refactor(tvix): split binary (REPL etc.) out from evaluator library

The tvix-eval project is independent from any *uses* of the evaluator,
such as the tvix-repl.

This functionality has been split out into a separate "tvix-cli"
crate. Note that this doesn't have to mean that this CLI crate is the
"final" CLI crate for tvix, the point of this is not "getting the CLI
structure right" but rather "getting the evaluator structure right".

This reshuffling is part of restructuring the way that functionality
like store communication is injected into language evaluation.

Note that at this commit the new CLI crate is not at feature-parity.

Change-Id: Id0af03dc8e07ef09a9f882a89612ad555eca8f93
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7541
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/cli/src/main.rs

@@ -0,0 +1,123 @@
+use std::{fs, path::PathBuf};
+
+use clap::Parser;
+use rustyline::{error::ReadlineError, Editor};
+use tvix_eval::Value; //{Error, EvalWarning, Evaluation, Value};
+
+#[derive(Parser)]
+struct Args {
+    /// Path to a script to evaluate
+    script: Option<PathBuf>,
+
+    #[clap(long, short = 'E')]
+    expr: Option<String>,
+    // TODO: port these options here directly
+    // #[clap(flatten)]
+    // eval_options: tvix_eval::Options,
+}
+
+/// Interprets the given code snippet, printing out warnings, errors
+/// and the result itself. The return value indicates whether
+/// evaluation succeeded.
+fn interpret(code: &str, path: Option<PathBuf>) -> bool {
+    let mut eval = tvix_eval::Evaluation::new(code, path);
+    let result = eval.evaluate();
+
+    let source_map = eval.source_map();
+    for error in &result.errors {
+        error.fancy_format_stderr(&source_map);
+    }
+
+    for warning in &result.warnings {
+        warning.fancy_format_stderr(&source_map);
+    }
+
+    if let Some(value) = result.value.as_ref() {
+        println_result(value, /* TODO raw = */ false);
+    }
+
+    // inform the caller about any errors
+    result.errors.is_empty()
+}
+
+fn main() {
+    let args = Args::parse();
+
+    if let Some(file) = args.script {
+        run_file(file /* TODO, args.eval_options*/)
+    } else if let Some(expr) = args.expr {
+        if !interpret(&expr, None) {
+            std::process::exit(1);
+        }
+    } else {
+        run_prompt(/* TODO args.eval_options */)
+    }
+}
+
+fn run_file(mut path: PathBuf /* TODO: , eval_options: tvix_eval::Options */) {
+    if path.is_dir() {
+        path.push("default.nix");
+    }
+    let contents = fs::read_to_string(&path).expect("failed to read the input file");
+
+    if !interpret(&contents, Some(path)) {
+        std::process::exit(1);
+    }
+}
+
+fn println_result(result: &Value, raw: bool) {
+    if raw {
+        println!("{}", result.to_str().unwrap().as_str())
+    } else {
+        println!("=> {} :: {}", result, result.type_of())
+    }
+}
+
+fn state_dir() -> Option<PathBuf> {
+    let mut path = dirs::data_dir();
+    if let Some(p) = path.as_mut() {
+        p.push("tvix")
+    }
+    path
+}
+
+fn run_prompt(/* TODO eval_options: tvix_eval::Options */) {
+    let mut rl = Editor::<()>::new().expect("should be able to launch rustyline");
+
+    let history_path = match state_dir() {
+        // Attempt to set up these paths, but do not hard fail if it
+        // doesn't work.
+        Some(mut path) => {
+            let _ = std::fs::create_dir_all(&path);
+            path.push("history.txt");
+            let _ = rl.load_history(&path);
+            Some(path)
+        }
+
+        None => None,
+    };
+
+    loop {
+        let readline = rl.readline("tvix-repl> ");
+        match readline {
+            Ok(line) => {
+                if line.is_empty() {
+                    continue;
+                }
+
+                rl.add_history_entry(&line);
+                interpret(&line, None);
+            }
+            Err(ReadlineError::Interrupted) | Err(ReadlineError::Eof) => break,
+
+            Err(err) => {
+                eprintln!("error: {}", err);
+                break;
+            }
+        }
+    }
+
+    if let Some(path) = history_path {
+        rl.save_history(&path).unwrap();
+    }
+}