Port

interface ITaxInfo {
  async GetTaxInfo(sin: SIN) -> TaxInfo?
  
  class Error {}
}

Port Tests

abstract class TaxInfoTests(subject: ITaxInfo) {
  Null_when_SIN_does_not_exist() {
    result = subject.GetTaxInfo(A_SIN_WHICH_DOES_NOT_EXIST);
    Assert.IsNull(result)
  }

  class Fake : TaxInfoTests(InMemoryTaxInfo()) {}

  @Explicit("Integration test")
  class Live : TaxInfoTests(CanadianRevenueAgencyTaxInfo()) {}
}

Real adapter

class CanadianRevenueAgencyTaxInfo : ITaxInfo? {

  async ITaxInfo.GetTaxInfo(sin: SIN) -> TaxInfo? => {
    val response = await HTTP.Request("http://cra.ca/taxinfo/?sin={sin}")
    if (!response.success) return None
    
    return Decode(await response.ReadContent());
  }
}

Simulators

class InMemoryTaxInfo : ITaxInfo {
  public TaxInfosBySSN: Map<SIN, TaxInfo> = {
    A_SIN -> A_TAX_INFO,
    ANOTHER_SIN -> ANOTHER_TAX_INFO,
  }
  
  async ITaxInfo.GetTaxInfo(sin: SIN) -> TaxInfo? => TaxInfosBySSN.TryGet(sin)
}

class NoTaxInfo : ITaxInfo {
  async ITaxInfo.GetTaxInfo(sin: SIN) -> TaxInfo? => None
}

class VerySlowTaxInfo : InMemoryTaxInfo {
  async ITaxInfo.GetTaxInfo(sin: SIN) -> TaxInfo? => {
    await Async.Wait(TimeSpan.Minutes(10));
    return await base.GetTaxInfo(sin);
  }
}

class OfflineTaxInfo : ITaxInfo {
  async ITaxInfo.GetTaxInfo(sin: SIN) -> TaxInfo? => throw ITaxInfo.Error()
}