Created a hierarchy of exceptions for grammar format issues. This allows the base catch-all exception to be caught for general handling, or more specific exceptions to be caught for more specific handling.

LLama/Exceptions/GrammarFormatException.cs

@@ -1,21 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.Text;
-
-namespace LLama.Exceptions
-{
-    public class GrammarFormatException
-        : Exception
-    {
-        public GrammarFormatException()
-        {
-
-        }
-
-        public GrammarFormatException(string message)
-            : base(message)
-        {
-
-        }
-    }
-}

LLama/Exceptions/GrammarFormatExceptions.cs

@@ -0,0 +1,125 @@
+using System;
+
+namespace LLama.Exceptions;
+
+/// <summary>
+/// Base class for all grammar exceptions
+/// </summary>
+public abstract class GrammarFormatException
+    : Exception
+{
+    internal GrammarFormatException(string message)
+        : base(message)
+    {
+    }
+}
+
+
+/// <summary>
+/// An incorrect number of characters were encountered while parsing a hex literal
+/// </summary>
+public class GrammarUnexpectedHexCharsCount
+    : GrammarFormatException
+{
+    internal GrammarUnexpectedHexCharsCount(int size, string source)
+        : base($"Expecting {size} hex chars at {source}")
+    {
+    }
+}
+
+/// <summary>
+/// Failed to parse a "name" element when one was expected
+/// </summary>
+public class GrammarExpectedName
+    : GrammarFormatException
+{
+    internal GrammarExpectedName(string source)
+        : base($"Expecting name at {source}")
+    {
+    }
+}
+
+/// <summary>
+/// An unexpected character was encountered after an escape sequence
+/// </summary>
+public class GrammarUnknownEscapeCharacter
+    : GrammarFormatException
+{
+    internal GrammarUnknownEscapeCharacter(string source)
+        : base($"Unknown escape at {source}")
+    {
+    }
+}
+
+/// <summary>
+/// End-of-file was encountered while parsing
+/// </summary>
+public class GrammarUnexpectedEndOfInput
+    : GrammarFormatException
+{
+    internal GrammarUnexpectedEndOfInput()
+        : base($"Unexpected end of input")
+    {
+    }
+}
+
+/// <summary>
+/// A specified string was expected when parsing
+/// </summary>
+public class GrammarExpectedNext
+    : GrammarFormatException
+{
+    internal GrammarExpectedNext(string expected, string source)
+        : base($"Expected '{expected}' at {source}")
+    {
+    }
+}
+
+/// <summary>
+/// A specified character was expected to preceded another when parsing
+/// </summary>
+public class GrammarExpectedPrevious
+    : GrammarFormatException
+{
+    internal GrammarExpectedPrevious(string expected, string source)
+        : base($"Expecting preceding item to be '{expected}' at {source}")
+    {
+    }
+}
+
+
+/// <summary>
+/// A CHAR_ALT was created without a preceding CHAR element
+/// </summary>
+public class GrammarUnexpectedCharAltElement
+    : GrammarFormatException
+{
+    internal GrammarUnexpectedCharAltElement(string ruleId, int index)
+        : base($"LLamaGrammarElementType.CHAR_ALT without preceding char: {ruleId},{index}")
+    {
+    }
+}
+
+/// <summary>
+/// A CHAR_RNG was created without a preceding CHAR element
+/// </summary>
+public class GrammarUnexpectedCharRngElement
+    : GrammarFormatException
+{
+    internal GrammarUnexpectedCharRngElement(string ruleId, int index)
+        : base($"LLamaGrammarElementType.CHAR_RNG_UPPER without preceding char: {ruleId},{index}")
+    {
+    }
+}
+
+/// <summary>
+/// An END was encountered before the last element
+/// </summary>
+public class GrammarUnexpectedEndElement
+    : GrammarFormatException
+{
+    internal GrammarUnexpectedEndElement(string ruleId, int index)
+        : base($"Unexpected LLamaGrammarElementType.END: {ruleId},{index}")
+    {
+    }
+}

LLama/Grammars/GBNFGrammarParser.cs

@@ -109,7 +109,7 @@ namespace LLama.Grammars
 
             if (pos != end)
             {
-                throw new GrammarFormatException($"Expecting {size} hex chars at {Encoding.UTF8.GetString(src.ToArray())}");
+                throw new GrammarUnexpectedHexCharsCount(size, Encoding.UTF8.GetString(src.ToArray()));
             }
             src = src.Slice(pos);
             return value;
@@ -146,7 +146,7 @@ namespace LLama.Grammars
             }
             if (pos == 0)
             {
-                throw new GrammarFormatException($"Expecting name at {Encoding.UTF8.GetString(src.ToArray())}");
+                throw new GrammarExpectedName(Encoding.UTF8.GetString(src.ToArray()));
             }
             return src.Slice(pos);
         }
@@ -177,7 +177,7 @@ namespace LLama.Grammars
                     case (byte)']':
                         return chr;
                     default:
-                        throw new GrammarFormatException("Unknown escape at " + Encoding.UTF8.GetString(src.ToArray()));
+                        throw new GrammarUnknownEscapeCharacter(Encoding.UTF8.GetString(src.ToArray()));
                 }
             }
             else if (!src.IsEmpty)
@@ -185,7 +185,7 @@ namespace LLama.Grammars
                 return DecodeUTF8(ref src);
             }
 
-            throw new GrammarFormatException("Unexpected end of input");
+            throw new GrammarUnexpectedEndOfInput();
         }
 
         private ReadOnlySpan<byte> ParseSequence(
@@ -258,17 +258,13 @@ namespace LLama.Grammars
                     // output reference to synthesized rule
                     outElements.Add(new LLamaGrammarElement(LLamaGrammarElementType.RULE_REF, subRuleId));
                     if (pos[0] != ')')
-                    {
-                        throw new GrammarFormatException($"Expecting ')' at {Encoding.UTF8.GetString(pos.ToArray())}");
-                    }
+                        throw new GrammarExpectedNext(")", Encoding.UTF8.GetString(pos.ToArray()));
                     pos = ParseSpace(pos.Slice(1), isNested);
                 }
                 else if (pos[0] == '*' || pos[0] == '+' || pos[0] == '?') // repetition operator
                 {
                     if (lastSymStart == outElements.Count)
-                    {
-                        throw new GrammarFormatException($"Expecting preceding item to */+/? at {Encoding.UTF8.GetString(pos.ToArray())}");
-                    }
+                        throw new GrammarExpectedPrevious("*/+/?", Encoding.UTF8.GetString(pos.ToArray()));
 
                     // apply transformation to previous symbol (lastSymStart to end) according to
                     // rewrite rules:
@@ -349,9 +345,8 @@ namespace LLama.Grammars
             string name = Encoding.UTF8.GetString(src.Slice(0, nameLen).ToArray());
 
             if (!(pos[0] == ':' && pos[1] == ':' && pos[2] == '='))
-            {
-                throw new GrammarFormatException($"Expecting ::= at {Encoding.UTF8.GetString(pos.ToArray())}");
-            }
+                throw new GrammarExpectedNext("::=", Encoding.UTF8.GetString(pos.ToArray()));
+
             pos = ParseSpace(pos.Slice(3), true);
 
             pos = ParseAlternates(state, pos, name, ruleId, false);
@@ -366,7 +361,7 @@ namespace LLama.Grammars
             }
             else if (!pos.IsEmpty)
             {
-                throw new GrammarFormatException($"Expecting newline or end at {Encoding.UTF8.GetString(pos.ToArray())}");
+                throw new GrammarExpectedNext("newline or EOF", Encoding.UTF8.GetString(pos.ToArray()));
             }
             return ParseSpace(pos, true);
         }

LLama/Grammars/Grammar.cs

@@ -70,10 +70,10 @@ namespace LLama.Grammars
         private void PrintGrammar(StringBuilder output)
         {
             for (var i = 0; i < Rules.Count; i++)
-                PrintRule(output, (uint)i, Rules[i]);
+                PrintRule(output, Rules[i]);
         }
 
-        private void PrintRule(StringBuilder output, uint ruleId, GrammarRule rule)
+        private void PrintRule(StringBuilder output, GrammarRule rule)
         {
             output.Append($"{rule.Name} ::= ");
 
@@ -82,37 +82,34 @@ namespace LLama.Grammars
                 var elem = rule.Elements[i];
                 switch (elem.Type)
                 {
+                    // GrammarRule has already verified that END is not being misused, no need to check again
                     case LLamaGrammarElementType.END:
-                        throw new GrammarFormatException($"Unexpected end of rule: {ruleId}, {i}");
+                        break;
+
                     case LLamaGrammarElementType.ALT:
                         output.Append("| ");
                         break;
+
                     case LLamaGrammarElementType.RULE_REF:
                         output.Append($"{Rules[(int)elem.Value].Name} ");
                         break;
+
                     case LLamaGrammarElementType.CHAR:
                         output.Append('[');
                         PrintGrammarChar(output, elem.Value);
                         break;
+
                     case LLamaGrammarElementType.CHAR_NOT:
                         output.Append("[^");
                         PrintGrammarChar(output, elem.Value);
                         break;
+
                     case LLamaGrammarElementType.CHAR_RNG_UPPER:
-                        if (i == 0 || !rule.Elements[i - 1].IsCharElement())
-                        {
-                            throw new GrammarFormatException(
-                                $"LLamaGrammarElementType.CHAR_RNG_UPPER without preceding char: {ruleId},{i}");
-                        }
                         output.Append('-');
                         PrintGrammarChar(output, elem.Value);
                         break;
+
                     case LLamaGrammarElementType.CHAR_ALT:
-                        if (i == 0 || !rule.Elements[i - 1].IsCharElement())
-                        {
-                            throw new GrammarFormatException(
-                                $"LLamaGrammarElementType.CHAR_ALT without preceding char: {ruleId},{i}");
-                        }
                         PrintGrammarChar(output, elem.Value);
                         break;
 

LLama/Grammars/GrammarRule.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using LLama.Exceptions;
 using LLama.Native;
 
 namespace LLama.Grammars
@@ -36,7 +37,7 @@ namespace LLama.Grammars
         private static void Validate(IReadOnlyList<LLamaGrammarElement> elements, string name)
         {
             if (elements.Count == 0)
-                throw new ArgumentException("Cannot create a GrammaRule with zero elements", nameof(elements));
+                throw new ArgumentException("Cannot create a GrammarRule with zero elements", nameof(elements));
             if (elements[elements.Count - 1].Type != LLamaGrammarElementType.END)
                 throw new ArgumentException("Last grammar element must be END", nameof(elements));
 
@@ -46,18 +47,16 @@ namespace LLama.Grammars
                 {
                     case LLamaGrammarElementType.END:
                         if (i != elements.Count - 1)
-                            throw new ArgumentException("Found more than one END grammar element", nameof(elements));
+                            throw new GrammarUnexpectedEndElement(name, i);
                         continue;
 
                     case LLamaGrammarElementType.CHAR_RNG_UPPER:
                         if (i == 0 || !elements[i - 1].IsCharElement())
-                            throw new ArgumentException($"LLamaGrammarElementType.CHAR_RNG_UPPER without preceding char: {name},{i}", nameof(elements));
+                            throw new GrammarUnexpectedCharRngElement(name, i);
                         break;
                     case LLamaGrammarElementType.CHAR_ALT:
                         if (i == 0 || !elements[i - 1].IsCharElement())
-                        {
-                            throw new ArgumentException($"LLamaGrammarElementType.CHAR_ALT without preceding char: {name},{i}", nameof(elements));
-                        }
+                            throw new GrammarUnexpectedCharAltElement(name, i);
                         break;
 
                     case LLamaGrammarElementType.ALT:
@@ -67,7 +66,7 @@ namespace LLama.Grammars
                         break;
 
                     default:
-                        throw new ArgumentException($"Unknown grammar element type: '{elements[i].Type}'");
+                        throw new ArgumentException($"Unknown grammar element type: '{elements[i].Type}'", nameof(elements));
                 }
             }
         }