explore: adopt anyhow, support CustomValue, remove help system (#12692)

This PR:
1. Adds basic support for `CustomValue` to `explore`. Previously `open
foo.db | explore` didn't really work, now we "materialize" the whole
database to a `Value` before loading it
2. Adopts `anyhow` for error handling in `explore`. Previously we were
kind of rolling our own version of `anyhow` by shoving all errors into a
`std::io::Error`; I think this is much nicer. This was necessary because
as part of 1), collecting input is now fallible...
3. Removes a lot of `explore`'s fancy command help system.
- Previously each command (`:help`, `:try`, etc.) had a sophisticated
help system with examples etc... but this was not very visible to users.
You had to know to run `:help :try` or view a list of commands with
`:help :`
- As discussed previously, we eventually want to move to a less modal
approach for `explore`, without the Vim-like commands. And so I don't
think it's worth keeping this command help system around (it's
intertwined with other stuff, and making these changes would have been
harder if keeping it).
4. Rename the `--reverse` flag to `--tail`. The flag scrolls to the end
of the data, which IMO is described better by "tail"
5. Does some renaming+commenting to clear up things I found difficult to
understand when navigating the `explore` code


I initially thought 1) would be just a few lines, and then this PR blew
up into much more extensive changes 😅


## Before
The whole database was being displayed as a single Nuon/JSON line 🤔 

![image](https://github.com/nushell/nushell/assets/26268125/6383f43b-fdff-48b4-9604-398438ad1499)


## After
The database gets displayed like a record

![image](https://github.com/nushell/nushell/assets/26268125/2f00ed7b-a3c4-47f4-a08c-98d07efc7bb4)


## Future work

It is sort of annoying that we have to load a whole SQLite database into
memory to make this work; it will be impractical for large databases.
I'd like to explore improvements to `CustomValue` that can make this
work more efficiently.

crates/nu-explore/src/lib.rs

@@ -6,20 +6,19 @@ mod pager;
 mod registry;
 mod views;
 
+use anyhow::Result;
+use commands::{ExpandCmd, HelpCmd, NuCmd, QuitCmd, TableCmd, TryCmd};
 pub use default_context::add_explore_context;
 pub use explore::Explore;
-
-use commands::{ExpandCmd, HelpCmd, HelpManual, NuCmd, QuitCmd, TableCmd, TryCmd};
 use nu_common::{collect_pipeline, has_simple_value, CtrlC};
 use nu_protocol::{
     engine::{EngineState, Stack},
     PipelineData, Value,
 };
 use pager::{Page, Pager, PagerConfig, StyleConfig};
-use registry::{Command, CommandRegistry};
-use std::io;
+use registry::CommandRegistry;
 use terminal_size::{Height, Width};
-use views::{BinaryView, InformationView, Orientation, Preview, RecordView};
+use views::{BinaryView, Orientation, Preview, RecordView};
 
 mod util {
     pub use super::nu_common::{create_lscolors, create_map, map_into_value};
@@ -31,7 +30,7 @@ fn run_pager(
     ctrlc: CtrlC,
     input: PipelineData,
     config: PagerConfig,
-) -> io::Result<Option<Value>> {
+) -> Result<Option<Value>> {
     let mut p = Pager::new(config.clone());
     let commands = create_command_registry();
 
@@ -45,18 +44,18 @@ fn run_pager(
         return p.run(engine_state, stack, ctrlc, view, commands);
     }
 
-    let (columns, data) = collect_pipeline(input);
+    let (columns, data) = collect_pipeline(input)?;
 
     let has_no_input = columns.is_empty() && data.is_empty();
     if has_no_input {
-        return p.run(engine_state, stack, ctrlc, information_view(), commands);
+        return p.run(engine_state, stack, ctrlc, help_view(), commands);
     }
 
     p.show_message("For help type :help");
 
     if let Some(value) = has_simple_value(&data) {
         let text = value.to_abbreviated_string(config.nu_config);
-        let view = Some(Page::new(Preview::new(&text), true));
+        let view = Some(Page::new(Preview::new(&text), false));
         return p.run(engine_state, stack, ctrlc, view, commands);
     }
 
@@ -67,6 +66,7 @@ fn run_pager(
 fn create_record_view(
     columns: Vec<String>,
     data: Vec<Vec<Value>>,
+    // wait, why would we use RecordView for something that isn't a record?
     is_record: bool,
     config: PagerConfig,
 ) -> Option<Page> {
@@ -75,17 +75,17 @@ fn create_record_view(
         view.set_orientation_current(Orientation::Left);
     }
 
-    if config.reverse {
+    if config.tail {
         if let Some((Width(w), Height(h))) = terminal_size::terminal_size() {
-            view.reverse(w, h);
+            view.tail(w, h);
         }
     }
 
-    Some(Page::new(view, false))
+    Some(Page::new(view, true))
 }
 
-fn information_view() -> Option<Page> {
-    Some(Page::new(InformationView, true))
+fn help_view() -> Option<Page> {
+    Some(Page::new(HelpCmd::view(), false))
 }
 
 fn binary_view(input: PipelineData) -> Option<Page> {
@@ -96,7 +96,7 @@ fn binary_view(input: PipelineData) -> Option<Page> {
 
     let view = BinaryView::new(data);
 
-    Some(Page::new(view, false))
+    Some(Page::new(view, true))
 }
 
 fn create_command_registry() -> CommandRegistry {
@@ -104,24 +104,16 @@ fn create_command_registry() -> CommandRegistry {
     create_commands(&mut registry);
     create_aliases(&mut registry);
 
-    // reregister help && config commands
-    let commands = registry.get_commands().cloned().collect::<Vec<_>>();
-    let aliases = registry.get_aliases().collect::<Vec<_>>();
-
-    let help_cmd = create_help_command(&commands, &aliases);
-
-    registry.register_command_view(help_cmd, true);
-
     registry
 }
 
 fn create_commands(registry: &mut CommandRegistry) {
-    registry.register_command_view(NuCmd::new(), false);
-    registry.register_command_view(TableCmd::new(), false);
+    registry.register_command_view(NuCmd::new(), true);
+    registry.register_command_view(TableCmd::new(), true);
 
-    registry.register_command_view(ExpandCmd::new(), true);
-    registry.register_command_view(TryCmd::new(), true);
-    registry.register_command_view(HelpCmd::default(), true);
+    registry.register_command_view(ExpandCmd::new(), false);
+    registry.register_command_view(TryCmd::new(), false);
+    registry.register_command_view(HelpCmd::default(), false);
 
     registry.register_command_reactive(QuitCmd);
 }
@@ -132,34 +124,3 @@ fn create_aliases(registry: &mut CommandRegistry) {
     registry.create_aliases("q", QuitCmd::NAME);
     registry.create_aliases("q!", QuitCmd::NAME);
 }
-
-fn create_help_command(commands: &[Command], aliases: &[(&str, &str)]) -> HelpCmd {
-    let help_manuals = create_help_manuals(commands);
-
-    HelpCmd::new(help_manuals, aliases)
-}
-
-fn create_help_manuals(cmd_list: &[Command]) -> Vec<HelpManual> {
-    cmd_list.iter().map(create_help_manual).collect()
-}
-
-fn create_help_manual(cmd: &Command) -> HelpManual {
-    let name = match cmd {
-        Command::Reactive(cmd) => cmd.name(),
-        Command::View { cmd, .. } => cmd.name(),
-    };
-
-    let manual = match cmd {
-        Command::Reactive(cmd) => cmd.help(),
-        Command::View { cmd, .. } => cmd.help(),
-    };
-
-    __create_help_manual(manual, name)
-}
-
-fn __create_help_manual(manual: Option<HelpManual>, name: &'static str) -> HelpManual {
-    manual.unwrap_or(HelpManual {
-        name,
-        ..HelpManual::default()
-    })
-}