Upgrading Stack to Cabal 2.2

[Oleg Grenrus]

Hi Michael,

thanks for your comments!

- The allBuildInfo change is
https://github.com/haskell/cabal/commit/8fc10320a5dc4898927c84ad6a2dce7965ef30db,
I agree with Herbert on this. New `allBuildInfo` implementation is
correct given the name. There was even a TODO to make that change.
`reallyAllBuildInfo` would been silly. I also didn't felt ok to change
the type to `Traversal` (we have lenses, please try out them too and
tell if something is missing!). We'll highlight the change in the
changelog, as it's easy to miss.

- The lack of SPDX changelog entry is my bad. It was a series of
patches, and the changelog + manual update is still not done. I try to
summarise:
    - `cabal-version: 2.2` files use SPDX expression syntax for
`license: ` field. PackageDescription's licenseRaw will be Left expr,
expr :: Distribution.SPDX.License.License
    - `cabal-version: 2.0` files still use old `License` syntax and
type, licenseRaw is `Right l`, l :: Distribution.License.License
    - I recommend treating the field as opaque. You can `either display
display` to show it
       - Next best choice is to use `SPDX.License` as in that direction
conversion is lossless (licenseToSPDX), but have to workaround lack of
e.g. `OtherLicense` as a concept (IIRC it's converted to
LicenseRef:OtherLicense which is valid SPDX-License-Id). This might be
necessary if you plan to read (human readable) text back.

Oleg

On 20.02.2018 15:47, Michael Snoyman wrote:
> Hi all,
>
> Oleg mentioned to me on Reddit that a 2.2 branch has been cut for
> Cabal, and recommended we try to upgrade Stack to it. I'm sharing
> information here on that process in case it's useful to others, either
> as feedback on the API changes, or help for others going through
> similar upgrades. If anyone would rather that I do or do not post such
> reports in the future, let me know.
>
> I've opened a PR for this change[1], which currently is a single
> commit[2]. As you can see from the change to stack.yaml[3], I needed
> to refer to the commit of Cabal in question, and turn on `allow-newer`
> for some packages (cabal-doctest and http-api-data) with restrictive
> upper bounds on Cabal. Both of these packages compiled without issue.
>
> No change was necessary to Stack's Setup.hs file.
>
> Most of the changes so far were compile-time errors, which is a Good
> Thing. Changes I had to make:
>
> * The build type is no longer a Maybe field. (As a user: this is a
> nice change, no longer a need to guess about what a Nothing value means.)
> * There are now two License types, the original one and the SPDX
> variant. I don't understand what this change is about, some further
> explanation in the docs or the ChangeLog would definitely be
> appreciated. But hacking my way through the types seems to have worked.
> * Parsing a package description now works on a ByteString instead of a
> String. This allowed me to remove some UTF8 conversion and
> BOM-stripping code, which is very nice.
> * The new parsing API exposes the cabal file version when that
> prevented the parse (at least, that's my understanding). The changes
> to accommodate the new API were relatively trivial, so another +1 here.
> * Also: getting file position information for warnings and errors is
> _very_ nice. +1
>
> I haven't done extensive testing yet, but I've so far only found one
> bug due to runtime changes: the behavior of allBuildInfo has changed
> to now include non-buildable components. This resulted in some
> confusion until I reviewed the changelog more closely. If I could make
> a request, it would be that, in the future, new runtime behavior come
> with a new function name, to convert runtime errors into compile time
> errors. This was _not_ a particularly difficult issue to work around
> though, in large part thanks to the changelog.
>
> I'll continue testing this branch of Stack. Our plans are to merge
> this to master, and release Stack 1.7.0 close to the Cabal 2.2 release
> date.
>
> Michael
>
> [1] https://github.com/commercialhaskell/stack/pull/3878/files
> [2]
> https://github.com/commercialhaskell/stack/pull/3878/commits/a101341d04213d6dd8e0cf16d2f2fef8e7ed5cd5
> [3]
> https://github.com/commercialhaskell/stack/pull/3878/commits/a101341d04213d6dd8e0cf16d2f2fef8e7ed5cd5#diff-fafd0cdcd559a7b124cc61c29413fb54
>
>
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