Yes, I agree with this, and I'd already snuck it into the rewrite of the Enum section that I had to do anyhow.
Would you like to read the section (6.3.4) and see if you think it is better specified now? Simon | -----Original Message----- | From: Josef Svenningsson [mailto:[EMAIL PROTECTED]] | Sent: 02 November 2001 15:54 | To: Simon Peyton-Jones | Cc: Ian Lynagh; [EMAIL PROTECTED] | Subject: RE: Stack overflow | | | Hi! | | OK, so the Int instance for Enum is a bit naughty in that | [bottom..] is bottom. But then one can argue that all | instances of Enum should be naughty in this respect. The | argument is as follows and uses the default declaration of | enumFrom in the Enum class (I've added some types with the | intention to clarify a bit what's happening): | | [bottom..] :: [Integer] | = { Syntactic sugar, section 3.10 } | enumFrom bottom :: [Integer] | = { Using the default definition of enumFrom in the Enum | class } map toEnum ([fromEnum bottom ..] :: [Int]) = { | fromEnum is strict } map toEnum ([bottom..] :: [Int]) = { | according to Simon } map toEnum (bottom :: [Int]) = { map is | strict in it's second argument } bottom | | Note that I've assumed that fromEnum is strict. I'd say this | is a pretty reasonable assumption. | | The key thing here is that the Report assumes that there is | an instance Enum Int which it uses in the default | declarations. One could wish that the report gave some more | detail about the Int instance of Enum at least. Then one | could use that to derive the behavour of the other instances, | using the default definitions like I did above. | | The bottom (:-) line, from how I interpret the Report, is | that the instances of Int and Integer should at least have | the same behaviour in this respect. But exactly how they | should behave is up to the implementor. I'd vote for the | strict version. | | Cheers, | | /Josef | | | On Fri, 2 Nov 2001, Simon Peyton-Jones wrote: | | > The [1..] generates [1,1+1,1+1+1,1+1+1+1, ...] | > Those thunks all go into the heap. Then last grabs | > the last one and evaluates it, so they all go on the | > stack. Result death. | > | > The Int instance for Enum is a bit naughty in GHC: | > it is strict. That is [bottom..] is bottom rather than | > being [bottom,bottom,bottom...]. (Actually the | > Report is vague on this point; sigh.) | > | > So the Int instance evaluates as it goes, while the | > Integer instance does not. Maybe we should | > make the Integer instance strict too. | > | > Simon | > | > | > | > | -----Original Message----- | > | From: Ian Lynagh [mailto:[EMAIL PROTECTED]] | > | Sent: 02 November 2001 14:32 | > | To: [EMAIL PROTECTED] | > | Subject: Stack overflow | > | | > | | > | | > | With | > | | > | main = putStrLn $ show $ last $ zip [1..] (replicate 100000 'a') | > | | > | I get | > | | > | Stack space overflow: current size 1048576 bytes. | > | Use `+RTS -Ksize' to increase it. | > | | > | If I Change the [1..] to ([1..] :: [Int]) then it works fine. | > | | > | | > | Ian | > | | > | | > | _______________________________________________ | > | Glasgow-haskell-bugs mailing list [EMAIL PROTECTED] | > | http://www.haskell.org/mailman/listinfo/glasgow-| haskell-bugs | > | | > | > _______________________________________________ | > Glasgow-haskell-bugs mailing list [EMAIL PROTECTED] | > http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs | > | | | | _______________________________________________ | Glasgow-haskell-bugs mailing list | [EMAIL PROTECTED] | http://www.haskell.org/mailman/listinfo/glasgow-| haskell-bugs | _______________________________________________ Glasgow-haskell-bugs mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
