The language

       • 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
         (https://github.com/oilshell/oil/wiki/ExternalResources)  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  (../learn/cookbook.html#navigation-mode),  summoned
           by  Ctrl-N,  mimics  Ranger (http://ranger.nongnu.org); 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.