diff --git a/src/builtins.rs b/src/builtins.rs
index 267349b..82ef8f8 100644
--- a/src/builtins.rs
+++ b/src/builtins.rs
@@ -1,9 +1,10 @@
-use std::fmt::Write;
+//! Language builtins, wired into the implementation.
 
-use thiserror::Error;
+use std::fmt::Write;
 
 use crate::evaluate::{Dictionary, Value};
 
+/// Builtin functions.
 #[derive(Debug, Copy, Clone)]
 pub enum Builtin {
     Print,
@@ -16,18 +17,10 @@ pub enum Builtin {
     LessThan,
 }
 
-#[derive(Debug, Error)]
-pub enum Error {
-    #[error("Tried to {0} a non-integer: {1}")]
-    NonInteger(&'static str, Value),
-}
-
-type Result<T> = std::result::Result<T, Error>;
-
-fn unwrap_int_value(op_name: &'static str) -> impl Fn(Value) -> Result<i64> {
+fn unwrap_int_value(op_name: &'static str) -> impl Fn(Value) -> i64 {
     move |value| match value {
-        Value::Integer(i) => Ok(i),
-        _ => Err(Error::NonInteger(op_name, value)),
+        Value::Integer(i) => i,
+        _ => panic!("got non-integer during {op_name}"),
     }
 }
 
@@ -37,8 +30,8 @@ macro_rules! int_op {
             $arguments
                 .into_iter()
                 .map(unwrap_int_value($op_name))
-                .reduce(|acc, v| Ok(acc? $op v?))
-                .unwrap()?,
+                .reduce(|acc, v| acc $op v)
+                .unwrap(),
         )
     };
 }
@@ -47,13 +40,14 @@ macro_rules! int_cmp {
     ($arguments:expr, $op:tt) => {{
         let mut arguments = $arguments;
         let unwrap = unwrap_int_value("compare");
-        let y = unwrap(arguments.pop().unwrap())?;
-        let x = unwrap(arguments.pop().unwrap())?;
+        let y = unwrap(arguments.pop().unwrap());
+        let x = unwrap(arguments.pop().unwrap());
         Value::Boolean(x $op y)
     }};
 }
 
 impl Builtin {
+    /// A mapping from runtime names to builtin sentinels.
     pub fn dictionary() -> Dictionary<Value> {
         [
             ("_print", Builtin::Print),
@@ -69,6 +63,7 @@ impl Builtin {
         .into()
     }
 
+    /// Minimum # of required arguments for a builtin.
     pub fn min_parameters(&self) -> usize {
         match self {
             Builtin::Print => 0,
@@ -81,6 +76,8 @@ impl Builtin {
             | Builtin::LessThan => 2,
         }
     }
+
+    /// Maximum # of parameters allowed, if any.
     pub fn max_parameters(&self) -> Option<usize> {
         match self {
             Builtin::Print
@@ -93,8 +90,9 @@ impl Builtin {
         }
     }
 
-    pub fn call(&self, arguments: Vec<Value>) -> Result<Value> {
-        let result = match self {
+    /// Execute the given builtin on the provided arguments.
+    pub fn call(&self, arguments: Vec<Value>) -> Value {
+        match self {
             Builtin::Print => {
                 let mut output = String::new();
                 let mut arguments = arguments.into_iter().peekable();
@@ -126,7 +124,6 @@ impl Builtin {
             }
             Builtin::GreaterThan => int_cmp!(arguments, >),
             Builtin::LessThan => int_cmp!(arguments, <),
-        };
-        Ok(result)
+        }
     }
 }
diff --git a/src/cli.rs b/src/cli.rs
index f3c5810..9991df4 100644
--- a/src/cli.rs
+++ b/src/cli.rs
@@ -1,8 +1,12 @@
-use std::{ffi::OsString, path::PathBuf};
+//! Command line interface for the language binary.
+
+use std::ffi::OsString;
+use std::path::PathBuf;
 
 use pico_args::Arguments;
 use thiserror::Error;
 
+/// Usage help text.
 pub const USAGE: &str = "
 USAGE:
     mul [-h|--help] (parse <FILE> | evaluate <FILE> | repl)
@@ -16,6 +20,7 @@ COMMANDS:
     repl                    Start the Read-Evaluate-Print-Loop
 ";
 
+/// Errors that may occur during command line parsing.
 #[derive(Debug, Error)]
 pub enum Error {
     #[error("Problem while parsing arguments, {0}")]
@@ -26,6 +31,7 @@ pub enum Error {
     UnrecognizedArguments(Vec<OsString>),
 }
 
+/// Possible top level commands.
 pub enum Command {
     Parse { path: PathBuf },
     Run { path: PathBuf },
@@ -33,8 +39,13 @@ pub enum Command {
 }
 
 impl Command {
+    /// Parse the command line from the current environment.
     pub fn from_environment() -> Result<Option<Self>, Error> {
-        let mut args = Arguments::from_env();
+        Self::from_arguments(Arguments::from_env())
+    }
+
+    /// Parse a command from raw arguments.
+    pub fn from_arguments(mut args: Arguments) -> Result<Option<Self>, Error> {
         if args.contains(["-h", "--help"]) {
             return Ok(None);
         }
diff --git a/src/evaluate.rs b/src/evaluate.rs
index 27ed287..4b1ea8d 100644
--- a/src/evaluate.rs
+++ b/src/evaluate.rs
@@ -1,12 +1,17 @@
-use std::{collections::HashMap, fmt::Display};
+//! Tree-walk interpreter for syntax.
+
+use std::collections::HashMap;
+use std::fmt::Display;
 
 use thiserror::Error;
 
-use crate::builtins::{self, Builtin};
+use crate::builtins::Builtin;
 use crate::syntax::{Block, Expression, InteractiveEntry, Item, Name, Program};
 
+/// Mapping of names to some T.
 pub type Dictionary<T> = HashMap<Name, T>;
 
+/// Runtime values.
 #[derive(Debug, Clone)]
 pub enum Value {
     Builtin(Builtin),
@@ -32,6 +37,7 @@ impl Display for Value {
     }
 }
 
+/// Errors that may occur during evaluation.
 #[derive(Debug, Error)]
 pub enum Error {
     #[error("The name '{0} was not bound to any value in this context")]
@@ -42,28 +48,30 @@ pub enum Error {
     ArgumentsMismatch(usize, usize),
     #[error("No function called main to run")]
     MissingMainFunction,
-    #[error("Problem with builtin call: {0}")]
-    BuiltinError(#[from] builtins::Error),
 }
 
 type Result<T> = std::result::Result<T, Error>;
 
+/// AST walking interpreter.
 #[derive(Debug, Clone)]
 pub struct Interpreter {
     environment: Dictionary<Value>,
 }
 
 impl Interpreter {
+    /// Create a fresh interpreter populated with builtins.
     pub fn new() -> Self {
         Interpreter {
             environment: Builtin::dictionary(),
         }
     }
 
+    /// Create a nested interpreter given an existing environment.
     fn nested(environment: Dictionary<Value>) -> Self {
         Interpreter { environment }
     }
 
+    /// Run a program. Expects a main function to exist.
     pub fn run(&mut self, program: &Program) -> Result<Value> {
         for item in program {
             self.interpret_item(item)?;
@@ -75,7 +83,7 @@ impl Interpreter {
         }
     }
 
-    pub fn interpret_item(&mut self, item: &Item) -> Result<Value> {
+    fn interpret_item(&mut self, item: &Item) -> Result<Value> {
         match item {
             Item::Fn {
                 name,
@@ -93,6 +101,7 @@ impl Interpreter {
         }
     }
 
+    /// Evaluate a code fragment from an interactive context.
     pub fn evaluate_interactive_entry(&mut self, entry: &InteractiveEntry) -> Result<Value> {
         match entry {
             InteractiveEntry::Item(item) => self.interpret_item(item),
@@ -103,7 +112,7 @@ impl Interpreter {
         }
     }
 
-    pub fn evaluate(&mut self, term: &Expression) -> Result<Value> {
+    fn evaluate(&mut self, term: &Expression) -> Result<Value> {
         match term {
             Expression::Variable(name) => self
                 .environment
@@ -158,7 +167,7 @@ impl Interpreter {
                     .iter()
                     .map(|term| self.evaluate(term))
                     .collect::<Result<Vec<_>>>()?;
-                b.call(arguments).map_err(Error::BuiltinError)
+                Ok(b.call(arguments))
             }
             _ => Err(Error::NotAFunction),
         }
diff --git a/src/parse.rs b/src/parse.rs
index 6152f99..8efe763 100644
--- a/src/parse.rs
+++ b/src/parse.rs
@@ -1,3 +1,5 @@
+//! Transform text into syntax.
+
 use lalrpop_util::{lalrpop_mod, lexer::Token, ParseError};
 
 use crate::syntax::{InteractiveEntry, Program};
@@ -6,10 +8,12 @@ lalrpop_mod!(parser, "/parse/parser.rs");
 
 type Result<'input, T> = std::result::Result<T, ParseError<usize, Token<'input>, &'static str>>;
 
+/// Parse a series of items.
 pub fn parse_program(input: &str) -> Result<Program> {
     parser::ProgramParser::new().parse(input)
 }
 
+/// Parse a fragment of code.
 pub fn parse_interactive_entry(input: &str) -> Result<InteractiveEntry> {
     parser::InteractiveEntryParser::new().parse(input)
 }