diff --git a/onihime/src/lexer/error.rs b/onihime/src/lexer/error.rs
index 303dbc7..6d8c9aa 100644
--- a/onihime/src/lexer/error.rs
+++ b/onihime/src/lexer/error.rs
@@ -46,8 +46,8 @@ impl std::fmt::Display for LexerError {
         use LexerErrorKind::*;
 
         match &self.kind {
-            InvalidEscape(c) => write!(f, "Unknown escape sequence '\\{c}' in string"),
-            InvalidNumber(n) => write!(f, "`{n}` is not a valid numeric literal"),
+            InvalidEscape(c) => write!(f, "Invalid escape sequence '\\{c}'"),
+            InvalidNumber(n) => write!(f, "Invalid numeric literal `{n}`"),
             InvalidString => write!(f, "Invalid string literal"),
             UnclosedChar => write!(f, "Unclosed character literal"),
             UnclosedString => write!(f, "Unclosed string literal"),
diff --git a/onihime/src/lexer/mod.rs b/onihime/src/lexer/mod.rs
index f423401..b0e808c 100644
--- a/onihime/src/lexer/mod.rs
+++ b/onihime/src/lexer/mod.rs
@@ -1,6 +1,6 @@
 use std::{str::Chars, sync::Arc};
 
-pub use self::{
+pub(crate) use self::{
     error::{LexerError, LexerErrorKind},
     symbol::Symbol,
     token::{Token, TokenKind},
@@ -30,7 +30,7 @@ pub(crate) struct Lexer<'lexer> {
 impl<'lexer> Lexer<'lexer> {
     /// Create a new lexer instance from a string.
     #[must_use]
-    pub fn new(input: &'lexer str) -> Self {
+    pub(crate) fn new(input: &'lexer str) -> Self {
         let source = Arc::new(Source::new(None, input.to_string()));
 
         Self {
@@ -41,20 +41,21 @@ impl<'lexer> Lexer<'lexer> {
     }
 
     /// Set the name of the [Source] being lexically analyzed.
-    pub fn set_name(&mut self, name: String) {
+    pub(crate) fn set_name(&mut self, name: String) {
         // TODO: Avoid unwrapping here (if possible?)
         Arc::get_mut(&mut self.source).unwrap().set_name(name);
     }
 
     /// The source being lexically analyzed.
+    #[cfg(test)]
     #[must_use]
-    pub fn source(&self) -> Arc<Source> {
+    pub(crate) fn source(&self) -> Arc<Source> {
         self.source.clone()
     }
 
     /// Get the current position of the lexer.
     #[must_use]
-    pub fn span(&self) -> Span {
+    pub(crate) fn span(&self) -> Span {
         Span::new(self.byte..self.byte, self.source.clone())
     }
 
@@ -103,7 +104,8 @@ impl<'lexer> Lexer<'lexer> {
     }
 
     /// Read the next token from the input.
-    pub fn read(&mut self) -> Result<Option<Token>, LexerError> {
+    #[must_use]
+    pub(crate) fn read(&mut self) -> Result<Option<Token>, LexerError> {
         // Eat whitespace until we encounter a meaningful character, or simply return if
         // we have reached the end of input and no additional characters can be read:
         let c = loop {
@@ -178,6 +180,7 @@ impl<'lexer> Lexer<'lexer> {
         Ok(Some(Token::new(kind, span)))
     }
 
+    #[must_use]
     fn line_comment(&mut self) -> TokenKind {
         // Line comments may start with any number of semicolons, so consume however
         // many are present at the beginning of the comment:
@@ -198,6 +201,7 @@ impl<'lexer> Lexer<'lexer> {
         TokenKind::LineComment(comment.trim().into())
     }
 
+    #[must_use]
     fn block_comment(&mut self) -> TokenKind {
         // TODO: This currently allows for unclosed block comments; do we care?
         self.advance(); // '#'
@@ -216,6 +220,7 @@ impl<'lexer> Lexer<'lexer> {
         TokenKind::BlockComment(comment.trim().into())
     }
 
+    #[must_use]
     fn float_literal(&self, word: String, span: Span) -> Result<TokenKind, LexerError> {
         let float = word.parse().map_err(|_| {
             LexerError::new(LexerErrorKind::InvalidNumber(word), span.join(&self.span()))
@@ -224,6 +229,7 @@ impl<'lexer> Lexer<'lexer> {
         Ok(TokenKind::Float(float))
     }
 
+    #[must_use]
     fn integer_literal(
         &self,
         word: String,
@@ -244,6 +250,7 @@ impl<'lexer> Lexer<'lexer> {
         Ok(TokenKind::Integer(integer))
     }
 
+    #[must_use]
     fn numeric_literal(&mut self, span: Span) -> Result<TokenKind, LexerError> {
         let word = self.read_word();
 
@@ -256,6 +263,7 @@ impl<'lexer> Lexer<'lexer> {
         Ok(kind)
     }
 
+    #[must_use]
     fn char_literal(&mut self, span: Span) -> Result<TokenKind, LexerError> {
         self.advance(); // '\''
 
@@ -300,6 +308,7 @@ impl<'lexer> Lexer<'lexer> {
         Ok(TokenKind::Char(c))
     }
 
+    #[must_use]
     fn string_literal(&mut self, span: Span) -> Result<TokenKind, LexerError> {
         self.advance(); // '"'
 
@@ -436,9 +445,10 @@ mod tests {
         Ok(TokenKind::Integer(255)),
     ]);
 
-    test!(error_parse_number: "1.1.1 0.x", [
+    test!(error_parse_number: "1.1.1 0.x 7b", [
         Err(LexerErrorKind::InvalidNumber("1.1.1".into())),
         Err(LexerErrorKind::InvalidNumber("0.x".into())),
+        Err(LexerErrorKind::InvalidNumber("7b".into())),
     ]);
 
     test!(char_literal: r"'x' '\n' '\r' '\t' '\e' '\\' '\q' 'b", [
@@ -479,6 +489,10 @@ mod tests {
         Err(LexerErrorKind::UnclosedString),
     ]);
 
+    test!(error_escape_unclosed_string: "\"oops\\", [
+        Err(LexerErrorKind::UnclosedString),
+    ]);
+
     test!(error_invalid_string: "\"hiii\"222", [
         Err(LexerErrorKind::InvalidString),
     ]);
diff --git a/onihime/src/lexer/symbol.rs b/onihime/src/lexer/symbol.rs
index b33cc5b..16c6a3d 100644
--- a/onihime/src/lexer/symbol.rs
+++ b/onihime/src/lexer/symbol.rs
@@ -1,10 +1,11 @@
 /// A symbol used to identify a function or variable.
 #[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
-pub struct Symbol(pub String);
+#[repr(transparent)]
+pub(crate) struct Symbol(String);
 
 impl Symbol {
     /// Create a new `Symbol` from a string.
-    pub fn from<S>(s: S) -> Self
+    pub(crate) fn from<S>(s: S) -> Self
     where
         S: Into<String>,
     {
diff --git a/onihime/src/lexer/token.rs b/onihime/src/lexer/token.rs
index 62c8a54..5c8641e 100644
--- a/onihime/src/lexer/token.rs
+++ b/onihime/src/lexer/token.rs
@@ -3,7 +3,7 @@ use crate::span::Span;
 
 /// Possible kinds of a [Token].
 #[derive(Debug, Clone, PartialEq)]
-pub enum TokenKind {
+pub(crate) enum TokenKind {
     /// Block comment, e.g. `#| ... |#`
     BlockComment(String),
     /// Line comment, e.g. `; ...`
@@ -32,10 +32,10 @@ pub enum TokenKind {
     Float(f64),
     /// Integer, e.g. `0`, `-1`, `+200`
     Integer(i64),
-    /// String, e.g. `"foo bar"`
-    String(String),
     /// Keyword, e.g. `:baz`
     Keyword(Symbol),
+    /// String, e.g. `"foo bar"`
+    String(String),
     /// Symbol, e.g. `qux`, `+`
     Symbol(Symbol),
     /// Nil, e.g. `nil`
@@ -50,17 +50,17 @@ pub enum TokenKind {
 /// [Source]: crate::span::Source
 /// [Location]: crate::span::Location
 #[derive(Debug, Clone, PartialEq)]
-pub struct Token {
+pub(crate) struct Token {
     /// The kind of token.
-    pub kind: TokenKind,
+    pub(crate) kind: TokenKind,
     /// The span in which the token occurs.
-    pub span: Span,
+    pub(crate) span: Span,
 }
 
 impl Token {
     /// Construct a new instance of `Token`.
     #[must_use]
-    pub const fn new(kind: TokenKind, span: Span) -> Self {
+    pub(crate) const fn new(kind: TokenKind, span: Span) -> Self {
         Self { kind, span }
     }
 }