pkl: type specialisation for validation

[amitu]

In pkl you can write:

module AcmeCICD

class Pipeline {
  name: String(nameRequiresBranchName)?

  hidden nameRequiresBranchName = (_) ->
      if (branchName == null)
        throw("Pipelines that set a 'name' must also set a 'branchName'.")
      else true

  branchName: String?
}

timeout: Int(this >= 3)

Point to note that they have "specialised types like Int(this >= 3) or String(nameRequiresBranchName)?, the () after the type gets a checker, which can be an expression, or a reference to a function / method.

We can do the same for us:

-- record { this.age < 40 } foo:
integer { this.age > 20 } age:

We have use {} so we can put arbitrarily long expressions there.