CD Notes

Syntax Checker Project

Build a comprehensive syntax and semantic analyzer with error reporting

Introduction and Motivation

A syntax checker is like a spell-checker for code — it reads your program and reports every error it can find, with helpful messages pointing to exactly where the problem is and what might fix it. Unlike a full compiler that stops at the first error, a good syntax checker continues analyzing after each error, potentially finding dozens of issues in a single pass. This project teaches you how to build robust error handling into a compiler and how static analysis tools like ESLint, pylint, and clang-tidy work under the hood.

The key insight is that useful error reporting requires more than just detecting errors — it requires recovering from them gracefully so the checker can find additional errors, and providing context-aware suggestions that help the programmer fix the problem quickly.

Project Architecture

Our syntax checker operates as a multi-pass analysis pipeline:

Source Code │ ▼ ┌───────────────────────┐ │ Lexer │ Tokenize, report lexical errors │ (continues on errors) │ (unterminated strings, invalid chars) └──────────┬────────────┘ ▼ ┌───────────────────────┐ │ Parser with Recovery │ Build AST, report syntax errors │ (panic mode + error │ (missing semicolons, unmatched braces) │ productions) │ └──────────┬────────────┘ ▼ ┌───────────────────────┐ │ Semantic Analyzer │ Walk AST, report semantic errors │ (type check, scope, │ (type mismatches, undeclared variables) │ usage analysis) │ └──────────┬────────────┘ ▼ ┌───────────────────────┐ │ Error Reporter │ Format and display all errors │ (sort, deduplicate, │ with source context and suggestions │ severity levels) │ └───────────────────────┘

Error Categories and Detection

Lexical Errors (Detected by Scanner)

c example

// Unterminated string literal
char *msg = "hello world;     // Error: string never closed

// Invalid character in source
int x = 5 § 3;               // Error: '§' not recognized

// Invalid numeric literal  
int y = 0x3G5;               // Error: 'G' invalid in hex

Syntax Errors (Detected by Parser)

c example

// Missing semicolon
int x = 5                     // Error: expected ';' after expression

// Unmatched parentheses
if (x > 5 {                  // Error: expected ')' before '{'

// Invalid expression
int z = * + 3;               // Error: unexpected '*' in expression

// Missing function body
void func()                   // Error: expected '{' after ')'
int y = 10;

Semantic Errors (Detected by Analyzer)

c example

// Undeclared variable
int y = x + 5;               // Error: 'x' not declared in this scope

// Type mismatch
int count = "hello";         // Error: cannot assign string to int

// Duplicate declaration
int x = 5;
int x = 10;                  // Error: 'x' already declared at line 1

// Function argument mismatch
int add(int a, int b);
add(1, 2, 3);                // Error: too many arguments (expected 2)

Error Recovery Strategies

The most important aspect of a syntax checker is continuing after errors. Here are the strategies implemented:

Panic Mode Recovery

When an error is detected, skip tokens until a synchronizing token (like ;, }, or a keyword) is found:

c example

void recover_to_synchronize(Parser *p) {
    // Skip tokens until we find a safe restart point
    while (p->current.type != TOKEN_SEMICOLON &&
           p->current.type != TOKEN_RBRACE &&
           p->current.type != TOKEN_EOF) {
        advance(p);
    }
    if (p->current.type == TOKEN_SEMICOLON)
        advance(p);  // Consume the synchronizing token
}

Error Productions in Grammar

Add grammar rules that explicitly handle common mistakes:

bison example

statement : expression ';'
          | expression error  { report("Missing semicolon"); yyerrok; }
          | error ';'         { report("Invalid statement"); yyerrok; }
          ;

if_stmt : IF '(' expression ')' block
        | IF expression block  { report("Missing parentheses around condition"); }
        | IF '(' expression block { report("Missing ')' after condition"); }
        ;

Brace Matching with Stack

Track opening braces on a stack to detect mismatches and provide helpful messages:

Example

typedef struct { char brace; int line; int column; } BraceEntry; BraceEntry brace_stack[256]; int brace_top = 0; void check_brace(char open, char close, int line) { if (brace_top == 0) { report_error("Unexpected '%c' without matching '%c'", close, open); return; } BraceEntry top = brace_stack[--brace_top]; if (top.brace != open) { report_error("Mismatched: '%c' at line %d closed by '%c' at line %d", top.brace, top.line, close, line); } }

The Error Object

Each detected error is stored in a structured format for later reporting:

Example

typedef enum { SEVERITY_WARNING, SEVERITY_ERROR, SEVERITY_FATAL } Severity; typedef struct { Severity severity; char category[16]; // "lexical", "syntax", "semantic", "type" char message[256]; // Human-readable description int line; int column; int end_column; // For underlining the error span char source_line[512]; // The actual source code line char suggestion[256]; // Optional fix suggestion char related_note[256]; // E.g., "first declared here at line 5" } Error; // Collect all errors during analysis Error errors[MAX_ERRORS]; int error_count = 0;

Pretty Error Output

The error reporter formats errors with source context, inspired by modern compilers like Rust's error messages:

12	int sum = count + value;
18	int result = x + y
19	return result;
7	int temp = 0;

Implementing "Did You Mean?" Suggestions

For undeclared variables, compute edit distance to all declared variables and suggest the closest match:

Example

int levenshtein(const char *s, const char *t) { int m = strlen(s), n = strlen(t); int dp[m+1][n+1]; for (int i = 0; i <= m; i++) dp[i][0] = i; for (int j = 0; j <= n; j++) dp[0][j] = j; for (int i = 1; i <= m; i++) for (int j = 1; j <= n; j++) { int cost = (s[i-1] != t[j-1]) ? 1 : 0; dp[i][j] = min3(dp[i-1][j]+1, dp[i][j-1]+1, dp[i-1][j-1]+cost); } return dp[m][n]; } char* suggest_variable(const char *unknown, SymbolTable *st) { char *best = NULL; int best_dist = INT_MAX; for (Symbol *s = st->first; s; s = s->next) { int dist = levenshtein(unknown, s->name); if (dist < best_dist && dist <= 2) { // Max 2 edits best_dist = dist; best = s->name; } } return best; }

Testing Strategy

Create test files for each error category:

Code example

test_clean.c         — Valid program, zero errors expected
test_semicolons.c    — Missing semicolons at various positions
test_braces.c        — Mismatched and missing braces
test_undeclared.c    — Variables used before declaration
test_types.c         — Type mismatch errors
test_duplicates.c    — Duplicate variable/function declarations
test_args.c          — Wrong number of function arguments
test_combined.c      — Multiple error types in one file
test_recovery.c      — Verify checker continues after errors

For each test file, maintain an expected output file and write an automated test that compares actual output against expected output. This regression testing catches any changes that break existing error detection.

Key Takeaways

Building a syntax checker teaches you that error handling is not an afterthought — it is a fundamental design consideration that affects every phase of the compiler. Good error recovery allows finding multiple errors per compilation, saving developers time. Helpful error messages with source context, suggestions, and related notes transform a frustrating debugging experience into a guided correction process. These techniques are used by every modern IDE and linting tool, making this knowledge immediately practical.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Syntax Checker Project.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Compiler Design topic.

Search Terms

compiler-design, compiler design, compiler, design, projects, syntax, checker, project

Continue learning this concept

ProjectsExpression Parser ProjectBuild an advanced mathematical expression parser with tree visualization ProjectsLexical Analyzer ProjectPractical project for building a lexical analyzer with token recognition ProjectsToy Compiler Project: Minimal Complete CompilerStep-by-step guide to building a complete toy compiler from scratch ProjectsCalculator Compiler ProjectDetailed project guide for building a mathematical expression evaluator ProjectsMini Programming Language ProjectComplete project building an interpreter for a small procedural language Intermediate Code GenerationSyntax Trees as Intermediate RepresentationGuide to syntax trees and abstract syntax trees as intermediate representations in compilers, covering construction, traversal, differences from parse trees, and translation to three-address code.

PreviousMini Programming Language Project NextToy Compiler Project: Minimal Complete Compiler

CD Notes

Syntax Checker Project

Build a comprehensive syntax and semantic analyzer with error reporting

Introduction and Motivation

Project Architecture

Our syntax checker operates as a multi-pass analysis pipeline:

Error Categories and Detection

Lexical Errors (Detected by Scanner)

c example

// Unterminated string literal
char *msg = "hello world;     // Error: string never closed

// Invalid character in source
int x = 5 § 3;               // Error: '§' not recognized

// Invalid numeric literal  
int y = 0x3G5;               // Error: 'G' invalid in hex

Syntax Errors (Detected by Parser)

c example

// Missing semicolon
int x = 5                     // Error: expected ';' after expression

// Unmatched parentheses
if (x > 5 {                  // Error: expected ')' before '{'

// Invalid expression
int z = * + 3;               // Error: unexpected '*' in expression

// Missing function body
void func()                   // Error: expected '{' after ')'
int y = 10;

Semantic Errors (Detected by Analyzer)

c example

// Undeclared variable
int y = x + 5;               // Error: 'x' not declared in this scope

// Type mismatch
int count = "hello";         // Error: cannot assign string to int

// Duplicate declaration
int x = 5;
int x = 10;                  // Error: 'x' already declared at line 1

// Function argument mismatch
int add(int a, int b);
add(1, 2, 3);                // Error: too many arguments (expected 2)

Error Recovery Strategies

The most important aspect of a syntax checker is continuing after errors. Here are the strategies implemented:

Panic Mode Recovery

When an error is detected, skip tokens until a synchronizing token (like ;, }, or a keyword) is found:

c example

void recover_to_synchronize(Parser *p) {
    // Skip tokens until we find a safe restart point
    while (p->current.type != TOKEN_SEMICOLON &&
           p->current.type != TOKEN_RBRACE &&
           p->current.type != TOKEN_EOF) {
        advance(p);
    }
    if (p->current.type == TOKEN_SEMICOLON)
        advance(p);  // Consume the synchronizing token
}

Error Productions in Grammar

Add grammar rules that explicitly handle common mistakes:

bison example

statement : expression ';'
          | expression error  { report("Missing semicolon"); yyerrok; }
          | error ';'         { report("Invalid statement"); yyerrok; }
          ;

if_stmt : IF '(' expression ')' block
        | IF expression block  { report("Missing parentheses around condition"); }
        | IF '(' expression block { report("Missing ')' after condition"); }
        ;

Brace Matching with Stack

Track opening braces on a stack to detect mismatches and provide helpful messages:

Example

The Error Object

Each detected error is stored in a structured format for later reporting:

Example

Pretty Error Output

The error reporter formats errors with source context, inspired by modern compilers like Rust's error messages:

12	int sum = count + value;
18	int result = x + y
19	return result;
7	int temp = 0;

Implementing "Did You Mean?" Suggestions

For undeclared variables, compute edit distance to all declared variables and suggest the closest match:

Example

Testing Strategy

Create test files for each error category:

Code example

test_clean.c         — Valid program, zero errors expected
test_semicolons.c    — Missing semicolons at various positions
test_braces.c        — Mismatched and missing braces
test_undeclared.c    — Variables used before declaration
test_types.c         — Type mismatch errors
test_duplicates.c    — Duplicate variable/function declarations
test_args.c          — Wrong number of function arguments
test_combined.c      — Multiple error types in one file
test_recovery.c      — Verify checker continues after errors

Key Takeaways

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Syntax Checker Project.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Compiler Design topic.

Search Terms

compiler-design, compiler design, compiler, design, projects, syntax, checker, project

Continue learning this concept

PreviousMini Programming Language Project NextToy Compiler Project: Minimal Complete Compiler