Re: [GHC] #1518: Make it possible to evaluate monadic actions when assigning record fields (-)

2007-07-22 Thread GHC 
#1518: Make it possible to evaluate monadic actions when assigning record fields
(-)
---+
Reporter:  [EMAIL PROTECTED]  |Owner: 
Type:  feature request |   Status:  new
Priority:  normal  |Milestone:  _|_
   Component:  Compiler|  Version:  6.6.1  
Severity:  normal  |   Resolution: 
Keywords:  |   Difficulty:  Unknown
  Os:  Unknown | Testcase: 
Architecture:  Unknown |  
---+
Changes (by igloo):

  * milestone:  = _|_

Comment:

 This is being discussed on the Haskell' list:
 http://www.haskell.org/pipermail/haskell-prime/2007-July/002265.html

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/1518
GHC http://www.haskell.org/ghc/
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Re: [GHC] #1518: Make it possible to evaluate monadic actions when assigning record fields (-)

2007-07-13 Thread Adde 
On Mon, 2007-07-09 at 23:28 +, GHC wrote:
 #1518: Make it possible to evaluate monadic actions when assigning record 
 fields
 (-)
 ---+
 Reporter:  [EMAIL PROTECTED]  |Owner: 
 Type:  feature request |   Status:  new
 Priority:  normal  |Milestone: 
Component:  Compiler|  Version:  6.6.1  
 Severity:  normal  |   Resolution: 
 Keywords:  |   Difficulty:  Unknown
   Os:  Unknown | Testcase: 
 Architecture:  Unknown |  
 ---+
 Comment (by [EMAIL PROTECTED]):
 
  If you wrote:
  {{{
  return Bar {
   barFoo - foo
   barQuux - quux
  }
  }}}
  which do you think should be executed first -- computation foo or
  computation quux?
 
 -- 
 Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/1518
 GHC http://www.haskell.org/ghc/
 The Glasgow Haskell Compiler

I would expect them to be executed in the declared order, why?

/Adde

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Re: [GHC] #1518: Make it possible to evaluate monadic actions when assigning record fields (-)

2007-07-13 Thread Tim Chevalier 

On 7/9/07, Adde [EMAIL PROTECTED] wrote:

On Mon, 2007-07-09 at 23:28 +, GHC wrote:
 #1518: Make it possible to evaluate monadic actions when assigning record 
fields
 (-)
 ---+
 Reporter:  [EMAIL PROTECTED]  |Owner:
 Type:  feature request |   Status:  new
 Priority:  normal  |Milestone:
Component:  Compiler|  Version:  6.6.1
 Severity:  normal  |   Resolution:
 Keywords:  |   Difficulty:  Unknown
   Os:  Unknown | Testcase:
 Architecture:  Unknown |
 ---+
 Comment (by [EMAIL PROTECTED]):

  If you wrote:
  {{{
  return Bar {
   barFoo - foo
   barQuux - quux
  }
  }}}
  which do you think should be executed first -- computation foo or
  computation quux?

 --
 Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/1518
 GHC http://www.haskell.org/ghc/
 The Glasgow Haskell Compiler

I would expect them to be executed in the declared order, why?



But when? At the point when the return Bar{...} expression is
demanded? OK, but then you're making the Bar constructor strict when
it wouldn't be otherwise. But if you let Bar be lazy as it normally
is, then you get effectful expressions being evaluated at
unpredictable times (e.g., in my example, maybe the foo field will
never be demanded and the quux field will be), which is generally
considered harmful.

Cheers,
Tim

--
Tim Chevalier* catamorphism.org *Often in error, never in doubt
I haven't got the slightest idea how to change people, but still I
keep a long list of prospective candidates just in case I should ever
figure it out.  --David Sedaris
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[GHC] #1518: Make it possible to evaluate monadic actions when assigning record fields (-)

2007-07-09 Thread GHC 
#1518: Make it possible to evaluate monadic actions when assigning record fields
(-)
-+--
  Reporter:  [EMAIL PROTECTED]  |  Owner: 
  Type:  feature request | Status:  new
  Priority:  normal  |  Milestone: 
 Component:  Compiler|Version:  6.6.1  
  Severity:  normal  |   Keywords: 
Difficulty:  Unknown | Os:  Unknown
  Testcase:  |   Architecture:  Unknown
-+--
It is currently not possible to build records from values resulting from
 monadic actions while still using the field-specifiers.

 foo :: IO Int
 ...

 data Bar = Bar {
   barFoo :: Int
 }

 buildBar :: IO ()
 buildBar = do
   return Bar {
 barFoo - foo --Evaluate foo to get the Int-value
   }

 I've found two possible ways of doing this:

 1) Using temporaries to evaluate the actions before assigning which
 doubles the number of lines:
 tmp - foo
 return Bar {
   barFoo = tmp
 }

 2) Lifting the record constructor which prevents you from using field
 specifiers (and you really need field specifiers when dealing with larger
 records):
 liftM Bar foo

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/1518
GHC http://www.haskell.org/ghc/
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Re: [GHC] #1518: Make it possible to evaluate monadic actions when assigning record fields (-)

2007-07-09 Thread GHC 
#1518: Make it possible to evaluate monadic actions when assigning record fields
(-)
---+
Reporter:  [EMAIL PROTECTED]  |Owner: 
Type:  feature request |   Status:  new
Priority:  normal  |Milestone: 
   Component:  Compiler|  Version:  6.6.1  
Severity:  normal  |   Resolution: 
Keywords:  |   Difficulty:  Unknown
  Os:  Unknown | Testcase: 
Architecture:  Unknown |  
---+
Comment (by [EMAIL PROTECTED]):

 If you wrote:
 {{{
 return Bar {
  barFoo - foo
  barQuux - quux
 }
 }}}
 which do you think should be executed first -- computation foo or
 computation quux?

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/1518
GHC http://www.haskell.org/ghc/
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Re: [GHC] #1518: Make it possible to evaluate monadic actions when assigning record fields (-)

2007-07-09 Thread Neil Mitchell 

Hi Adde,


#1518: Make it possible to evaluate monadic actions when assigning record fields

It is currently not possible to build records from values resulting from
 monadic actions while still using the field-specifiers.


This is quite a significant change to the language, which isn't
precise enough. Perhaps if you discussed it on the haskell-cafe@ or
haskell-prime@ mailing lists first you might be able to firm up what
you are asking for, including the desugaring rules etc. You might also
find that people have developed something to get round this.

Thanks

Neil
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