feat: Add a default Vi prompt

src/binary/prompt.rs

 use super::InteractiveShell;
 use ion_shell::{
     expansion::{self, Expander},
-    IonError, PipelineError,
+    IonError, PipelineError, Shell,
 };
-use liner::Prompt;
+use liner::{KeyBindings::*, Prompt, ViPromptMode, ViStatus};
 
 impl<'a> InteractiveShell<'a> {
     /// Generates the prompt that will be used by Liner.
@@ -38,9 +38,55 @@ impl<'a> InteractiveShell<'a> {
                 }
             });
             shell.set_previous_status(previous_status); // Set the previous exit code again
-            Prompt::from(out)
+            let key_bindings = self.context.borrow().key_bindings;
+            match key_bindings {
+                Emacs => Prompt::from(out),
+                Vi => {
+                    let normal = vi_prompt_indicator(&mut shell, ViPromptMode::Normal);
+                    let insert = vi_prompt_indicator(&mut shell, ViPromptMode::Insert);
+                    Prompt {
+                        prompt:    out,
+                        vi_status: Some(ViStatus::new(ViPromptMode::Insert, normal, insert)),
+                    }
+                }
+            }
         } else {
             Prompt::from("    ".repeat(blocks))
         }
     }
 }
+
+/// Returns the default indicators for a given mode in absence of a configuration.
+///
+/// NOTE: This should be made `const` once this functionality is stabilized:
+/// https://github.com/rust-lang/rust/issues/49146
+fn default_vi_indicator(mode: ViPromptMode) -> &'static str {
+    match mode {
+        ViPromptMode::Insert => "${c::dim}${c::greenbg}${c::black} I ${c::reset} ",
+        ViPromptMode::Normal => "${c::dim}${c::light_graybg}${c::black} N ${c::reset} ",
+    }
+}
+
+/// Returns the configuration variable for a given mode.
+///
+/// NOTE: This should be made `const` once this functionality is stabilized:
+/// https://github.com/rust-lang/rust/issues/49146
+fn vi_indicator_variable(mode: ViPromptMode) -> &'static str {
+    match mode {
+        ViPromptMode::Insert => "VI_INSERT",
+        ViPromptMode::Normal => "VI_NORMAL",
+    }
+}
+
+fn vi_prompt_indicator<'a>(shell: &mut Shell<'a>, mode: ViPromptMode) -> String {
+    shell
+        .variables()
+        .get_str(vi_indicator_variable(mode))
+        .or_else(|_| Ok(small::String::from(default_vi_indicator(mode))))
+        .and_then(|prompt| shell.get_string(&prompt))
+        .map(|expanded| expanded.to_string())
+        .unwrap_or_else(|e| {
+            eprintln!("{:?}", e);
+            String::new()
+        })
+}