The development of Elvish is driven by a set of ideas, a design philosophy.
Elvish should be a real, expressive programming language.
Shells are often considered domain-specific languages (DSL), but Elvish does not restrict itself to this notion. It embraces such concepts as namespaces, first-class functions and exceptions. Whatever you may find in a modern general-purpose programming language is likely to be found in Elvish.
Elvish is not alone in this respect. There are multiple ongoing efforts; this page on the wiki of oilshell (which is one of the efforts) is a good reference.
Elvish should try to preserve and extend traditional shell programming techniques, as long as they don’t conflict with the previous tenet. Some examples are:
Barewords are simply strings.
Prefix notation dominates, like Lisp. For example, arithmetics is done like
+ 10 (/ 105 5).
Pipeline is the main tool for function composition. To make pipelines suitable for complex data manipulation, Elvish extends them to be able to carry structured data (as opposed to just bytes).
Output capture is the auxiliary tool for function composition. Elvish functions may write structured data directly to the output, and capturing the output yields the same structured data.
The user interface
The user interface should be usable without any customizations. It should be simple and consistent by default:
Prefer to extend well-known functionalities in other shell to inventing brand new ones. For instance, in Elvish Ctrl-R summons the “history listing” for searching history, akin to how Ctrl-R works in bash, but more powerful.
When a useful feature has no prior art in other shells, borrow from other programs. For instance, the “navigation mode”, summoned by Ctrl-N, mimics Ranger; while the “location mode” used for quickly changing location, mimics location bars in GUI browsers (and is summoned by the same key combination Ctrl-L).
Customizability should be achieved via progammability, not an enormous inventory of options that interact with each other in obscure ways.