Compile empty association list to object rather than list?

[wbadart]

Right now, unless --noMaps is specified, we expect dhall-to-json and dhall-to-yaml to compile values of type List { mapKey = t, mapValue = u } to objects. This works when the list is populated, but it seems that whatever process transforms non-empty association lists to objects is skipped for empty ones, so you end up with a different type in the compiled file. For example:

$ dhall-to-json --version
1.5.0

$ dhall-to-json <<< '[{ mapKey = "foo", mapValue = "bar" }]'
{
  "foo": "bar"
}

$ dhall-to-json <<< '[] : List { mapKey : Text, mapValue : Text }'
[]

Expected:

$ dhall-to-json <<< '[] : List { mapKey : Text, mapValue : Text }'
{}

If this is actually the desired behavior, please let me know! But if it is, I think it would still make sense to add this behavior as an option (maybe add a command line flag like --transformEmptyAssocs or something).

There are definitely practical applications of this new behavior. In my case, I'm compiling a docker compose configuration where I represent services and networks and such as association lists, e.g.

{ services = [
  { mapKey = "my_service"
  , mapValue = Service::
    { image = "dhallhaskell/dhall-json"
    , ports = [ "8000:80" ]
    }
  } ]
, networks = [
  { mapKey = "my_net", mapValue = Network.default } ]
}

==>

services:
  my_service:
    image: "dhallhaskell/dhall-json"
    ports: [ "8000:80" ]
networks:
  my_net:
    driver: overlay

In my case, if no, say, networks have been specified, it needs to be an object rather than a list, or docker will reject it:

networks: {}

I have a work-around using Optional, but the ergonomics are dramatically better if I can allow networks = [] : List { mapKey = Text, mapValue = Network.Type } to be empty.

[sjakobi]

Thanks for the report! Looks like a regression back from 0ee6ce6.

[wbadart]

Glad to hear my intuition on that behavior was on track. Thanks for the quick turnaround!