From 44715b99f4f5f54f6b4681de896f3fe8274fcbc8 Mon Sep 17 00:00:00 2001
From: Grant Hutchins <git@nertzy.com>
Date: Tue, 23 Jun 2026 17:20:33 -0500
Subject: [PATCH] Raise Unit::ParseError instead of built-in SyntaxError

Malformed unit expressions (unbalanced parentheses, dangling operators)
previously raised Ruby's built-in SyntaxError. That is the wrong class for
a bad argument value: SyntaxError is a ScriptError, so it is not a
StandardError and slips through a plain `rescue`, and it conflates a
malformed input string with a Ruby-level parse failure.

Introduce Unit::ParseError < ArgumentError and raise it from the
expression parser instead. A malformed unit string is a bad argument, so
ArgumentError is the correct lineage: it is a StandardError (caught by a
bare rescue) and is distinct from Unit::IncompatibleUnitError (a
TypeError), keeping value errors and type errors separate.
---
 lib/unit/class.rb  |  1 +
 lib/unit/system.rb |  8 ++++----
 spec/error_spec.rb | 14 ++++++++++++++
 3 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/lib/unit/class.rb b/lib/unit/class.rb
index 5635d52..aae71cf 100644
--- a/lib/unit/class.rb
+++ b/lib/unit/class.rb
@@ -2,6 +2,7 @@
 class Unit < Numeric
   attr_reader :value, :normalized, :unit, :system
 
+  class ParseError < ArgumentError; end
   class IncompatibleUnitError < TypeError; end
 
   def initialize(value, unit, system)
diff --git a/lib/unit/system.rb b/lib/unit/system.rb
index 3783b39..82bc124 100644
--- a/lib/unit/system.rb
+++ b/lib/unit/system.rb
@@ -83,7 +83,7 @@ def validate_unit(units)
     # Unrecognized glyphs survive lexing (see +SYMBOL+) and fail loudly as an
     # "Undefined unit" +TypeError+ during validation rather than being dropped.
     #
-    # Raises +SyntaxError+ on unbalanced parentheses.
+    # Raises +Unit::ParseError+ on unbalanced parentheses.
     def parse_unit(expr)
       stack, result, implicit_mul = [], [], false
       expr.to_s.scan(TOKENIZER).each do |tok|
@@ -92,7 +92,7 @@ def parse_unit(expr)
           implicit_mul = false
         elsif tok == ')'
           compute(result, stack.pop) while !stack.empty? && stack.last != '('
-          raise(SyntaxError, 'Unexpected token )') if stack.empty?
+          raise(Unit::ParseError, 'Unexpected token )') if stack.empty?
           stack.pop
           implicit_mul = true
         elsif OPERATOR.key?(tok)
@@ -151,12 +151,12 @@ def symbol_to_unit(symbol)
 
     def compute(result, op)
       b = result.pop
-      a = result.pop || raise(SyntaxError, "Unexpected token #{op}")
+      a = result.pop || raise(Unit::ParseError, "Unexpected token #{op}")
       result << case op
                 when '*' then a + b
                 when '/' then a + Unit.power_unit(b, -1)
                 when '^' then Unit.power_unit(a, b[0][1])
-                else raise SyntaxError, "Unexpected token #{op}"
+                else raise Unit::ParseError, "Unexpected token #{op}"
                 end
     end
 
diff --git a/spec/error_spec.rb b/spec/error_spec.rb
index d0e55a1..8e6a3b9 100644
--- a/spec/error_spec.rb
+++ b/spec/error_spec.rb
@@ -49,4 +49,18 @@
     end
   end
 
+  describe "parse failures for a malformed unit expression" do
+    it "raises Unit::ParseError for a dangling operator" do
+      expect { Unit(1, "m").in!("m//s") }.to raise_error(Unit::ParseError, "Unexpected token /")
+    end
+
+    it "raises Unit::ParseError for an unbalanced opening parenthesis" do
+      expect { Unit(1, "m").in!("(s") }.to raise_error(Unit::ParseError, "Unexpected token (")
+    end
+
+    it "raises Unit::ParseError for an unbalanced closing parenthesis" do
+      expect { Unit(1, "m").in!(")") }.to raise_error(Unit::ParseError, "Unexpected token )")
+    end
+  end
+
 end