Add support for cabal-version field values in the form ">= 1.10"

[jneira]

Hi, it seems dhall-to-cabal dont support the >= 1.10 value in the field, only a simple version like 2.0. However cabal files referencing cabal specs below 1.12 must use the form >= 1.10. Although packages should update those versions i guess there are many of them using it yet. Not sure about how to implement it in dhall-to-cabal and cabal-to-dhall:

• an option could change the dhall type of cabal-version to < Left : ./Version.dhall | Right : ./VersionRange.dhall >
  • like the cabal lib underlying type specVersionRaw :: Either Version VersionRange
  • but in fact cabal only allow simple versions or >= 1.10

Thanks!

[ocharles]

Hi. You're right, dhall-to-cabal only takes inputs where the cabal version is exactly specified. I've done this because of this comment:

The version of the Cabal spec that this package description uses. For historical reasons this is specified with a version range but only ranges of the form >= v make sense. We are in the process of transitioning to specifying just a single version, not a range. See also specVersion.

This is on specVersionRaw here.

Before we look at solutions, I haven't entirely understand why this constraint is causing a problem. Could you elaborate as to why you need >= 1.10?

[quasicomputational]

I think the problem is that old Cabal spec versions do strictly require the >= (i.e., writing cabal-vesion: 1.10 won't work). But it probably makes more sense to have a check for being less than 1.10 and inserting the >= in that case unconditionally anyway, and not otherwise, rather than leaving it up to the user.

[ocharles]

I'm not entirely sure it's in the remit of dhall-to-cabal to do that validation (there are various ways you can construct a Cabal file that parses, but isn't accepted by cabal semantically). That said, adding the >= seems reasonable.

[jneira]

• For completeness, this is the warning from cabal check with cabal-version = 1.10:

  
  PS C:\dhall\dhall-to-cabal> cabal --version
  cabal-install version 2.2.0.0
  compiled using version 2.2.0.1 of the Cabal library
  PS C:\dhall\dhall-to-cabal> type .\dhall-to-cabal.cabal | sls "cab
  al-version"
  cabal-version: 1.10
  PS C:\dhall\dhall-to-cabal> cabal check
  The following warnings are likely to affect your build negatively:

• With Cabal 1.10 or earlier, the 'cabal-version' field must use range syntax rather than a simple version number. Use 'cabal-version: >= 1.10'.

Hackage would reject this package.

Although it is a warning the last sentence sounds really bad 😉 

Moreover, `cabal-to-dhall` does not support cabal files with `cabal-version: >= 1.10`