When to use Match or Tap?

[lloydjatkinson]

I'm unsure where to use Tap vs Match when using Result. These two blocks are equivalent in this use case. When should one be used over the other? I'm performing side effects in two areas (logging), does that impact the decision?

foreach (var feed in feeds)
{
    var result = await this.syndicationFeedHttpClient.GetSyndicationFeedAsync(feed.Uri);

    result
        .Tap(success => successfulFeeds.Add(feed))
        .TapError(failed => failedFeeds.Add(feed));

    result
        .Tap(success => this.logger.LogInformation("Parsed feed {feedUri} [{feedItemsCount} items]", feed.Uri, successFeed.Items.Count()))
        .TapError(failed => this.logger.LogError("Failed to parse feed {feedUri}", feed.Uri));
}

foreach (var feed in feeds)
{
    var result = await this.syndicationFeedHttpClient.GetSyndicationFeedAsync(feed.Uri);

    result.Match(
        success => successfulFeeds.Add(feed),
        failed => failedFeeds.Add(feed)
    );
    
    result.Match(
        success => this.logger.LogInformation("Parsed feed {feedUri} [{feedItemsCount} items]", feed.Uri, success.Items.Count()),
        failed => this.logger.LogError("Failed to parse feed {feedUri}", feed.Uri)
    );
}

[hankovich]

The idea here is that Match is used to terminate the result chain, i.e. you can't do anything with the result after Match since it returns plain value, not result, while Tap/TapError are used to add a side effect and do not modify the result.

The provided logging example perfectly matches with the side effect use case.

And CSharpFunctionExtensions forces you to use extensions, so I'll rewrite your code to

foreach (var feed in feeds)
{
    await this.syndicationFeedHttpClient
        .GetSyndicationFeedAsync(feed.Uri);
        .Tap(success => successfulFeeds.Add(feed))
        .TapError(failed => failedFeeds.Add(feed))
        .Tap(success => this.logger.LogInformation("Parsed feed {feedUri} [{feedItemsCount} items]", feed.Uri, successFeed.Items.Count()))
        .TapError(failed => this.logger.LogError("Failed to parse feed {feedUri}", feed.Uri));
}