If you have a Linux or Mac handy, you can trying it out! It’s…kinda wild. If you know some Vim commands that start with :, there’s a good chance they’ll work in ed, except you don’t type : itself (effectively you’re always in “command mode”).
There’s also a novelty Twitter account, @ed1conf, that tweets about ed.
Some coworkers told me a story about a previous job candidate who said his preferred editor was ed. They thought it would be really interesting to see someone actually use it. But during the actual interview, when he opened ed, he didn’t recognize or understand it; he was actually accustomed to a graphical editor that he thought was called ed because he apparently did all his work on a system where someone had symlinked or aliased ed to a modern tool.
ed, the “standard editor” (according to its man page) and the predecessor of vi (the “visual editor”), is a terminal editor that doesn’t automatically display any of the text you’re working on; you have to use the p (“print”) command to display the lines your wish to see.
I think the “ten days” explanation has the merit of being charitable, because it implies that Brendan Eich wouldn’t have made such short-sighted design choices under more favorable circumstances.
(I do not believe that it’s a “sensible principle” to treat text as such a fundamental form of data that a basic language feature like the equality operator should be entirely shaped around it. Surely the consequences of building an entire language around text manipulation should be apparent by considering how awkward Posix-style shells are for any nontrivial scripts.)