This example requires some input to be stored for some time within a common workflow of a user (perhaps within 15 minutes, within a session). How does this concise example within Arc behave if there are requirements about the input (not just the reappearing unicode argument but perhaps some sort of identification key that is only numeric or stored within a database) and about the state beyond server reboots (adding an item to a cart to be checked out and paid for in the third step, which might have days between them).
Rails shines in making common crud actions coded easily when handling models that can be expressed in a model stored in one row of a database. This arc challenge thus far only shows me that arc shines in a wizard like behavior that is carrying an input across 3 pages (with the layout of those three pages being very concise or not with a standard beyond web 0.1 layout)
The power of a language is not expressed in one application (web based interfaces without an obvious use in this example case). It is expressed, I feel, in the easy a devoloper can solve the problem which allows another developer to evolve the current code to solve evolving requirements. This places a burden on the base language and the libraries expressed with this language.
The real challenge for a new (if that) programming language is to provide case studies that evolved over time with consistent maintainability and quality. (Software quality is another debate involving the power of users or clients to recognize quality of software or why firefox is not yet the most standard browser).
This challenge isn't it.
edit: I apologize for the abundance of parentheses within my comment.