more help, unify error handling

Cargo.toml

@@ -2,6 +2,7 @@
 name = "fast-markov-chain"
 version = "0.1.0"
 edition = "2024"
+description = "Markov Chain generator and querier."
 
 [dependencies]
 bstr = "1.11.3"

src/main.rs

@@ -1,7 +1,6 @@
 use std::{
     error::Error,
     io::{self, Read, Write},
-    process,
 };
 
 use bstr::ByteSlice;
@@ -10,12 +9,24 @@ use rand::seq::IndexedRandom;
 use rustc_hash::FxHashMap;
 
 #[derive(clap::Parser)]
+#[command(
+    version,
+    about,
+    long_about = "Markov Chain generator and querier.
+Reads all of STDIN, tokenizes with order ORDER, and outputs *up to* COUNT tokens, starting with FIRST_WORD or a random word from the corpus.
+Tokens are currently lower-cased words."
+)]
 struct Args {
     // TODO: multi-word prefix
     first_word: String,
-    #[arg(short, long)]
+    #[arg(short, long, help = "maximum number of tokens to output")]
     count: usize,
-    #[arg(short = 'n', long, default_value_t = 2, value_parser = clap::value_parser!(u8).range(2..=5))]
+    #[arg(
+        short = 'n', long,
+        default_value_t = 2,
+        value_parser = clap::value_parser!(u8).range(2..=8),
+        help = "the length of sample windows; higher orders increase coherence but also repetition",
+    )]
     order: u8,
     // DONE: higher-order chains
     // TODO: save chains
@@ -29,6 +40,7 @@ fn main() -> Result<(), Box<dyn Error>> {
         mut count,
         order,
     } = Args::parse();
+    let first_word = first_word.to_lowercase();
 
     // printing the first word immediately gives an intuitive idea of how long freq construction takes
     print!("{first_word} ");
@@ -53,10 +65,10 @@ fn main() -> Result<(), Box<dyn Error>> {
     }
 
     let mut rng = rand::rng();
-    let Some(mut context) = freq.keys().find(|k| k[0] == first_word) else {
-        eprintln!("well *I* didn't find {first_word} in the corpus");
-        process::exit(1);
-    };
+    let mut context = freq
+        .keys()
+        .find(|k| k[0] == first_word)
+        .ok_or(format!("well *I* didn't find {first_word} in the corpus"))?;
     while let Some(nexts) = freq.get(context) {
         if count == 0 {
             break;