implement the Haskell' proposal for polymorphic pattern bindings

[GHC]

#2357: Implement the Haskell' proposal for polymorphic pattern bindings
---+
  Reporter:  simonmar   
   |  Owner:  
  Type:  task   
   | Status:  closed  
  Priority:  high   
   |  Milestone:  7.4.1   
 Component:  Compiler (Type checker)
   |Version:  6.8.2   
Resolution:  fixed  
   |   Keywords:  
  Testcase:  typecheck/should_compile/T1897a, 
indexed_types/should_compile/T1897b  |  Blockedby:  
Difficulty:  Unknown
   | Os:  Unknown/Multiple
  Blocking: 
   |   Architecture:  Unknown/Multiple
   Failure:  None/Unknown   
   |  
---+

Comment(by simonpj):

 Concerning (c) see http://www.haskell.org/pipermail/glasgow-haskell-
 users/2011-June/020489.html

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Re: [GHC] #2357: Implement the Haskell' proposal for polymorphic pattern bindings

[GHC]

#2357: Implement the Haskell' proposal for polymorphic pattern bindings
---+
  Reporter:  simonmar   
   |  Owner:  
  Type:  task   
   | Status:  closed  
  Priority:  high   
   |  Milestone:  7.4.1   
 Component:  Compiler (Type checker)
   |Version:  6.8.2   
Resolution:  fixed  
   |   Keywords:  
  Testcase:  typecheck/should_compile/T1897a, 
indexed_types/should_compile/T1897b  |  Blockedby:  
Difficulty:  Unknown
   | Os:  Unknown/Multiple
  Blocking: 
   |   Architecture:  Unknown/Multiple
   Failure:  None/Unknown   
   |  
---+
Changes (by simonpj):

  * status:  new => closed
  * testcase:  => typecheck/should_compile/T1897a,
   indexed_types/should_compile/T1897b
  * resolution:  => fixed


Comment:

 Free at last, free at last.  This long standing change is finally done!

 Simon

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Re: [GHC] #2357: Implement the Haskell' proposal for polymorphic pattern bindings

[GHC]

#2357: Implement the Haskell' proposal for polymorphic pattern bindings
+---
Reporter:  simonmar |Owner:  
Type:  task |   Status:  new 
Priority:  high |Milestone:  7.4.1   
   Component:  Compiler (Type checker)  |  Version:  6.8.2   
Keywords:   | Testcase:  
   Blockedby:   |   Difficulty:  Unknown 
  Os:  Unknown/Multiple | Blocking:  
Architecture:  Unknown/Multiple |  Failure:  None/Unknown
+---

Comment(by simonpj@…):

 commit 49dbe60558deee5ea6cd2c7730b7c591d15559c8
 {{{
 Author: Simon Peyton Jones 
 Date:   Tue Aug 16 10:23:52 2011 +0100

 Major improvement to pattern bindings

 This patch makes a number of related improvements

 a) Implements the Haskell Prime semantics for pattern bindings
(Trac #2357).  That is, a pattern binding p = e is typed
just as if it had been written
 t = e
 f = case t of p -> f
 g = case t of p -> g
 ... etc ...
where f,g are the variables bound by p. In paricular it's
ok to say
   (f,g) = (\x -> x, \y -> True)
and f and g will get propertly inferred types
   f :: a -> a
   g :: a -> Int

 b) Eliminates the MonoPatBinds flag altogether.  (For the moment
it is deprecated and has no effect.)  Pattern bindings are now
generalised as per (a).  Fixes Trac #2187 and #4940, in the
way the users wanted!

 c) Improves the OutsideIn algorithm generalisation decision.
Given a definition without a type signature (implying "infer
the type"), the published algorithm rule is this:
   - generalise *top-level* functions, and
   - do not generalise *nested* functions
The new rule is
   - generalise a binding whose free variables have
 Guaranteed Closed Types
   - do not generalise other bindings

Generally, a top-level let-bound function has a Guaranteed
Closed Type, and so does a nested function whose free vaiables
are top-level functions, and so on. (However a top-level
function that is bitten by the Monomorphism Restriction does
not have a GCT.)

Example:
  f x = let { foo y = y } in ...
Here 'foo' has no free variables, so it is generalised despite
being nested.

 d) When inferring a type f :: ty for a definition f = e, check that
the compiler would accept f :: ty as a type signature for that
same definition.  The type is rejected precisely when the type
is ambiguous.

Example:
   class Wob a b where
 to :: a -> b
 from :: b -> a

   foo x = [x, to (from x)]
GHC 7.0 would infer the ambiguous type
   foo :: forall a b. Wob a b => b -> [b]
but that type would give an error whenever it is called; and
GHC 7.0 would reject that signature if given by the
programmer.  The new type checker rejects it up front.

Similarly, with the advent of type families, ambiguous types are
easy to write by mistake.  See Trac #1897 and linked tickets for
many examples.  Eg
   type family F a :: *
   f ::: F a -> Int
   f x = 3
This is rejected because (F a ~ F b) does not imply a~b.
 Previously
GHC would *infer* the above type for f, but was unable to check it.
Now even the inferred type is rejected -- correctly.

 The main implemenation mechanism is to generalise the abe_wrap
 field of ABExport (in HsBinds), from [TyVar] to HsWrapper. This
 beautiful generalisation turned out to make everything work nicely
 with minimal programming effort.  All the work was fiddling around
 the edges; the core change was easy!

  compiler/deSugar/DsBinds.lhs|   62 +++-
  compiler/deSugar/DsExpr.lhs |6 +-
  compiler/hsSyn/HsBinds.lhs  |   28 +++-
  compiler/hsSyn/HsUtils.lhs  |   32 +++--
  compiler/main/DynFlags.hs   |   13 +-
  compiler/rename/RnBinds.lhs |6 +-
  compiler/typecheck/TcBinds.lhs  |  282
 --
  compiler/typecheck/TcClassDcl.lhs   |   12 +-
  compiler/typecheck/TcEnv.lhs|  117 +--
  compiler/typecheck/TcErrors.lhs |   32 +++--
  compiler/typecheck/TcHsSyn.lhs  |   14 +-
  compiler/typecheck/TcInstDcls.lhs   |   19 ++-
  compiler/typecheck/TcMType.lhs  |6 +-
  compiler/typecheck/TcRnDriver.lhs   |   14 +-
  compiler/typecheck/TcRnMonad.lhs|2 +-
  compiler/typecheck/TcRnTypes.lhs|   11 +-
  compiler/ty

Re: [GHC] #2357: Implement the Haskell' proposal for polymorphic pattern bindings

[GHC]

#2357: Implement the Haskell' proposal for polymorphic pattern bindings
+---
Reporter:  simonmar |Owner:  
Type:  task |   Status:  new 
Priority:  high |Milestone:  7.4.1   
   Component:  Compiler (Type checker)  |  Version:  6.8.2   
Keywords:   | Testcase:  
   Blockedby:   |   Difficulty:  Unknown 
  Os:  Unknown/Multiple | Blocking:  
Architecture:  Unknown/Multiple |  Failure:  None/Unknown
+---
Changes (by igloo):

  * milestone:  7.2.1 => 7.4.1


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Re: [GHC] #2357: Implement the Haskell' proposal for polymorphic pattern bindings

[GHC]

#2357: Implement the Haskell' proposal for polymorphic pattern bindings
+---
Reporter:  simonmar |Owner:  
Type:  task |   Status:  new 
Priority:  high |Milestone:  7.2.1   
   Component:  Compiler (Type checker)  |  Version:  6.8.2   
Keywords:   | Testcase:  
   Blockedby:   |   Difficulty:  Unknown 
  Os:  Unknown/Multiple | Blocking:  
Architecture:  Unknown/Multiple |  Failure:  None/Unknown
+---
Changes (by simonmar):

  * priority:  normal => high


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Re: [GHC] #2357: Implement the Haskell' proposal for polymorphic pattern bindings

[GHC]

#2357: Implement the Haskell' proposal for polymorphic pattern bindings
+---
Reporter:  simonmar |Owner:  
Type:  task |   Status:  new 
Priority:  normal   |Milestone:  6.14.1  
   Component:  Compiler (Type checker)  |  Version:  6.8.2   
Keywords:   |   Difficulty:  Unknown 
  Os:  Unknown/Multiple | Testcase:  
Architecture:  Unknown/Multiple |  Failure:  None/Unknown
+---
Changes (by kfrdbs):

 * cc: kfr...@… (added)


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Re: [GHC] #2357: Implement the Haskell' proposal for polymorphic pattern bindings

[GHC]

#2357: Implement the Haskell' proposal for polymorphic pattern bindings
+---
Reporter:  simonmar |Owner:  
Type:  task |   Status:  new 
Priority:  normal   |Milestone:  6.14.1  
   Component:  Compiler (Type checker)  |  Version:  6.8.2   
Keywords:   |   Difficulty:  Unknown 
  Os:  Unknown/Multiple | Testcase:  
Architecture:  Unknown/Multiple |  Failure:  None/Unknown
+---
Changes (by simonmar):

  * failure:  => None/Unknown
  * milestone:  6.12.3 => 6.14.1


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Re: [GHC] #2357: Implement the Haskell' proposal for polymorphic pattern bindings

[GHC]

#2357: Implement the Haskell' proposal for polymorphic pattern bindings
-+--
 Reporter:  simonmar |  Owner: 
 Type:  task | Status:  new
 Priority:  normal   |  Milestone:  6.12 branch
Component:  Compiler (Type checker)  |Version:  6.8.2  
 Severity:  normal   | Resolution: 
 Keywords:   | Difficulty:  Unknown
 Testcase:   |   Architecture:  Unknown
   Os:  Unknown  |  
-+--
Changes (by igloo):

  * milestone:  6.10.1 => 6.12 branch

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Re: [GHC] #2357: Implement the Haskell' proposal for polymorphic pattern bindings

[GHC]

#2357: Implement the Haskell' proposal for polymorphic pattern bindings
-+--
 Reporter:  simonmar |  Owner: 
 Type:  task | Status:  new
 Priority:  normal   |  Milestone:  6.10.1 
Component:  Compiler (Type checker)  |Version:  6.8.2  
 Severity:  normal   | Resolution: 
 Keywords:   | Difficulty:  Unknown
 Testcase:   |   Architecture:  Unknown
   Os:  Unknown  |  
-+--
Comment (by simonpj):

 See also #2187

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