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...
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...
Correct: it is not lazy, and it is safe to use inside withFile.
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 Perhaps this todo is obsolete and needs to be removed?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Well spotted, that comment is out of date and wrong.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. Cheers, Simon