Lean

A guided introduction to Lean for theorem proving, formalization, and proof engineering.

4 items

Chapter 1. Core Language and Environment

  1. Installation and toolchain
  2. Project structure with Lean and Lake
  3. Files, namespaces, and modules
  4. Basic syntax and expressions
  5. Definitions with def
  6. Theorems with theorem and lemma
  7. Types and universes
  8. Functions and lambda abstraction
  9. Implicit and explicit arguments
  10. Notation and infix operators
  11. Comments and documentation
  12. Evaluation with #eval
  13. Checking types with #check
  14. Simple rewriting
  15. Pattern matching basics
  16. Inductive types introduction
  17. Structures and records
  18. Basic tactic mode
  19. Term mode vs tactic mode
  20. Holes and goals
  21. Error messages and debugging
  22. Imports and dependencies
  23. Interactive development workflow
  24. Editor integration (VSCode)
  25. Minimal working examples
Chapter 2. Propositions and Proofs
  1. Propositions as types
  2. Implication and functions
  3. Conjunction
  4. Disjunction
  5. Negation
  6. False and contradiction
  7. True and trivial proofs
  8. Equality basics
  9. Rewriting with equality
  10. Symmetry and transitivity
  11. Existential quantifiers
  12. Universal quantifiers
  13. Classical vs constructive logic
  14. Proof by contradiction
  15. Case analysis
  16. Introduction and elimination rules
  17. Proof structuring patterns
  18. Naming conventions
  19. Local context management
  20. Using assumptions effectively
  21. Forward reasoning
  22. Backward reasoning
  23. Combining tactics and terms
  24. Small proof refactoring
  25. Proof readability patterns

Chapter 3. Tactic Framework

  1. The intro tactic
  2. The apply tactic
  3. The exact tactic
  4. The assumption tactic
  5. The rw tactic
  6. The simp tactic
  7. The simp set control
  8. The cases tactic
  9. The induction tactic
  10. The constructor tactic
  11. The have tactic
  12. The let tactic
  13. The show tactic
  14. The refine tactic
  15. The calc block
  16. The conv tactic
  17. The change tactic
  18. The generalize tactic
  19. The revert tactic
  20. The clear tactic
  21. The rename tactic
  22. The simp_all tactic
  23. The aesop tactic
  24. Combining tactics
  25. Debugging tactic scripts

Chapter 4. Equality and Rewriting

  1. Definitional equality
  2. Propositional equality
  3. The rfl proof
  4. Congruence lemmas
  5. Rewriting direction control
  6. Chained rewriting
  7. Rewriting under binders
  8. Rewriting with hypotheses
  9. Dependent rewriting
  10. Substitution patterns
  11. Transport across equality
  12. Heterogeneous equality
  13. simp normalization
  14. Custom simp lemmas
  15. Avoiding rewrite loops
  16. Controlled rewriting strategies
  17. Equality in inductive types
  18. Equality of functions
  19. Extensionality
  20. Proof irrelevance
  21. Decidable equality
  22. Boolean equality bridges
  23. Rewriting with equivalences
  24. Rewriting in structures
  25. Common pitfalls

Chapter 5. Inductive Types

  1. Defining inductive types
  2. Constructors and recursion
  3. Pattern matching deep dive
  4. Structural recursion
  5. Well-founded recursion
  6. Induction principles
  7. Custom induction schemes
  8. Mutual inductive types
  9. Nested inductive types
  10. Indexed families
  11. Finite types
  12. Enumerations
  13. Trees and recursion
  14. Lists and sequences
  15. Options and sums
  16. Products and pairs
  17. Proofs by induction
  18. Eliminators
  19. Recursors
  20. Dependent pattern matching
  21. Inductive propositions
  22. Encoding logical rules
  23. Proof objects
  24. Design patterns
  25. Performance considerations

Chapter 6. Structures and Typeclasses

  1. Structures definition
  2. Field access
  3. Instances
  4. Typeclass basics
  5. Instance search
  6. Coercions
  7. Inheritance patterns
  8. Bundled vs unbundled
  9. Algebraic structures
  10. Custom instances
  11. Overlapping instances
  12. Priority control
  13. Local instances
  14. Deriving mechanisms
  15. Reusable interfaces
  16. Notation with structures
  17. Canonical structures
  18. Class inference debugging
  19. Mixins
  20. Parametric structures
  21. Typeclass design patterns
  22. Automation via classes
  23. Interoperability
  24. Extending libraries
  25. Pitfalls

Chapter 7. Functions and Recursion

  1. Function definitions
  2. Higher-order functions
  3. Currying and uncurrying
  4. Recursion basics
  5. Tail recursion
  6. Termination checking
  7. Measure functions
  8. Structural recursion patterns
  9. Partial functions
  10. Option-returning functions
  11. Error handling
  12. Monadic style
  13. Dependent functions
  14. Function extensionality
  15. Mapping and folding
  16. Composition patterns
  17. Lazy evaluation patterns
  18. Efficiency concerns
  19. Memoization ideas
  20. Rewriting recursive definitions
  21. Equational reasoning
  22. Proofs about functions
  23. Parametricity
  24. Abstraction patterns
  25. Refactoring functions

Chapter 8. Lists and Collections

  1. Lists basics
  2. List recursion
  3. Mapping
  4. Filtering
  5. Folding
  6. Zipping
  7. Concatenation
  8. Membership
  9. Sublist relations
  10. Permutations
  11. Sorting
  12. Multisets
  13. Arrays
  14. Finite sets
  15. Maps and dictionaries
  16. Indexing
  17. Iterators
  18. Collection proofs
  19. Algebraic properties
  20. Complexity reasoning
  21. Efficient representations
  22. Custom containers
  23. Interoperability
  24. Common lemmas
  25. Design patterns

Chapter 9. Arithmetic and Algebra

  1. Natural numbers
  2. Integers
  3. Rational numbers
  4. Real numbers interface
  5. Arithmetic tactics
  6. Ring reasoning
  7. Linear arithmetic
  8. Inequalities
  9. Divisibility
  10. Modular arithmetic
  11. Algebraic identities
  12. Polynomial reasoning
  13. Structures (semigroup, monoid)
  14. Groups
  15. Rings
  16. Fields
  17. Homomorphisms
  18. Algebraic rewriting
  19. Canonical forms
  20. Decision procedures
  21. Automation
  22. Custom algebraic tactics
  23. Proof patterns
  24. Optimization
  25. Integration with libraries

Chapter 10. Logic Engineering

  1. Encoding syntax
  2. Encoding semantics
  3. Abstract syntax trees
  4. Evaluation functions
  5. Substitution
  6. Variable binding
  7. De Bruijn indices
  8. Alpha equivalence
  9. Beta reduction
  10. Small-step semantics
  11. Big-step semantics
  12. Proof systems
  13. Soundness
  14. Completeness
  15. Decidability
  16. Normalization
  17. Encoding inference rules
  18. Automated reasoning
  19. Reflection
  20. Verified interpreters
  21. Verified compilers
  22. Logical frameworks
  23. DSL design
  24. Meta-theory proofs
  25. Case studies

Chapter 11. Metaprogramming

  1. The Lean metaprogramming model
  2. Syntax trees
  3. Elaborator basics
  4. Tactic writing
  5. Custom tactics
  6. Macros
  7. Attributes
  8. Environment inspection
  9. Quotation
  10. Anti-quotation
  11. Reflection
  12. Code generation
  13. Custom commands
  14. Interactive tools
  15. Debugging meta code
  16. Performance tuning
  17. Automation pipelines
  18. Proof search
  19. Custom simplifiers
  20. Domain-specific tactics
  21. Integrating with libraries
  22. Tooling extensions
  23. Safe metaprogramming
  24. Testing meta code
  25. Design patterns

Chapter 12. Automation

  1. Simplification strategies
  2. Rewriting automation
  3. Decision procedures
  4. Search-based tactics
  5. Heuristic design
  6. Controlling search
  7. Custom simp sets
  8. Combining automation
  9. Proof reconstruction
  10. External solvers
  11. SMT integration
  12. Arithmetic automation
  13. Logic automation
  14. Rewriting engines
  15. Performance tuning
  16. Debugging automation
  17. Domain-specific automation
  18. Automation boundaries
  19. Proof minimization
  20. Trusted kernels
  21. Reliability concerns
  22. Incremental automation
  23. Benchmarking
  24. Scaling automation
  25. Case studies

Chapter 13. Formalizing Mathematics

  1. Sets and functions
  2. Relations
  3. Orders
  4. Topology basics
  5. Analysis basics
  6. Algebra formalization
  7. Category theory basics
  8. Combinatorics formalization
  9. Graph theory
  10. Probability basics
  11. Number theory
  12. Linear algebra
  13. Abstract structures
  14. Reusable lemmas
  15. Naming conventions
  16. Library navigation
  17. Documentation patterns
  18. Bridging informal to formal
  19. Formal proof strategies
  20. Refactoring libraries
  21. Dependency management
  22. Cross-domain reuse
  23. Collaboration workflows
  24. Review processes
  25. Case studies

Chapter 14. Large Projects

  1. Project architecture
  2. Module boundaries
  3. Naming systems
  4. Dependency graphs
  5. Build performance
  6. Incremental compilation
  7. Testing strategies
  8. Continuous integration
  9. Documentation systems
  10. Versioning
  11. Refactoring large codebases
  12. Code review practices
  13. Style guides
  14. Collaboration models
  15. Packaging
  16. Distribution
  17. Benchmarking
  18. Profiling
  19. Debugging large systems
  20. Error isolation
  21. Scaling proofs
  22. Automation pipelines
  23. Library evolution
  24. Migration strategies
  25. Case studies

Chapter 15. Interoperability

  1. Calling external code
  2. FFI basics
  3. Data exchange
  4. Serialization
  5. JSON handling
  6. Interfacing with C
  7. Interfacing with Rust
  8. Interfacing with Python
  9. Command line tools
  10. File IO
  11. Network IO
  12. External solvers
  13. Database interaction
  14. Embedding Lean
  15. Extracting programs
  16. Verified pipelines
  17. Toolchain integration
  18. Build system bridges
  19. Testing interoperability
  20. Performance concerns
  21. Security considerations
  22. Sandboxing
  23. Deployment
  24. Monitoring
  25. Case studies

Chapter 16. Performance and Optimization

  1. Evaluation model
  2. Memory usage
  3. Lazy vs strict
  4. Profiling tools
  5. Bottleneck analysis
  6. Optimizing recursion
  7. Efficient data structures
  8. Avoiding recomputation
  9. Caching
  10. Parallelism concepts
  11. Compilation strategies
  12. Kernel performance
  13. Tactic performance
  14. Simplifier tuning
  15. Instance search tuning
  16. Reducing proof size
  17. Code generation efficiency
  18. Benchmarking
  19. Micro-optimizations
  20. Macro-optimizations
  21. Trade-offs
  22. Regression testing
  23. Performance metrics
  24. Scaling strategies
  25. Case studies

Chapter 17. Debugging and Diagnostics

  1. Reading error messages
  2. Type mismatch diagnosis
  3. Unification failures
  4. Missing instances
  5. Infinite loops
  6. Tactic failures
  7. Simplifier issues
  8. Trace tools
  9. Logging
  10. Inspecting goals
  11. Context inspection
  12. Stepwise debugging
  13. Binary search debugging
  14. Minimizing examples
  15. Reproducing bugs
  16. Fix strategies
  17. Testing fixes
  18. Debugging meta code
  19. Debugging automation
  20. Performance debugging
  21. Tooling support
  22. Editor features
  23. Reporting issues
  24. Common pitfalls
  25. Debugging checklist

Chapter 18. Proof Patterns

  1. Direct proofs
  2. Contradiction
  3. Contrapositive
  4. Induction patterns
  5. Case splits
  6. Invariants
  7. Structural recursion proofs
  8. Algebraic proofs
  9. Combinatorial proofs
  10. Constructive proofs
  11. Classical proofs
  12. Equational reasoning
  13. Diagram chasing
  14. Reduction arguments
  15. Encoding arguments
  16. Abstraction patterns
  17. Layered proofs
  18. Reusable lemmas
  19. Proof compression
  20. Proof expansion
  21. Readability patterns
  22. Naming patterns
  23. Refactoring proofs
  24. Anti-patterns
  25. Case studies

Chapter 19. Domain-Specific Libraries

  1. Mathlib overview
  2. Navigating mathlib
  3. Extending mathlib
  4. Custom libraries
  5. DSL construction
  6. Domain modeling
  7. Interfaces
  8. Reuse strategies
  9. Version compatibility
  10. Documentation
  11. Testing
  12. Benchmarking
  13. Distribution
  14. Collaboration
  15. Review processes
  16. Stability
  17. Deprecation
  18. Migration
  19. Tooling
  20. Packaging
  21. Publication
  22. Community standards
  23. Maintenance
  24. Governance
  25. Case studies

Chapter 20. End-to-End Case Studies

  1. Verified arithmetic library
  2. Verified data structure
  3. Verified parser
  4. Verified interpreter
  5. Verified compiler
  6. Verified algorithm
  7. Formal combinatorics project
  8. Formal algebra project
  9. Formal logic system
  10. DSL implementation
  11. Automation pipeline
  12. Custom tactic suite
  13. Large proof development
  14. Library extension
  15. Interoperability project
  16. Performance optimization
  17. Debugging case study
  18. Refactoring case study
  19. Collaboration case study
  20. Teaching example
  21. Research prototype
  22. Industrial application
  23. Deployment scenario
  24. Maintenance workflow
  25. Lessons learned
Programming › Lean ›

Preface

This book is a working manual for the Lean proof assistant.
lean proof-assistant type-theory May 3, 2026
Programming › Lean ›

Chapter 1. Core Language and Environment

This chapter establishes the operational model of the Lean system.
lean proof-assistant type-theory May 3, 2026
Programming › Lean ›

Chapter 2. Propositions and Proofs

This chapter introduces the proof layer of Lean.
lean proof-assistant type-theory May 3, 2026
Programming › Lean ›

Chapter 4. Equality and Rewriting

This chapter isolates the operational core of reasoning in Type Theory as implemented by Lean: equality and its use as a transport mechanism.
lean proof-assistant type-theory May 4, 2026