Convert union alternatives to directory tree #1757
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This only works for alternatives with suitable non-nullary constructors. For
example, running `dhall to-directory-tree` on this expression:
let UnionType : Type = < Left : Text | Right : Text >
in { foo = UnionType.Left "one", bar = UnionType.Right "two" }
will result in the output directory containing two regular files, `foo` and
`bar`, with the contents "one" and "two", respectively.
The following union type, however, cannot be converted to a directory tree:
let UnionType2 : Type = < Left : Text | Middle | Right : Text>
because the nullary `Middle` constructor has no associated value.
This change is of particular use when dealing with union types whose
constructors take values of differently structured types:
let Thinkpad : Type = { series : Text, model : Text }
let MacBookPro : Type = { year : Text }
let Laptop : Type = < TP : Thinkpad | MBP : MacBookPro >
let flywheel : Thinkpad = { series = "T", model = "460" }
let splinch : MacBookPro = { year = "2007" }
let myLaptops = {
flywheel = Laptop.TP flywheel,
splinch = Laptop.MBP splinch
}
in myLaptops
With this change, the above expression can be converted into the following
directory tree:
<output dir>
|- flywheel
| |- series
| \- model
\- splinch
\- year
Previously, this would have required creating a record structure which had an
`Optional Text` field for every possible member, something like:
let Laptop2 : Type = { series : Optional Text, model : Optional Text
, year : Optional Text }
which has the issue that it would be possible to introduce an invalid instance
of the `Laptop2` type.
sjakobi
approved these changes
Apr 21, 2020
|
@sysvinit: Thank you for contributing this! 🙂 |
|
Thanks for merging this! Do you have any idea when the next release (including this change) is likely to happen? |
|
@sysvinit: We typically release monthly, so around May 1 |
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This change adds the ability to convert union alternatives with non-nullary constructors of appropriate type into directory trees.
The commit message describes the details of the change, but my motivation here is that I'd like to build s6-rc service directories using Dhall as a source format. The
s6-rcservice directory format is described here.There are three types of
s6-rcservice directory: "longrun", "oneshot" and "bundle". I've modelled these as below (with some simplification and details omitted for brevity):I've then defined a
Servicetype, which is a sum type over the above types.I'd like to be able to construct an expression of the
Servicetype for each service in my database, and construct a record representing the database, e.g.:At this point it would be really convenient to feed the record definition into
dhall to-directory-treein order to automatically generate the service database directory tree, however the union type means that (without the change in this commit) I need to convert theServicetype into some suitable record type, with only (Optional)Textfields, and not all combinations of these fields may be valid.Hence, it would be really useful for my case to be able to convert (appropriately-typed) union types into directory trees.