[Haskell-cafe] Re: Is hGetLine lazy like hGetContents? And what about this todo item?

[Jason Dagit]

On Thu, 2010-03-25 at 21:38 +0000, Simon Marlow wrote:
> On 25/03/10 17:07, Jason Dagit wrote:
> > On Thu, 2010-03-25 at 16:13 +0000, Simon Marlow wrote:
> >> On 25/03/2010 15:40, Jason Dagit wrote:
> >>> Hello,
> >>>
> >>> I was trying to figure out if hGetLine is safe to use inside of
> >>> withFile.  Specifically, I want to return the line I read and use it
> >>> later, without inspecting it before withFile calls hClose.
> >>>
> >>> If you want to understand the concern I have, look here:
> >>> http://www.haskell.org/haskellwiki/Maintaining_laziness#Input_and_Output
> >>>
> >>> There is a bit of explanation showing that hGetContents can be
> >>> problematic with withFile.
> >>>
> >>> I can tell from reading the source of hGetContents that it uses
> >>> unsafeInterleaveIO so this make sense to me why that wiki page talks
> >>> about hGetContents:
> >>> http://www.haskell.org/ghc/docs/latest/html/libraries/base/src/GHC-IO-Handle-Text.html#hGetContents
> >>>
> >>> When I read the source of hGetLine, it is less clear to me if I need to
> >>> be concerned.  I believe it is not lazy in the sense of lazy IO above.
> >>> Could someone else please comment?
> >>> http://www.haskell.org/ghc/docs/latest/html/libraries/base/src/GHC-IO-Handle-Text.html#hGetLine
> >>
> >> Correct: it is not lazy, and it is safe to use inside withFile.
> >
> > Great!  I did a few simple tests in GHCi and it seemed safe, but I
> > wanted to be extra prudent.  Thanks.
> >
> >>
> >>> Then I notice this 'todo' item in the description:
> >>> -- ToDo: the unbuffered case is wrong: it doesn't lock the handle for
> >>> -- the duration.
> >>>
> >>> The code itself looks to me like it only handles the buffered case.
> >>> Perhaps this todo is obsolete and needs to be removed?  If it's not
> >>> obsolete, do we need to create a ticket for this?
> >>
> >> Well spotted, that comment is out of date and wrong.  There used to be a
> >> version of hGetLine written in terms of hGetChar which was used when the
> >> Handle was unbuffered, but I think I removed it in the recent rewrite.
> >
> > What is the next step for getting rid of the obsolete comment?  Did you
> > already nuke it?  If not, I could try to get a copy of the ghc repo and
> > see if I can figure out the right protocol for submitting a patch.
> 
> Already nuked in my working tree, it'll filter through into the repo in 
> due course.  The library submission process would be way overkill for that!

I received your email about 5 minutes too late :)

Feel free to delete my library submission ticket.

Thanks,
Jason
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