Implement Caret-style version range operator

[hvr]

This implements a new syntactic sugar: The version range operator
^>= which is equivalent to >= intersected with an
automatically inferred major upper bound.

This new syntax is only allowed for cabal-version: >=2.0, and
allows to describe the most common use of version
bounds more conveniently:

build-depends: foo ^>= 1.2.3.4,
               bar ^>= 1

The declaration above is exactly equivalent to

build-depends: foo >= 1.2.3.4 && < 1.3,
               bar >= 1 && < 1.1

The ^-symbol was chosen because it can serve as a mnemonic when the
> sign is rotated and interpreted as "less than ceiling"

Moreover, ^ appears to become a quasi-standard to denote morally
equivalent operator that way in other language ecosystems which similiar
to Haskell have adopted semantic versioning:

• Node: https://nodesource.com/blog/semver-tilde-and-caret/
• Bower: https://bower.io/docs/api/#install
• PHP: https://getcomposer.org/doc/articles/versions.md#caret

Ruby, on the other hand, uses a Tilde operator (~>) for that
purpose (but with a less robust semantic):

• https://blog.codeship.com/optimists-guide-pessimistic-library-versioning

And Python is currently planing to use an ~= operator:

• https://www.python.org/dev/peps/pep-0440/#compatible-release

[phadej]

LGTM. But I'll like to see tests. (which would break in parsec branch)

[23Skidoo]

/cc @dcoutts

[23Skidoo]

Would be nice to add a changelog note.

[hvr]

I've changed the syntax from ^> to ^>= because it's more accurate/self-descriptive (since it's more evident that ^+>= translates to < + >= than the original syntax, and it's easier to convert between one and the other in a text-editor) in my opinion.

@23Skidoo changelog entry added

@phadej finally brought myself to try to come up with a simple parser test... getting there took longer than it should have...

[23Skidoo]

Thanks! So I think this can be merged once Travis is green.

[unclechu]

That’s a bit confusing since in other platforms ^ has a different meaning:

semver.toComparators('^1.2.3')
// [ [ '>=1.2.3-0', '<2.0.0-0' ] ]

// compare upper limit for ~
semver.toComparators('~1.2.3')
// [ [ '>=1.2.3-0', '<1.3.0-0' ] ]

I’d prefer to see ~>= instead. But it seems it’s too late to change.

[unclechu]

Or maybe I’m missing something?

[gbaz]

Or maybe I’m missing something?

In the haskell pvp (https://pvp.haskell.org/), which predates semver btw, the first two components indicate major version, while in semver, only the first component indicates major version. So semantically the caret operator means the same thing in both cases -- picking the range up to the next major version. It just so happens that "major version" covers a different number of components.

[unclechu]

@gbaz thanks for this clarification!