elvish/doc/error.md

There will be three classes of errors:

1. Static programming errors. The code doesn't pass the static checker and there is no way to handle it other than change the code.

2. Dynamic programming errors. Some pre-condition not enforced by the static checker is violated. Examples:

   1. fn f a b {}; var args = [1]; f @$args

   2. / 1 0

3. Exceptional conditions that are anticipated but could not be reliably avoided. Examples:

   1. echo >./out but ./out is not writable

   2. ffmpeg -i a.mp4 a.ogg but a.mp4 is not valid MP4. Non-zero return values of external commands always fall into this category.