[cli] Part 3: {load,save}_history as appropriate.

It's possible that we should be saving more aggressively -- perhaps
after each entered command -- but we can add that later.

tools/cli/src/mentat_cli/input.rs

@@ -77,6 +77,11 @@ impl InputReader {
     /// Constructs a new `InputReader` reading from `stdin`.
     pub fn new(interface: Option<Interface<DefaultTerminal>>) -> InputReader {
         if let Some(ref interface) = interface {
+            // It's fine to fail to load history.
+            let p = ::history_file_path();
+            let loaded = interface.load_history(&p);
+            debug!("history read from {}: {}", p.display(), loaded.is_ok());
+
             let mut r = interface.lock_reader();
             // Handle SIGINT (Ctrl-C)
             r.set_report_signal(Signal::Interrupt, true);
@@ -221,5 +226,15 @@ impl InputReader {
         if let Some(ref interface) = self.interface {
             interface.add_history(line);
         }
+        self.save_history();
+    }
+
+    pub fn save_history(&self) -> () {
+        if let Some(ref interface) = self.interface {
+            let p = ::history_file_path();
+            // It's okay to fail to save history.
+            let saved = interface.save_history(&p);
+            debug!("history saved to {}: {}", p.display(), saved.is_ok());
+        }
     }
 }

tools/cli/src/mentat_cli/lib.rs

@@ -10,6 +10,10 @@
 
 #![crate_name = "mentat_cli"]
 
+use std::path::{
+    PathBuf,
+};
+
 #[macro_use] extern crate failure_derive;
 #[macro_use] extern crate log;
 #[macro_use] extern crate lazy_static;
@@ -36,6 +40,17 @@ use termion::{
     color,
 };
 
+static HISTORY_FILE_PATH: &str = ".mentat_history";
+
+/// The Mentat CLI stores input history in a readline-compatible file like "~/.mentat_history".
+/// This accords with main other tools which prefix with "." and suffix with "_history": lein,
+/// node_repl, python, and sqlite, at least.
+pub(crate) fn history_file_path() -> PathBuf {
+    let mut p = ::std::env::home_dir().unwrap_or_default();
+    p.push(::HISTORY_FILE_PATH);
+    p
+}
+
 static BLUE: color::Rgb = color::Rgb(0x99, 0xaa, 0xFF);
 static GREEN: color::Rgb = color::Rgb(0x77, 0xFF, 0x99);
 

tools/cli/src/mentat_cli/repl.rs

@@ -252,6 +252,8 @@ impl Repl {
                 Err(e) => eprintln!("{}", e.to_string()),
             }
         }
+
+        self.input_reader.save_history();
     }
 
     fn cache(&mut self, attr: String, direction: CacheDirection) {