Haskell sentence segregation
While your code works and uses type signatures, it's missing documentation. It's not clear from your description or your code what splitter's intended result will be on a given input. Documentation and tests are therefore highly welcome.

Also, it's not clear why you've added an underscore to replace_. And your code is missing at least one include for =~. I assume that you just forgot to include that import line in your question and it is in your actual code.

That being said, the fullstop library is—according to its own documentation—a placeholder library:


  Note that this package is mostly a placeholder. I hope the Haskell/NLP
  communities will run with it and upload a more sophisticated (family
  of) segmenter(s) in its place. Patches (and new maintainers) would be
  greeted with delight!


Your quarrel about the line endings also comes from segment, since it hard-codes the allowed punctuations:

-- https://hackage.haskell.org/package/fullstop-0.1.4/docs/src/NLP-FullStop.html#stopPunctuation
stopPunctuation :: [Char]
stopPunctuation = [ '.', '?', '!' ] -- &lt;&lt;&lt;&lt;


Unfortunately, you cannot expand stopPunctuation, since content in parentheses (like this) does not lead to a new sentence. Note that .) and ." aren't valid in some languages, though, they require ). and "., so it's not clear what you try to achieve there (see comment above documentation above).

So all in all, well written, but without additional explanation or documentation there is no way to check whether the function actually does what you want. I also suggest you to add some tests.