Re: [Haskell-cafe] How to split this string.

Steve Horne writes:

On 02/01/2012 11:12, Jon Fairbairn wrote:

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max writes:

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I want to write a function whose behavior is as follows:

foo "string1\nstring2\r\nstring3\nstring4" = ["string1", "string2\r\nstring3", "string4"]

Note the sequence "\r\n", which is ignored. How can I do this? cabal install split

then do something like

import Data.List (groupBy) import Data.List.Split (splitOn)

rn '\r' '\n' = True rn _ _ = False

required_function = fmap concat . splitOn ["\n"] . groupBy rn

(though that might be an abuse of groupBy)

Sadly, it turns out that not only is this an abuse of groupBy, but it has (I think) a subtle bug as a result.

It does indeed. Thanks. That was pretty much what I feared.

Explanation (best guess) - the function passed to groupBy, according to the docs, is meant to test whether two values are 'equal'. I'm guessing the assumption is that the function will effectively treat values as belonging to equivalence classes. That implies some rules such as...

Right. This issue has come up from time to time since groupBy was first written, and someone pops up to justify the present behaviour, but I can never remember why.

In the context of this \r\n test function, this behaviour will I guess result in \r\n\n being combined into one group. The second \n will therefore not be seen as a valid splitting point.

Correct. In my defence, I did say “do something like” :-)

Personally, I think this is a tad disappointing. Given that groupBy cannot check or enforce that it's test respects equivalence classes, it should ideally give results that make as much sense as possible either way. That said, even if the test was always given adjacent elements, there's still room for a different order of processing the list (left-to-right or right-to-left) to give different results - and in any case, maybe it's more efficient the way it is.

Looking back at the libraries list, I get the impression that there was a suggestion to change the behaviour of groupBy, but it doesn’t seem to have happened. -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk http://www.chaos.org.uk/~jf/Stuff-I-dont-want.html (updated 2010-09-14)