# ANTLR Grammar Review and Suggestions

This document provides a review of the ANTLR grammar files (`sequenceLexer.g4` and `sequenceParser.g4`) with suggestions for improvement in readability, maintainability, and performance.

## General Observations

*   **Good Use of Channels:** You're effectively using channels (`COMMENT_CHANNEL`, `MODIFIER_CHANNEL`, `HIDDEN`) to separate different types of tokens, which is great for keeping the parser grammar clean.
*   **Error Tolerance:** The grammar has several rules designed to handle incomplete code, which is excellent for use in an editor context. This improves the user experience by providing better error recovery.
*   **Performance Notes:** It's good to see performance tuning notes in the grammar. This indicates that performance is a consideration, and it provides a history of what has been tried.

## `sequenceLexer.g4` - Suggestions

The lexer is generally well-structured and there are no major issues.

### 1. Readability: Keyword Tokens

The rules for keywords like `TRUE`, `FALSE`, `IF`, etc., are defined as separate tokens. This is clear and works well. For larger grammars, sometimes grouping them under a single `KEYWORD` rule can be beneficial, but for the current size, the existing approach is perfectly fine.

### 2. `STRING` Literal Rule

The `STRING` rule is well-designed for an editor context:

```antlr
STRING
 : '"' (~["\r\n] | '""')* ('"'|[\r\n])?
 ;
```

This rule gracefully handles unclosed strings that end at a newline, which is a good strategy for error recovery and improving the user experience in an editor.

### 3. `DIVIDER` Rule

The `DIVIDER` rule uses a semantic predicate to ensure it only matches at the beginning of a line:

```antlr
DIVIDER: {this.column === 0}? WS* '==' ~[\r\n]*;
```

This is a powerful ANTLR feature that is used correctly here. The comment in the code explaining this is also very helpful.

### 4. Lexer Modes

The use of modes for `EVENT` and `TITLE_MODE` is a clean and efficient way to handle context-sensitive lexing.

## `sequenceParser.g4` - Suggestions

The parser grammar is also in good shape, but a few rules could be refactored for better readability and maintainability.

### 1. Readability & Maintainability: Left-Factoring `group` rule

The `group` rule has multiple alternatives that can be simplified by left-factoring.

**Current `group` rule:**
```antlr
group
 : GROUP name? OBRACE participant* CBRACE
 | GROUP name? OBRACE
 | GROUP name?
 ;
```

**Suggested Improvement:**
```antlr
group
 : GROUP name? (OBRACE participant* CBRACE?)?
 ;
```

This change makes the rule more concise and easier to understand. The optional `CBRACE?` maintains the error tolerance for incomplete blocks.

### 2. Readability: Simplify `parExpr` rule

The `parExpr` rule is written in a way that handles various stages of user input, which is good for an editor. However, it can be expressed more concisely.

**Current `parExpr` rule:**
```antlr
parExpr
 : OPAR condition CPAR
 | OPAR condition
 | OPAR CPAR
 | OPAR
 ;
```

**Suggested Improvement:**
```antlr
parExpr
 : OPAR (condition (CPAR)? | CPAR)?
 ;
```

This simplified version covers all the original cases:
*   `(condition)`
*   `(condition` (incomplete)
*   `()`
*   `(` (incomplete)

This change improves readability without altering the parser's behavior.

### 3. Performance: `stat` and `expr` rules

You have already included performance notes about the `stat` and `expr` rules, which is great.

*   **`expr`:** The expression rule uses the standard pattern for handling operator precedence with left-recursion, which ANTLR handles well.
*   **`stat`:** The `stat` rule has many alternatives. The order of these alternatives can sometimes affect performance, especially in cases of ambiguity. Placing the most frequently matched statements earlier in the rule *might* provide a small performance boost, but ANTLR's prediction mechanism is generally very effective, so this is not a critical change.

## Summary of Recommendations

1.  **`sequenceParser.g4`:**
    *   **Left-factor the `group` rule** for better readability and maintainability.
    *   **Simplify the `parExpr` rule** to be more concise.

2.  **`sequenceLexer.g4`:**
    *   The lexer is well-designed, and no changes are recommended.

These suggestions aim to improve the grammar's clarity and maintainability while preserving its excellent error-recovery capabilities.
