Add a TrimmedString.trim method

[ceedubs]

This is somewhat similar to FiniteStringOps.truncate in that it's
guaranteed to turn any String into a TrimmedString.

There are some funny characters that get removed by String.trim but
don't match on \s in regular expressions. I've added an example to
demonstrate that these characters can lead to a String being "trimmed"
a bit more than would technically be necessary to match the
TrimmedString regex.

[ceedubs]

Oops I have a broken test :(

[ceedubs]

Strings are hard. I'm not sure what to do here.

[fthomas]

So the culprit is \u0085. The regex engine and String#trim disagree about if it is whitespace or not. It is for the regex engine but not for String#trim. Hmm... let me think about that.

[fthomas]

So String#trim removes all leading and trailing characters with a code less than or equal to \u0020. I think we can replace \s in the regex of TrimmedString to [\x{0000}-\x{0020}] to mimic the semantics of String#trim. I'm in favor of doing this although it changes the semantics of the refined type. But breaking the expectation that every s.trim is a valid TrimmedString is much worse IMO.

[ceedubs]

@fthomas thanks for the guidance!

I pushed 9a9a877 which hopefully addresses the issue. I used your advice but also had to make an additional slight change. See the commit message for more details.

[ceedubs]

Okay I've upgraded to a NullPointerException as my build failure. Hmmm.

[ceedubs]

I'm not sure what the build failure is about. Any ideas? Also locally I have a change to add a scalacheck generator for trimmed strings -- should I add that here or would you prefer that as a separate PR afterwards?

[fthomas]

The test passes on the JVM but fails with JS. Oh no. :-( This is not the first time that we have difficulties writing regexes that should work on the JVM and with JS (see #453 (comment) and #357 (comment)). I've no idea if the current regex can be changed to work on both platforms. I'm at a point where I would suggest adding a Trimmed predicate that is implemented with String#trim and redefine TrimmedString as String Refined Trimmed. I bet this would be easier than changing the regex to work on both platforms and it would probably be more robust because it doesn't depend on the peculiarities of the underlying regex engine.

I'm sorry this is not a straightforward change as it should have been.

[fthomas]

Instead of adding Trimmed now you could also make the failing test JVM-only, and we add that predicate later. I could imagine that the original definition of TrimmedString already does not work with JS which is possible because there are currently no tests. So merging this PR now would not make anything worse.

WRT the ScalaCheck instances I'm fine with both.

[ceedubs]

Thanks again for the guidance @fthomas! I'm inclined to agree with your suggestion of adding a Trimmed predicate. I'll plan on making that change. I'll go ahead and close this PR out and start a fresh new one.