Class communication tool

To facilitate class communication (e.g., course announcements, Q&A) and course logistics such as assignment submissions, the instructor created a private Slack team. Slack is a team communication tool initially popularized by tech startups and more recently adopted by educators. Compared to a traditional threaded discussion format, Slack's key strength for fostering learner discourse is its free-flow style of communication. Specifically, instead of having students posting in a thread each week, the class hosted several Slack channels for different general purposes (e.g., discussion of course concepts, assignment submission and feedback, general sharing of 'cool stuff'). The linear progression of discourse in each channel helps participants stay current on the most recent posts while they could also easily revitalize an idea posted weeks earlier (Goal 2), mitigating boundaries set by traditional discussion threads. In addition, various social features in Slack, such as mentioning a user and reacting to a post with emojis, could benefit socio-emotional ties in this online course. 
Slack is not merely an alternative to discussion forums. Its support for file sharing and management enables students to contribute various types of conceptual artifacts (e.g., an essay, a code snippet, a Google Doc) flexibly to the whole class, a specific channel, or an individual (Goals 1 & 4). In addition, Slack's open API brings about a wide range of plugins that serves various purposes ranging from integrations with external tools (e.g., Hypothesis, Twitter), chat bots, to GIF importers. These possibilities, many of which were untapped in this design case, offers interoperability and personalization capabilities unseen in most LMS solutions \cite{Brown2015-wh}.
To support privacy and intimacy of formal course participants, the Slack team was set to private and not accessible to open participants (Goal 4). Students submitted course assignments in a dedicated "assignments" channel and made their submissions visible to all Slack members. This design decision was made based on the course context where assignments were largely personalized based on each student's research interests, access to datasets, and familiarity with different tools. So posting assignments publicly in the private Slack space would less likely jeopardize independent work. As a matter of fact, such openness in the closed environment benefited the collaborative discourse, as students quickly started to share project ideas and codes and commented on each other's work actively. Even though they were working on distinctive course projects, their overlapping problem spaces offered them motivations and opportunities to collaborate in various manners, including debugging understanding of key concepts, sharing generic codes, and critiquing each other's project findings. Moreover, the class' Hypothesis activities were synced into a dedicated Slack channel named "Hypo Feed" via the Hypothesis' API; as such, social annotations containing the course hashtag (made by both formal or open participants) were notified on Slack as well, and Slack participants could react to a Hypothesis annotation by following a link provided in the notification (Goals 2, 3 & 4). 
Table 1 summarizes how these three components came together to achieve the design goals, and Figure 3 illustrates course activities collectively supported by these components. Overall, the Bookdown course website served as a portal bringing various web objects into learner discourse and enabled two pathways of participation. Hypothesis supported interactions on various web objects (e.g., course textbook) in the form of web annotations, allowed ideas posted in annotations to be shared elsewhere, facilitated varied mechanisms to review and participate in the discourse, and promoted two participation pathways as well as boundary-crossing between them. Slack provided a replacement of traditional threaded discussion forums but with stronger support for participants to monitor discourse, revisit earlier ideas, and repurpose ideas in the ever-evolving discourse process. Slack also provided entry points to the discourse by integrating Hypothesis annotations and further supports internal public-private boundary-crossing in the formal pathway. Beyond these three tools, a standalone gradebook was used to keep track of student grades, a feature commonly found in an LMS.