apply client/cli polish (#15)

Continuation of #11. Refines the client structure and implements GET.

`clap` is decoupled from the lib code. This is done to avoid any CLI
parsing concerns to leak into the lib. The main motivation for this is
to allow the reader to focus on Tokio concerns and not CLI parsing
concerns.

src/bin/cli.rs

@@ -0,0 +1,98 @@
+use mini_redis::{client, DEFAULT_PORT};
+
+use bytes::Bytes;
+use clap::Clap;
+use std::num::ParseIntError;
+use std::str;
+use std::time::Duration;
+
+#[derive(Clap, Debug)]
+#[clap(name = "mini-redis-cli", version = env!("CARGO_PKG_VERSION"), author = env!("CARGO_PKG_AUTHORS"), about = "Issue Redis commands")]
+struct Cli {
+    #[clap(subcommand)]
+    command: Command,
+
+    #[clap(name = "hostname", long = "--host", default_value = "127.0.0.1")]
+    host: String,
+
+    #[clap(name = "port", long = "--port", default_value = DEFAULT_PORT)]
+    port: String,
+}
+
+#[derive(Clap, Debug)]
+enum Command {
+    /// Get the value of key.
+    Get {
+        /// Name of key to get
+        key: String
+    },
+    /// Set key to hold the string value.
+    Set {
+        /// Name of key to set
+        key: String,
+
+        /// Value to set.
+        #[clap(parse(from_str = bytes_from_str))]
+        value: Bytes,
+
+        /// Expire the value after specified amount of time
+        #[clap(parse(try_from_str = duration_from_ms_str))]
+        expires: Option<Duration>,
+    },
+}
+
+/// Entry point for CLI tool.
+///
+/// The `[tokio::main]` annotation signals that the Tokio runtime should be
+/// started when the function is called. The body of the function is executed
+/// within the newly spawned runtime.
+///
+/// `basic_scheduler` is used here to avoid spawning background threads. The CLI
+/// tool use case benefits more by being lighter instead of multi-threaded.
+#[tokio::main(basic_scheduler)]
+async fn main() -> mini_redis::Result<()> {
+    // Enable logging
+    tracing_subscriber::fmt::try_init()?;
+
+    // Parse command line arguments
+    let cli = Cli::parse();
+
+    // Get the remote address to connect to
+    let addr = format!("{}:{}", cli.host, cli.port);
+
+    // Establish a connection
+    let mut client = client::connect(&addr).await?;
+
+    match cli.command {
+        Command::Get { key } => {
+            if let Some(value) = client.get(&key).await? {
+                if let Ok(string) = str::from_utf8(&value) {
+                    println!("\"{}\"", string);
+                } else {
+                    println!("{:?}", value);
+                }
+            } else {
+                println!("(nil)");
+            }
+        }
+        Command::Set { key, value, expires: None } => {
+            client.set(&key, value).await?;
+            println!("OK");
+        }
+        Command::Set { key, value, expires: Some(expires) } => {
+            client.set_expires(&key, value, expires).await?;
+            println!("OK");
+        }
+    }
+
+    Ok(())
+}
+
+fn duration_from_ms_str(src: &str) -> Result<Duration, ParseIntError> {
+    let ms = src.parse::<u64>()?;
+    Ok(Duration::from_millis(ms))
+}
+
+fn bytes_from_str(src: &str) -> Bytes {
+    Bytes::from(src.to_string())
+}