How to lift a pure value into the effect-space inside an async function

[dirkil]

Hi there,

I have the following function that creates an offer. In case the call succeeds it returns the Guid of the created offer, otherwise an Error. Here is the code for that:

Aff<Guid> CreateOffer(Guid mandantId, string offerJson)
{
        return Aff(async () =>
        {
            var content = new StringContent(immobilieJson, Encoding.UTF8, "application/json");
            var response = await _httpClient.PostAsync($"api/offers/{mandantId}", content);
            if (response.StatusCode == HttpStatusCode.OK)
            {
                return await response.Content.ReadFromJsonAsync<Guid>();
            }

            var errorContent = await response.Content.ReadAsStringAsync();

            var error = Error.New((int)response.StatusCode, errorContent);
            return FailAff<Guid>(error);
        });
}

Unfortunately, this code does not compile, and it has to do with the last return statement and/or the return Aff( async() =>...) at the beginning. The compilation error is:

Error CS0411 : The type arguments for method Prelude.Aff<RT, A>(Func<RT, ValueTask>) cannot be inferred from the usage.

As far as I understand it, I have to lift up a pure value into the effect space which I thought that FailAff does for me. I guess it has to do with the fact that the method must return a ValueTask but I have no idea how to do that.

Can anyone help me, please?

[harrhp]

Prelude.Aff can't return non-exceptional errors, you should use Prelude.AffMaybe for that. It expects Fin<A> as return value, but Prelude.FailAff returns Aff<A>, so you have to use Prelude.FinFail instead

Aff<Guid> CreateOffer(Guid mandantId, string offerJson)
{
  return AffMaybe(async () =>
  {
    var content = new StringContent(immobilieJson, Encoding.UTF8, "application/json");
    var response = await _httpClient.PostAsync($"api/offers/{mandantId}", content);
    if (response.StatusCode == HttpStatusCode.OK)
    {
      return await response.Content.ReadFromJsonAsync<Guid>();
    }
    var errorContent = await response.Content.ReadAsStringAsync();
    var error = Error.New((int)response.StatusCode, errorContent);
    return FinFail<Guid>(error);
  });
}

[dirkil]

Hi harrhp,

thank you very much for your answer. It solved my problem! And many thanks for the explanation. Now I know what AffMaybe is for, I have always wondered...