Report on CALCO 2013

the beautiful setting of the Old Library building on the historical campus of the University of Warsaw. The first day was dedicated to the Calco Early Ideas workshop, devoted to presentation of work in progress, with a particular emphasis on talks by PhD students and young researchers. This workshop is the follower of Calco-jnr. The other workshop affiliated with Calco, Calco-Tools, is dedicated to work on tools based on algebraic and/or coalgebraic principles. This year it took place in-lined with the main conference which led to discussion during the business meeting about the possibility of making Calco-Tools a track of the main conference instead of a separate workshop. There were four invited talks: Andrej Bauer opened the conference proper with an invited talk about how effects can be treated in a strikingly elegant way within a functional setting. The ideas of Plotkin and Power, who used Lawvere theories instead of monads, have led Andrej and his collaborators to design a programming language where effects can sometimes be combined in an easier and more transparent way, as was illustrated with a number of compelling examples. The second morning session concerned coalgebrac logics and consisted of four stimulating talks. The afternoon featured the first session of Calco Tools, including a couple of talks with a strong Haskell flavour, and closed with a session on Categorical Structures. In the evening we were treated to a reception at the Tyszkiewicz-Potocki Palace on campus, featuring an official welcome by Andrzej Tarlecki. The second day started with Damien Pous' invited talk on Coalgebraic up-to techniques. Damien talked about several enhancements to the bisimulation proof method that enable shorter proofs. He illustrated the application of the new techniques with basic examples from automata theory which made the talk very accessible (but not less impressive!).