Use hpack's phrasing in dhall to write cabal files.
Install with cabal install hpack-dhall or install from
source.
There are two main reasons why you'd use hpack-dhall, convenience and safety.
Get the convenience of hpack. Don't bother to state what can be inferred or
defaulted, easing the burden of completing a package description by hand. For
example other-modules can be inferred by taking the set difference between
modules on disk and the set of exposed-modules.
Get the safety of dhall's programmable configuration: typed fields, safe imports and functions.
Another convenience is that files are easily formatted. The cabal file is formatted on write and the package dhall file can be formatted too with:
> dhall format --inplace package.dhall
We use hpack phrasing. The vocabulary of hpack's fields and values differs only slightly from cabal's properties. They're close Haskell packaging dialects.
This very package is described in
package.dhall
and produces
hpack-dhall.cabal
with the command:
> dhall-hpack-cabal
generated hpack-dhall.cabal
The generated cabal file can then inform both stack and cabal-install what modules to package into libraries, executables and test suites.
There is more you can do.
If you use stack to generate a .cabal file from a package.yaml file, you
can also use hpack to do the same. The dhall-hpack-cabal executable will go
from .dhall to .cabal but you can use the dhall-hpack-yaml executable to
generate package.yaml from a package.dhall. That's how we'll manage the
conversion.
If you already have a package.yaml file then you can rename this as
package.dhall, do the conversion to .dhall syntax manually, run dhall format package.dhall to catch syntax errors and then run dhall-hpack-yaml > package.yaml and keep iterating until the generated package.yaml is as close
to the original as you need it to be, using something like git diff to check
for differences.
It will be harder to write a package.dhall file from scratch than it is to
convert from a package.yaml file but it is doable. All of the field names in
.dhall are the same as for .yaml. Start small and keep iterating with
dhall-hpack-cabal until the generated .cabal file is like the original. You
may need to go back to the original to change the formatting and reorder fields
to make comparisons with the generated .cabal file easier.
Value the dhall file as a way to produce the cabal file. If both files are be checked into source control but the cabal file gets ahead of the dhall file then it should be possible to get them in sync again.
