Introduction

Representing programs as text strings makes programming harder then it has to be. The source text of a program is far removed from its behavior. Bridging this conceptual gulf is what makes programming so inhumanly difficult – we are not compilers. Subtext is a new medium in which the representation of a program is the same thing as its execution. Like a spreadsheet, a program is visible and alive, constantly executing even as it is edited; edits are coherent semantic transformations.

Approach

Instead of parsing text with a grammar, a semantic model is maintained through a direct-manipulation user interface – WYSIWYG semantics. This approach is merely the standard model/view architecture applied to programming language design. It frees the representation of programs from the limitations of paper-centric notations such as text and diagrams. Instead these notations become a presentation layer on top of much richer underlying structures. The insight is that text is a great user-interface technique, but a poor data structure for encoding program semantics.

The primary structure of the model is a tree, and the metaphor of the user interface is correspondingly an indented outline. There are two forms of structure that cross-cut the tree: links and copies. Links equate pairs of nodes in the tree, serving as constants, variables, and arguments. Any subtree can be copied anywhere else in the tree (including recursively within itself, which is lazily infinite). Both links and copies are constructed in the user interface with drag-and-drop or copy-and-paste operations. Execution is through reactive mutation: changes to inputs cause changes to outputs in response. Notably absent are symbolic names, the workhorse of textual notation, which are replaced by immediately-bound explicit links. This framework unifies traditionally distinct programming tools and concepts: there is no difference between run-time and edit-time, and no need for separate debugging, testing, building, and version-control tools. Programming becomes more akin to using a spreadsheet than a keypunch.

Progress

An experimental prototype implements an untyped first-order lazy pure functional language. The language has a novel blend of prototypical and functional semantics. This experiment serves as a proof of concept of direct-manipulation semantics without grammar or names.

[1] Jonathan Edwards. Example Centric Programming. In Companion to the 19th annual ACM SIGPLAN conference on Object Oriented Programming Systems, Languages, and Applications (OOPSLA'04 Onward) Vancouver, BC, CANADA, October 2004. [PDF]

[2] Jonathan Edwards. Subtext: Uncovering the Simplicity of Programming. Submitted to Object Oriented Programming Systems Languages and Applications Onward 2005 (OOPSLA'05 Onward). [PDF]