Support SPDX license identifiers (see #292)

[sol]

• Require cabal-version: 2.2 when SPDX license identifiers are used
• Map cabal-style licenses to SPDX license identifiers when
  cabal-version is 2.2 or higher

[quasicomputational]

LGTM, one ignorable suggestion aside.

[quasicomputational]

I think it might be clearer to write these cases like this:

  Right l | isRight cabalLicense -> CanSPDX l
  Right l -> MustSPDX l
  Left  _ -> case cabalLicense of
    Right l -> CanSPDX (Cabal.licenseToSPDX l)
    Left _ -> DontTouch

Then we can also get rid of the hard-coded knocnLicenses list.

[quasicomputational]

Oh, wait, that'll go wrong because UnknownLicense exists. Ugh, annoying.

[sol]

Yes, exactly. Right now we don't touch unknown stuff so that Cabal will fail on typos in SPDX license identifiers. Otherwise e.g. GPL-2.0-olny would be converted to LicenseRef-GPL-2.0-olny, which I assume we don't want.

[sol]

I added one more test to make it more clear what's happening here: 4d23731

[quasicomputational]

CI should be fixed by just bumping the stack resolver to something recent.

[sol]

Anybody knows what the AppVeyor build issue is about? Apparently due to network?! @kazu-yamamoto

[tfausak]

Using the LTS 12.0 resolver instead of a nightly might fix the network build problem on AppVeyor.

[quasicomputational]

Oh, I do know this one. In fact I helped fix it in Cabal HEAD! commercialhaskell/stack#3944 has all the details, but the TLDR is that setting TMP: "c:\\tmp" in the AppVeyor config fixes it.

[sol]

@quasicomputational do you know if a different stackage snapshot as suggested by @tfausak will also help?

[quasicomputational]

No, I don't think it will. The problem is that the temporary directory generated by AppVeyor looks like C:\1\blah - note the backslashes, which cause problems for the configure script. Manually overriding the temporary directory works because the specific problem is a backslash followed by a number; there's a fuller fix in Cabal HEAD (swap backslashes for slashes on Windows), but this is a good workaround on AppVeyor today.

[tfausak]

Ah, yup. LTS 12 doesn't fix this problem on AppVeyor: https://ci.appveyor.com/project/TaylorFausak/rattletrap/build/446#L336

[kazu-yamamoto]

Very strange. network 2.6 should be buildable on AppVeyor: https://ci.appveyor.com/project/eborden/network/build/453

We did not change building system for 2.6.

[sol]

there's a fuller fix in Cabal HEAD (swap backslashes for slashes on Windows), but this is a good workaround on AppVeyor today.

@quasicomputational this sounds scary. I'm surprised that this wasn't included in a bug fix release yet.

[sol]

Still failing on Windows, not sure what the gist of it is...

[sol]

I oppened #295 for the failing AppVeyor build.

[sol]

Merged! @quasicomputational thanks for the review :)