How do I combine multiple TryAsync results?

[MrYossu]

Suppose I have the following methods...

TryAsync<File> GetFolder(string folderId)
TryAsync<string> CreateFolder(string folderName, string parentFolderId)
TryAsync<string> UploadFile(Stream file, string fileName, string mimeType, string folderId)

...where File is a Google type for files/folders in a Google Drive.

I can call each of these individually, no problem. However, I sometimes want to call more than one, say to get the File object for a specific folder, then create a new subfolder and upload a file to it. I know I can do this as follows (air code, so please ignore any typos)...

(await GetFolder("123"))
  .Match(async folder => {
    (await CreateFolder("New folder", folder.Id))
      .Match(async newFolder => {
        (await UploadFile(stream, "New file name.txt", "text/text", newFolder.Id))
          .Match(fileId => /* do whatever with the uploaded file's Id */, ex => /* report exception */);
      }, ex => /* report exception */);
  }, ex => /* report exception */);

As you can see, this is very painful. I am sure that you are supposed to be able to chain monads together, I think using Bind, so you end up with something more like this (again, air code)...

(await GetFolder("123"))
  .Bind(folder => CreateFolder("New folder", folder.Id))
  .Bind(newFolder => UploadFile(stream, "New file name.txt", "text/text", newFolder.Id))
  .Match(fileId => /* do whatever with the uploaded file's Id */, ex => /* report exception */);

However, I can't get any code to compile like this.

Anyone able to advise how I can improve the nested monads I have? Thanks

[Malgefor]

You bind the TryAsync operations together like this:

            var bindWayResult = await GetFolder("123")
                .Bind(folder => CreateFolder("New folder", folder))
                .Bind(createdFolderResult => UploadFile(Stream.Null, "New file name.txt", "text/text", createdFolderResult))
                .Match(successValue => successValue, exception => $"Exception: {exception.Message}");
            
            var linqWayOperation =
                from folder in GetFolder("123")
                from createdFolderResult in CreateFolder("New folder", folder)
                from uploadFileResult in UploadFile(Stream.Null, "New file name.txt", "text/text", createdFolderResult)
                select uploadFileResult;

            var linqWayOperationResult = await linqWayOperation
                .Match(successValue => successValue, exception => $"Exception: {exception.Message}");

You can also use Aff instead of TryAsync. I think this is the way going forward since release 4 of Language.Ext.
Here is more documentation

[MrYossu]

@Malgefor Thanks for the reply, but unfortunately, it didn't work. I copied your first example (as I prefer the method syntax), and changed the second parameter to the CreateFolder method to folder.Id, as the CreateFolder method returns a File object, but CreateFolder expects a string (the folder Id) as its second parameter. This gave...

var bindWayResult = await GoogleDriveHelper.GetFolder("123")
  .Bind(folder => GoogleDriveHelper.CreateFolder("New folder", folder.Id))
  .Bind(createdFolderResult => GoogleDriveHelper.UploadFile(Stream.Null, "New file name.txt", "text/text", createdFolderResult))
  .Match(successValue => successValue, exception => $"Exception: {exception.Message}");

However, that gave me a compiler error on the first call to Bind...

The type arguments for method 'TryAsyncExtensions.Bind<A, B>(TryAsync, Func<A, TryAsync>)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

Hovering over the folder parameter at the beginning of the lambda shows it to be of the File type

Any ideas? Thanks again.

[Malgefor]

When I copy your code it compiles just fine. I think the problem is something else than the Bind-method.

• Are you sure that you are using the same File-type everywhere? Not accidentally a mix between the Google-type and System.IO.File?
• Are the method signatures still the same as in your original question? What is the return type of CreateFolder in your first Bind-call?

[MrYossu]

@Malgefor Duh, you got it! I had forgotten that when I first wrote these methods, I went through a few iterations of using the wrong monads. The previous iteration returned an Either<Exception, T>. I then realised that TryAsync catches and handles the exception for you, reducing the amount of code you need. I converted all the methods except one to use TryAsync, but had trouble with CreateFolder as I couldn't work out how to write it. I ended up leaving it returning an Either<Exception, T>, but copied the wrong bit of code into my question.

With a dummy body for CreateFolder, the code you showed compiles fine. All I need to do now is work out how to convert that last method to use TryAsync.

Thanks again. If I get stuck on converting, I'll open a new issue.

[MrYossu]

@Malgefor Got stuck with that last method. I opened a new issue, so if there's any chance you could take a look, I'd really appreciate it.

Thanks again