ParserContext now directly manages its Token property.

Author: RyanLamansky

SqlServerSimulator/Parser/Expression.cs

@@ -48,11 +48,11 @@ public static Expression Parse(ParserContext context)
                     switch (reservedKeyword.Keyword)
                     {
                         case Keyword.As:
-                            if (expression is null || !context.TryMoveNext(out context.Token) || context.Token is not Name alias)
+                            if (expression is null || context.GetNextOptional() is not Name alias)
                                 throw SimulatedSqlException.SyntaxErrorNearKeyword(reservedKeyword);
 
                             expression = new NamedExpression(expression, alias.Value);
-                            _ = context.TryMoveNext(out context.Token);
+                            context.MoveNextOptional();
                             return expression;
                         case Keyword.From:
                             if (expression is null)
@@ -71,12 +71,12 @@ public static Expression Parse(ParserContext context)
                 case Plus:
                     if (expression is null)
                     {
-                        context.Token = context.RequireNext();
+                        context.MoveNextRequired();
                         expression = Expression.Parse(context);
                         break;
                     }
 
-                    context.Token = context.RequireNext();
+                    context.MoveNextRequired();
 
                     {
                         var parsed = Parse(context);
@@ -90,13 +90,13 @@ public static Expression Parse(ParserContext context)
                 case Minus:
                     if (expression is null)
                     {
-                        context.Token = context.RequireNext();
+                        context.MoveNextRequired();
                         expression = Parse(context);
                         expression = new Subtract(new Value(0), expression);
                         break;
                     }
 
-                    context.Token = context.RequireNext();
+                    context.MoveNextRequired();
 
                     {
                         var parsed = Parse(context);
@@ -116,7 +116,7 @@ public static Expression Parse(ParserContext context)
                         if (expression is not Reference reference)
                             throw new NotSupportedException("Simulated expression parser doesn't know how to handle '.' here.");
 
-                        reference.AddMultiPartComponent(context.RequireNext<Name>());
+                        reference.AddMultiPartComponent(context.GetNextRequired<Name>());
                     }
                     break;
                 case Comma:
@@ -128,15 +128,15 @@ public static Expression Parse(ParserContext context)
                     {
                         if (expression is not Reference reference)
                             throw SimulatedSqlException.SyntaxErrorNear(context.Token);
-                        context.Token = context.RequireNext(); // Move past (
+                        context.MoveNextRequired(); // Move past (
                         expression = ResolveBuiltIn(reference.Name, context);
-                        _ = context.TryMoveNext(out context.Token); // Move past )
+                        context.MoveNextOptional(); // Move past )
                         return expression;
                     }
                 default:
                     throw new NotSupportedException($"Simulated expression parser doesn't know how to handle '{context.Token}'.");
             }
-        } while ((tokenWasRead && context.Token is not null) || context.TryMoveNext(out context.Token));
+        } while ((tokenWasRead && context.Token is not null) || context.GetNextOptional() is not null);
 
         return expression;
     }

SqlServerSimulator/Parser/ParserContext.cs

@@ -1,7 +1,5 @@
 using System.Collections.Frozen;
 using System.Data.Common;
-using System.Diagnostics;
-using System.Diagnostics.CodeAnalysis;
 
 namespace SqlServerSimulator.Parser;
 
@@ -16,7 +14,7 @@ internal sealed class ParserContext : IDisposable
 #else
     private readonly IEnumerator<Token> tokens;
 #endif
-    public Token? Token;
+    public Token? Token { get; private set; }
     private readonly FrozenDictionary<string, (string Name, (DataType type, object? Value) TypeValue)> variables;
 
     public ParserContext(SimulatedDbCommand command)
@@ -54,45 +52,84 @@ public ParserContext(SimulatedDbCommand command)
         ? value.TypeValue.Value
         : throw new SimulatedSqlException($"Must declare the scalar variable \"@{name}\".");
 
+    /// <summary>
+    /// Advances <see cref="Token"/> to the next token, if one exists.
+    /// </summary>
+    public void MoveNextOptional()
+    {
+        var enumerator = this.tokens;
+        this.Token = enumerator.MoveNext() ? enumerator.Current : null;
+    }
 
     /// <summary>
-    /// Moves to the next item in an enumeration (if possible) and transfers <see cref="IEnumerator{T}.Current"/> to <paramref name="current"/>.
+    /// Returns the next token in the enumeration, or null.
+    /// Also updates <see cref="Token"/> to the new value.
     /// </summary>
-    /// <param name="current">Receives the <see cref="IEnumerator{T}.Current"/> value, or the types default if advancement isn't possible.</param>
-    /// <returns>True if the enumerator was advanced, otherwise false.</returns>
-    public bool TryMoveNext([NotNullWhen(true)] out Token? current)
+    /// <returns>The next token if the enumerator was advanced, otherwise null.</returns>
+    public Token? GetNextOptional()
     {
         var enumerator = this.tokens;
-        bool moved;
-        current = (moved = enumerator.MoveNext()) ? enumerator.Current : default;
-        return moved;
+        return this.Token = enumerator.MoveNext() ? enumerator.Current : null;
     }
 
     /// <summary>
     /// Returns the next token in the enumeration, throwing an exception if the end was reached instead.
+    /// Also updates <see cref="Token"/> to the new value.
     /// </summary>
     /// <returns>The next token.</returns>
     /// <exception cref="SimulatedSqlException">Incorrect syntax near '{token}'.</exception>
-    public Token RequireNext()
+    public Token GetNextRequired()
     {
         var enumerator = this.tokens;
-        Debug.Assert(enumerator.Current is not null);
         var previous = enumerator.Current;
-        return enumerator.MoveNext() ? enumerator.Current : throw SimulatedSqlException.SyntaxErrorNear(previous);
+        return enumerator.MoveNext() ? this.Token = enumerator.Current : throw SimulatedSqlException.SyntaxErrorNear(previous);
     }
 
     /// <summary>
     /// Returns the next token in the enumeration, throwing an exception if the end was reached instead or the token is the wrong type.
+    /// Also updates <see cref="Token"/> to the new value.
     /// </summary>
+    /// <typeparam name="T">The expected type of the new token.</typeparam>
     /// <returns>The next token.</returns>
     /// <exception cref="SimulatedSqlException">Incorrect syntax near '{token}'.</exception>
-    public T RequireNext<T>()
+    public T GetNextRequired<T>()
         where T : Token
     {
         var enumerator = this.tokens;
-        Debug.Assert(enumerator.Current is not null);
         var previous = enumerator.Current;
-        return enumerator.MoveNext() && enumerator.Current is T current ? current : throw SimulatedSqlException.SyntaxErrorNear(previous);
+
+        if (enumerator.MoveNext() && enumerator.Current is T current)
+        {
+            this.Token = current;
+            return current;
+        }
+
+        throw SimulatedSqlException.SyntaxErrorNear(previous);
+    }
+
+    /// <summary>
+    /// Advances <see cref="Token"/> to the next token in the enumeration, throwing an exception if the end was reached instead or the token is the wrong type.
+    /// </summary>
+    /// <typeparam name="T">The expected type of the new token.</typeparam>
+    /// <exception cref="SimulatedSqlException">Incorrect syntax near '{token}'.</exception>
+    public void MoveNextRequired<T>()
+        where T : Token
+    {
+        var enumerator = this.tokens;
+        var previous = enumerator.Current;
+
+        this.Token = enumerator.MoveNext() && enumerator.Current is T current ? (Token)current : throw SimulatedSqlException.SyntaxErrorNear(previous);
+    }
+
+    /// <summary>
+    /// Advances <see cref="Token"/> to the next token in the enumeration, throwing an exception if the end was reached instead.
+    /// </summary>
+    /// <exception cref="SimulatedSqlException">Incorrect syntax near '{token}'.</exception>
+    public void MoveNextRequired()
+    {
+        var enumerator = this.tokens;
+        var previous = enumerator.Current;
+        this.Token = enumerator.MoveNext() ? enumerator.Current : throw SimulatedSqlException.SyntaxErrorNear(previous);
     }
 
     private bool isDisposed;

SqlServerSimulator/Parser/Selection.cs

@@ -21,24 +21,24 @@ internal sealed class Selection
     /// <exception cref="NotSupportedException">A condition was encountered that may be valid but can't currently be parsed.</exception>
     public static Selection Parse(ParserContext context, uint depth)
     {
-        context.Token = context.RequireNext();
+        context.MoveNextRequired();
 
         int? topCount = null;
 
         if (context.Token is ReservedKeyword { Keyword: Keyword.Top })
         {
             // SQL Server doesn't require outer parentheses.
             // When Expression.Parse supports them, the checks for them here should be removed.
-            context.Token = context.RequireNext<OpenParentheses>();
-            context.Token = context.RequireNext();
+            context.MoveNextRequired<OpenParentheses>();
+            context.MoveNextRequired();
 
             var resolvedExpression = Expression.Parse(context).Run(name => throw SimulatedSqlException.ColumnReferenceNotAllowed(name));
             topCount = resolvedExpression is int unboxed ? unboxed : throw SimulatedSqlException.TopFetchRequiresInteger();
 
             if (context.Token is not null and not CloseParentheses)
                 throw SimulatedSqlException.SyntaxErrorNear(context.Token);
 
-            context.Token = context.RequireNext();
+            context.MoveNextRequired();
         }
 
         List<Expression> expressions = [];
@@ -76,13 +76,13 @@ public static Selection Parse(ParserContext context, uint depth)
                     if (expectFrom.Keyword != Keyword.From)
                         throw new NotSupportedException("Simulated selection processor expected a `from`.");
 
-                    switch (context.Token = context.RequireNext())
+                    switch (context.GetNextRequired())
                     {
                         case StringToken tableName:
                             if (!context.Simulation.Tables.TryGetValue(tableName.Value, out var table) && !context.Simulation.SystemTables.Value.TryGetValue(tableName.Value, out table))
                                 throw SimulatedSqlException.InvalidObjectName(tableName);
 
-                            if (context.TryMoveNext(out context.Token))
+                            if (context.GetNextOptional() is not null)
                             {
                                 if (context.Token is ReservedKeyword { Keyword: Keyword.As })
                                 {
@@ -105,13 +105,13 @@ public static Selection Parse(ParserContext context, uint depth)
                                 ));
 
                         case OpenParentheses:
-                            if ((context.Token = context.RequireNext()) is not ReservedKeyword { Keyword: Keyword.Select })
+                            if (context.GetNextRequired() is not ReservedKeyword { Keyword: Keyword.Select })
                                 throw SimulatedSqlException.SyntaxErrorNear(context.Token);
 
                             {
                                 var derived = Selection.Parse(context, depth + 1).Results;
 
-                                if ((context.Token = context.RequireNext()) is ReservedKeyword { Keyword: Keyword.As })
+                                if (context.GetNextRequired() is ReservedKeyword { Keyword: Keyword.As })
                                 {
                                     if (context.Token is not ReservedKeyword)
                                         break;
@@ -136,7 +136,7 @@ public static Selection Parse(ParserContext context, uint depth)
             }
 
             throw new NotSupportedException($"Simulated selection processor doesn't know what to do with {context.Token}.");
-        } while (context.TryMoveNext(out context.Token));
+        } while (context.GetNextOptional() is not null);
 
         throw new NotSupportedException($"Simulated selection reached the end of the command before expected.");
     }

SqlServerSimulator/SimulatedSqlException.cs

@@ -108,7 +108,7 @@ internal static SimulatedSqlException ColumnReferenceNotAllowed(IEnumerable<stri
 
     internal static SimulatedSqlException SyntaxErrorNearKeyword(ReservedKeyword token) => new($"Incorrect syntax near the keyword '{token}'.", 156, 15, 1);
 
-    internal static SimulatedSqlException SyntaxErrorNear(Token token) => new($"Incorrect syntax near '{token}'.", 102, 15, 1);
+    internal static SimulatedSqlException SyntaxErrorNear(Token? token) => new($"Incorrect syntax near '{token}'.", 102, 15, 1);
 
     internal static SimulatedSqlException ThereIsAlreadyAnObject(string name) => new($"There is already an object named '{name}' in the database.", 2714, 16, 6);