How to create a dhall schema with arbitrary key but typed values?

Question

In dhall, how can I create a schema for the following yaml?

environment:
    local:
        path: "/path/to/env"
        envvars:
            - var1
            - var2
    stage:
        path: "/path/to/env"
        envvars:
            - var1
            - var2
    prod:
        path: "/path/to/env"
        envvars:
            - var1
            - var2

As you can see, it has arbitrary key ("local", "stage", etc) but same type for the values. I believe this is a fairly common use case, but I didn't see it discussed in the tutorial.

Uncommitted

Answer 1

You can use Map, defined in the Prelude and discussed in the Language Tour.

let Prelude = http://prelude.dhall-lang.org/v16.0.0/package.dhall

let Environment = { path : Text, envvars : List Text }

let EnvironmentSet = { environment : Prelude.Map.Type Text Environment }

in    { environment = toMap
          { local = { path = "/path/to/env", envvars = [ "var1", "var2" ] }
          , stage = { path = "/path/to/env", envvars = [ "var1", "var2" ] }
          , prod = { path = "/path/to/env", envvars = [ "var1", "var2" ] }
          }
      }
    : EnvironmentSet

dhall-to-yaml produces the original YAML (modulo key order):

% dhall-to-yaml < tmp.dhall
environment:
  local:
    envvars:
      - var1
      - var2
    path: /path/to/env
  prod:
    envvars:
      - var1
      - var2
    path: /path/to/env
  stage:
    envvars:
      - var1
      - var2
    path: /path/to/env

---

You can also get rid of much of the boilerplate by defining some functions to create the records and maps.

let Map = http://prelude.dhall-lang.org/v16.0.0/Map/package.dhall

let Environment = { path : Text, envvars : List Text }

let makeEnv =
        λ(name : Text)
      → λ(path : Text)
      → λ(vars : List Text)
      → Map.keyValue Environment name { path = path, envvars = vars }

let makeEnvironmentSet =
      λ(envs : List (Map.Entry Text Environment)) → { environment = envs }

in  makeEnvironmentSet
      [ makeEnv "local" "/path/to/local" [ "localvar1", "localvar2" ]
      , makeEnv "stage" "/path/to/stage" [ "stagevar1", "stagevar2" ]
      , makeEnv "prod" "/path/to/prod" [ "prodvar1", "prodvar2" ]
      ]

dhall-to-yaml converts a record of type {mapKey: k, mapValue: v} to a YAML object.

Rancidity

Nice answer! Side note: Preserving the key order for Map is being discussed in github.com/dhall-lang/dhall-haskell/issues/1813. – Sardinian

Actually creating a value of type EnvironmentSet has a lot of boilerplate; I'm not sure of the best way to abstract that out to a function. – Rancidity