[Haskell] line-based interactive program
wolfgang at jeltsch.net
Sat Jul 9 11:01:06 EDT 2005
Am Samstag, 9. Juli 2005 08:31 schrieb Bulat Ziganshin:
> Hello Wolfgang,
> Friday, July 08, 2005, 11:55:48 PM, you wrote:
> > As part of my diploma thesis, I'm working on a small collection of modules
> > which provides safe I/O interleaving. The key point is to split the state
> > of the world since I/O on different parts of the world can be interleaved
> > arbitrarily. If someone is interested, I can post more details.
> yes, i am interested. i feel that this would be very interesting and useful
> for real programs
the idea is to have different monads for I/O on different resources. A simple
example is to have the two monads WorldIO and FileIO and a type FileIOHandle.
A file is a part of the world. You have the following functions:
readChar :: FileIO Char
writeChar :: Char -> FileIO ()
runFileIO :: FilePath -> FileIO a -> WorldIO (a, FileIOHandle)
finishFileIO :: FileIOHandle -> WorldIO ()
readChar and writeChar should be self-explanatory. At first, runFileIO does
nothing instead of opening the file and returning the result. Whenever parts
of the first component of the result pair are evaluated, as much readChar and
writeChar actions of the file I/O action are executed as are needed to
produce the desired parts of the result. finishFileIO executes the remainder
of the file I/O and closes the file afterwards.
An extended version of this approach shall also handle situations like pure
reading of files where not all read operations have to be carried out if they
are not needed to produce the desired result. In this case, finishFileIO
would just close the file without previously executing the remainder of the
file I/O action. The problem is that it cannot be assured that as yet
unevaluated parts of the result aren't evaluated after exeuction of
finishFileIO. Therefore, if evaluation after finishing demands the execution
of read operations these operations shall not actually be executed but
instead _|_ shall be returned.
I also plan to provide a general framework for working on parts of the state
interleaved with working on the remainder of the state. The framework shall
be hierarchical in the sense that you cannot just work on parts of the world
but also on parts of parts of the world and so on. It will probably use
multi-parameter classes to describe which thing is part of which other thing.
When my diploma thesis and the corresponding talk are finished (which will
probably at the end of September), I may post a more detailed description on
this list and also provide some code.
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