Fusing Direct Manipulations into Functional Programs.

Bidirectional live programming systems (BLP) enable developers to modify a program by directly manipulating the program output, so that the updated program can produce the manipulated output. One state-of-the-art approach to BLP systems is operation-based, which captures the developer's intention of program modifications by taking how the developer manipulates the output into account. The program modifications are usually hard coded for each direct manipulation in these BLP systems, which are difficult to extend. Moreover, to reflect the manipulations to the source program, these BLP systems trace the modified output to appropriate code fragments and perform corresponding code transformations. Accordingly, they require direct manipulation users be aware of the source code and how it is changed, making "direct" manipulation (on output) be "indirect". In this paper, we resolve this problem by presenting a novel operation-based framework for bidirectional live programming, which can automatically fuse direct manipulations into the source code, thus supporting code-insensitive direct manipulations. Firstly, we design a simple but expressive delta language DM capable of expressing common direct manipulations for output values. Secondly, we present a fusion algorithm that propagates direct manipulations into the source functional programs and applies them to the constants whenever possible; otherwise, the algorithm embeds manipulations into the "proper positions" of programs. We prove the correctness of the fusion algorithm that the updated program executes to get the manipulated output. To demonstrate the expressiveness of DM and the effectiveness of our fusion algorithm, we have implemented FuseDM, a prototype SVG editor that supports GUI-based operations for direct manipulation, and successfully designed 14 benchmark examples starting from blank code using FuseDM.