Re: [GHC] #2470: read for StdGen fails for arbitrary string

[GHC]

#2470: read for StdGen fails for arbitrary string
--+-
 Reporter:  guest |  Owner: 
 Type:  bug   | Status:  closed 
 Priority:  normal|  Milestone: 
Component:  libraries/random  |Version:  6.8.3  
 Severity:  normal| Resolution:  invalid
 Keywords:  StdGen read   | Difficulty:  Unknown
 Testcase:|   Architecture:  x86
   Os:  Windows   |  
--+-
Comment (by Isaac Dupree):

 The Haskell 98 report is buggy; that behavior is unimplementable when
 applying (`read::String->StdGen`) to an infinite list.  Actually, no,
 that's not quite right.  (`readsPrec::Int->ReadS StdGen`) ==
 (`readsPrec::Int->String->[(StdGen,String)]`).  So readsPrec could be
 defined to read only a finite amount of the string, and unconditionally
 return `[(result,"")]` so that "read" always succeeded.  But that breaks
 the read/show identities (e.g. `(read.show)::[StdGen]->[StdGen]` would
 break), and in general breaks the expected behavior for the `reads`
 functions, so I don't think it's worth it to pretend the standard isn't
 broken there.

 -Isaac

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Re: [GHC] #2470: read for StdGen fails for arbitrary string

[GHC]

#2470: read for StdGen fails for arbitrary string
--+-
 Reporter:  guest |  Owner: 
 Type:  bug   | Status:  closed 
 Priority:  normal|  Milestone: 
Component:  libraries/random  |Version:  6.8.3  
 Severity:  normal| Resolution:  invalid
 Keywords:  StdGen read   | Difficulty:  Unknown
 Testcase:|   Architecture:  x86
   Os:  Windows   |  
--+-
Comment (by ross):

 Replying to [comment:2 igloo]:
 > Strings that are the result of showing a StdGen work fine as part of
 structures [...]
 >
 > For reading a random string you can use `reads` to avoid the exception.

 The description quoted in the bug report, including "It guarantees to
 succeed on any string" is in the Haskell 98 Report.  Not implementing that
 would be a bug, wouldn't it?

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Re: [GHC] #2470: read for StdGen fails for arbitrary string

[GHC]

#2470: read for StdGen fails for arbitrary string
--+-
 Reporter:  guest |  Owner: 
 Type:  bug   | Status:  closed 
 Priority:  normal|  Milestone: 
Component:  libraries/random  |Version:  6.8.3  
 Severity:  normal| Resolution:  invalid
 Keywords:  StdGen read   | Difficulty:  Unknown
 Testcase:|   Architecture:  x86
   Os:  Windows   |  
--+-
Comment (by Isaac Dupree):

 IMHO it would be appropriate to specify in the documentation that
 (read::String->StdGen) is only guaranteed to work on the result of
 (show::StdGen->String).  The "read 6 characters" can be mentioned as an
 implementation detail.  (Then it's just like all other read/show pairs: it
 works fine in tuples.)  That leaves us free to / forces us to come up with
 a more proper suite of functions for initializing a generator (for
 example, taking all the bits in a finite list into account, initializing
 from an Int, from an Integer, etc.)

 -Isaac

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Re: [GHC] #2470: read for StdGen fails for arbitrary string

[GHC]

#2470: read for StdGen fails for arbitrary string
--+-
 Reporter:  guest |  Owner: 
 Type:  bug   | Status:  closed 
 Priority:  normal|  Milestone: 
Component:  libraries/random  |Version:  6.8.3  
 Severity:  normal| Resolution:  invalid
 Keywords:  StdGen read   | Difficulty:  Unknown
 Testcase:|   Architecture:  x86
   Os:  Windows   |  
--+-
Comment (by guest):

 igloo:  would it be appropriate to document the "read 6 characters"?  Just
 a final thought.  At a minimum, my interpretation of the phrase "using
 different strings is likely to result in different generators" is strongly
 violated by a dependency on the first six characters, unless that's
 documented.

 No worries either way.

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Re: [GHC] #2470: read for StdGen fails for arbitrary string

[GHC]

#2470: read for StdGen fails for arbitrary string
--+-
 Reporter:  guest |  Owner: 
 Type:  bug   | Status:  closed 
 Priority:  normal|  Milestone: 
Component:  libraries/random  |Version:  6.8.3  
 Severity:  normal| Resolution:  invalid
 Keywords:  StdGen read   | Difficulty:  Unknown
 Testcase:|   Architecture:  x86
   Os:  Windows   |  
--+-
Changes (by igloo):

  * status:  new => closed
  * difficulty:  => Unknown
  * resolution:  => invalid

Comment:

 Strings that are the result of showing a StdGen work fine as part of
 structures:
 {{{
 Prelude System.Random> let x = (mkStdGen 100, mkStdGen 200)
 Prelude System.Random> x
 (101 1,201 1)
 Prelude System.Random> let y = show x
 Prelude System.Random> y
 "(101 1,201 1)"
 Prelude System.Random> let z = read y :: (StdGen,StdGen)
 Prelude System.Random> z
 (101 1,201 1)
 Prelude System.Random>
 }}}

 For reading a random string you can use `reads` to avoid the exception.

 We've discussed this before, although I can't find the ticket now (it
 might have been on a mailing list or something), and concluded that given
 the "finite portion" requirement the current behaviour is the best.

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Re: [GHC] #2470: read for StdGen fails for arbitrary string

[GHC]

#2470: read for StdGen fails for arbitrary string
-+--
Reporter:  guest |Owner: 
Type:  bug   |   Status:  new
Priority:  normal|Milestone: 
   Component:  libraries/random  |  Version:  6.8.3  
Severity:  normal|   Resolution: 
Keywords:  StdGen read   | Testcase: 
Architecture:  x86   |   Os:  Windows
-+--
Comment (by guest):

 With further thought, I wonder if I misunderstand the specification.  From
 reading the code, it looks like the read instance is using the first 6
 characters to construct the StdGen result.  However, because the rest of
 the string is "left over" and unprocessed, the read call fails whenever
 there are more than 6 characters in the passed string.

 Two notes:  (1) The number 6 is undocumented, making the behavior of read
 incompehensible without examining the code; (2) it's unclear how this
 specification interacts with compound structures, like the derived
 instance of read :: String -> (StdGen,StdGen).  What would we expect read
 "(longword,longerword,longestword)" :: (StdGen,StdGen) to do?  How do we
 know which commas mean what?

 If whoever looks at this decides this is an enhancement rather than a bug,
 I won't be too terribly hurt.  Again, you can contact me at
 [EMAIL PROTECTED]  Exposing stdFromString (perhaps with a different
 name) might make everything cleaner.

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[GHC] #2470: read for StdGen fails for arbitrary string

[GHC]

#2470: read for StdGen fails for arbitrary string
+---
Reporter:  guest|   Owner:  
Type:  bug  |  Status:  new 
Priority:  normal   |   Component:  libraries/random
 Version:  6.8.3|Severity:  normal  
Keywords:  StdGen read  |Testcase:  
Architecture:  x86  |  Os:  Windows 
+---
 According to the docs for StdGen:

 "In addition, read may be used to map an arbitrary string (not necessarily
 one produced by show) onto a value of type StdGen. In general, the read
 instance of StdGen has the following properties:

 * It guarantees to succeed on any string.
 * It guarantees to consume only a finite portion of the string.
 * Different argument strings are likely to result in different
 results."

 Here's what happens on my system:

 C:\odm\ghc\lib>ghci
 GHCi, version 6.8.3: http://www.haskell.org/ghc/  :? for help
 Loading package base ... linking ... done.
 Prelude> :mod +System.Random
 Prelude System.Random> (read "tasty tofu") :: StdGen
 Loading package old-locale-1.0.0.0 ... linking ... done.
 Loading package old-time-1.0.0.0 ... linking ... done.
 Loading package random-1.0.0.0 ... linking ... done.
 *** Exception: Prelude.read: no parse
 Prelude System.Random>

 You can contact me at [EMAIL PROTECTED] if you need more information.

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