[Haskell-cafe] Read lines of a file into a Vector

Oleg Grenrus oleg.grenrus at iki.fi
Thu Aug 11 07:27:14 UTC 2016

Thanks to the lazy IO, deforestration and fusion framework in `Vector`,

    readLines h = fromList . lines <$> hGetContents h

shouldn’t create intermediate lists or/and read the whole file at once into memory.

- Oleg

> On 11 Aug 2016, at 10:03, Jake <jake.waksbaum at gmail.com> wrote:
> I want to write a function that will read the contents of a file and return a Vector of its lines:
> readLines :: Handle -> IO (Vector String)
> I have an implementation that works but seems to naive and inefficient to me because it reads the entire file contents, then splits it into lines, and then converts it to a Vector from a list.
> readLines h = fromList . lines <$> hGetContents h
> I would like to a) use hGetLine and continue until I get an isEOFError and b) read directly into a Vector instead of a list. I started something using Data.Vector.unfoldr but I the presence of the IO monad around my String was causing issues I couldn't figure out how to solve.
> I'd also be curious to see how to do either one of these things separately and I assume they'd each help make my program more efficient on their own. Or, maybe my implementation is not as terrible as I thought because some laziness/fusion/optimization magic is happening behind the scenes?
> Thanks,
> Jake Waksbaum
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