Re: [GHC] #7479: ArrowChoice unit law in haddock seems to be wrong

2012-12-06 Thread GHC 
#7479: ArrowChoice unit law in haddock seems to be wrong
+---
  Reporter:  pminten|  Owner:  
  Type:  bug| Status:  closed  
  Priority:  normal |  Milestone:  
 Component:  libraries/base |Version:  7.7 
Resolution:  fixed  |   Keywords:  
Os:  Unknown/Multiple   |   Architecture:  Unknown/Multiple
   Failure:  Documentation bug  | Difficulty:  Unknown 
  Testcase: |  Blockedby:  
  Blocking: |Related:  
+---
Changes (by igloo):

  * status:  merge = closed
  * difficulty:  = Unknown
  * resolution:  = fixed


Comment:

 Merged as 56c8295c638a03676a9be8d34195e6be945ddc2c.

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7479#comment:2
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[GHC] #7479: ArrowChoice unit law in haddock seems to be wrong

2012-12-04 Thread GHC 
#7479: ArrowChoice unit law in haddock seems to be wrong
--+-
Reporter:  pminten|  Owner:  
Type:  bug| Status:  new 
Priority:  normal |  Component:  libraries/base  
 Version:  7.7|   Keywords:  
  Os:  Unknown/Multiple   |   Architecture:  Unknown/Multiple
 Failure:  Documentation bug  |  Blockedby:  
Blocking: |Related:  
--+-
 In the haddock documentation of !ArrowChoice there is an (unnamed) law:
 {{{left f  arr Left = arr Left  f}}}. In Ross Paterson's
 [http://www.soi.city.ac.uk/~ross/papers/fop.html Arrows and Computations]
 paper there is a similar law named the unit law for !ArrowChoice that goes
 {{{pure Left  left f = f  pure Left}}} (pure in that paper is
 arr in the base library).

 Reordering the unit law from the paper and replacing pure with arr the
 unit law from the paper is {{{f  arr Left = arr Left  left f}}}
 where the law from haddock is {{{left f  arr Left = arr Left 
 f}}}. The laws are similar but left is used in different places.

 The law from haddock appears to be invalid, there does not appear to be a
 way to create an arrow f in such a way that it works with both sides of
 the equation. This can be verified by typing the following lines in GHCi:

 {{{
 import Control.Arrow
 let foo f = left f  arr Left
 let bar f = arr Left  f
 let same :: a b c - a b c - (); same _ _ = ()
 \f - same (foo f) (bar f)
 }}}

 The last line gives a type error.

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7479
GHC http://www.haskell.org/ghc/
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Re: [GHC] #7479: ArrowChoice unit law in haddock seems to be wrong

2012-12-04 Thread GHC 
#7479: ArrowChoice unit law in haddock seems to be wrong
--+-
Reporter:  pminten|  Owner:  
Type:  bug| Status:  merge   
Priority:  normal |  Component:  libraries/base  
 Version:  7.7|   Keywords:  
  Os:  Unknown/Multiple   |   Architecture:  Unknown/Multiple
 Failure:  Documentation bug  |  Blockedby:  
Blocking: |Related:  
--+-
Changes (by ross):

  * status:  new = merge


Comment:

 Thanks for spotting this.  Fixed with
 {{{
 commit 96889ddccdcfc1fb852eeb3fdce7d4124f52376d
 Author: Ross Paterson r...@soi.city.ac.uk
 Date:   Tue Dec 4 19:32:40 2012 +

 fix #7479: ArrowChoice unit law in doc comment
 }}}

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7479#comment:1
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler

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