11. GHC Language Features

• 11.1. Language options
• 11.2. Unboxed types and primitive operations
  • 11.2.1. Unboxed types
  • 11.2.2. Unboxed type kinds
  • 11.2.3. Unboxed tuples
  • 11.2.4. Unboxed sums
• 11.3. Syntactic extensions
  • 11.3.1. Unicode syntax
  • 11.3.2. The magic hash
  • 11.3.3. Negative literals
  • 11.3.4. Fractional looking integer literals
  • 11.3.5. Binary integer literals
  • 11.3.6. Hexadecimal floating point literals
  • 11.3.7. Pattern guards
  • 11.3.8. View patterns
  • 11.3.9. n+k patterns
  • 11.3.10. The recursive do-notation
    • 11.3.10.1. Recursive binding groups
    • 11.3.10.2. The mdo notation
  • 11.3.11. Applicative do-notation
    • 11.3.11.1. Strict patterns
    • 11.3.11.2. Things to watch out for
  • 11.3.12. Parallel List Comprehensions
  • 11.3.13. Generalised (SQL-like) List Comprehensions
  • 11.3.14. Monad comprehensions
  • 11.3.15. New monadic failure desugaring mechanism
  • 11.3.16. Rebindable syntax and the implicit Prelude import
    • 11.3.16.1. Things unaffected by RebindableSyntax
  • 11.3.17. Postfix operators
  • 11.3.18. Tuple sections
  • 11.3.19. Lambda-case
  • 11.3.20. Empty case alternatives
  • 11.3.21. Multi-way if-expressions
  • 11.3.22. Local Fixity Declarations
  • 11.3.23. Import and export extensions
    • 11.3.23.1. Hiding things the imported module doesn't export
    • 11.3.23.2. Package-qualified imports
    • 11.3.23.3. Safe imports
    • 11.3.23.4. Explicit namespaces in import/export
  • 11.3.24. Summary of stolen syntax
• 11.4. Extensions to data types and type synonyms
  • 11.4.1. Data types with no constructors
  • 11.4.2. Data type contexts
  • 11.4.3. Infix type constructors, classes, and type variables
  • 11.4.4. Type operators
  • 11.4.5. Liberalised type synonyms
  • 11.4.6. Existentially quantified data constructors
    • 11.4.6.1. Why existential?
    • 11.4.6.2. Existentials and type classes
    • 11.4.6.3. Record Constructors
    • 11.4.6.4. Restrictions
  • 11.4.7. Declaring data types with explicit constructor signatures
  • 11.4.8. Generalised Algebraic Data Types (GADTs)
• 11.5. Extensions to the record system
  • 11.5.1. Traditional record syntax
  • 11.5.2. Record field disambiguation
  • 11.5.3. Duplicate record fields
    • 11.5.3.1. Selector functions
    • 11.5.3.2. Record updates
    • 11.5.3.3. Import and export of record fields
  • 11.5.4. Record puns
  • 11.5.5. Record wildcards
  • 11.5.6. Record field selector polymorphism
    • 11.5.6.1. Solving HasField constraints
    • 11.5.6.2. Virtual record fields
• 11.6. Extensions to the "deriving" mechanism
  • 11.6.1. Deriving instances for empty data types
  • 11.6.2. Inferred context for deriving clauses
  • 11.6.3. Stand-alone deriving declarations
  • 11.6.4. Deriving instances of extra classes (Data, etc.)
    • 11.6.4.1. Deriving Functor instances
    • 11.6.4.2. Deriving Foldable instances
    • 11.6.4.3. Deriving Traversable instances
    • 11.6.4.4. Deriving Data instances
    • 11.6.4.5. Deriving Typeable instances
    • 11.6.4.6. Deriving Lift instances
  • 11.6.5. Generalised derived instances for newtypes
    • 11.6.5.1. Generalising the deriving clause
    • 11.6.5.2. A more precise specification
    • 11.6.5.3. Associated type families
  • 11.6.6. Deriving any other class
  • 11.6.7. Deriving strategies
    • 11.6.7.1. Default deriving strategy
• 11.7. Pattern synonyms
  • 11.7.1. Record Pattern Synonyms
  • 11.7.2. Syntax and scoping of pattern synonyms
  • 11.7.3. Import and export of pattern synonyms
  • 11.7.4. Typing of pattern synonyms
  • 11.7.5. Matching of pattern synonyms
• 11.8. Class and instances declarations
  • 11.8.1. Class declarations
    • 11.8.1.1. Multi-parameter type classes
    • 11.8.1.2. The superclasses of a class declaration
    • 11.8.1.3. Constrained class method types
    • 11.8.1.4. Default method signatures
    • 11.8.1.5. Nullary type classes
  • 11.8.2. Functional dependencies
    • 11.8.2.1. Rules for functional dependencies
    • 11.8.2.2. Background on functional dependencies
      • 11.8.2.2.1. An attempt to use constructor classes
      • 11.8.2.2.2. Adding functional dependencies
  • 11.8.3. Instance declarations
    • 11.8.3.1. Instance resolution
    • 11.8.3.2. Relaxed rules for the instance head
    • 11.8.3.3. Relaxed rules for instance contexts
    • 11.8.3.4. Instance termination rules
    • 11.8.3.5. Undecidable instances
    • 11.8.3.6. Overlapping instances
    • 11.8.3.7. Instance signatures: type signatures in instance declarations
  • 11.8.4. Overloaded string literals
  • 11.8.5. Overloaded labels
  • 11.8.6. Overloaded lists
    • 11.8.6.1. The IsList class
    • 11.8.6.2. Rebindable syntax
    • 11.8.6.3. Defaulting
    • 11.8.6.4. Speculation about the future
  • 11.8.7. Undecidable (or recursive) superclasses
• 11.9. Type families
  • 11.9.1. Data families
    • 11.9.1.1. Data family declarations
    • 11.9.1.2. Data instance declarations
    • 11.9.1.3. Overlap of data instances
  • 11.9.2. Synonym families
    • 11.9.2.1. Type family declarations
    • 11.9.2.2. Type instance declarations
    • 11.9.2.3. Closed type families
    • 11.9.2.4. Type family examples
    • 11.9.2.5. Compatibility and apartness of type family equations
    • 11.9.2.6. Decidability of type synonym instances
  • 11.9.3. Wildcards on the LHS of data and type family instances
  • 11.9.4. Associated data and type families
    • 11.9.4.1. Associated instances
    • 11.9.4.2. Associated type synonym defaults
    • 11.9.4.3. Scoping of class parameters
    • 11.9.4.4. Instance contexts and associated type and data instances
  • 11.9.5. Import and export
    • 11.9.5.1. Examples
    • 11.9.5.2. Instances
  • 11.9.6. Type families and instance declarations
  • 11.9.7. Injective type families
    • 11.9.7.1. Syntax of injectivity annotation
    • 11.9.7.2. Verifying injectivity annotation against type family equations
• 11.10. Datatype promotion
  • 11.10.1. Motivation
  • 11.10.2. Overview
  • 11.10.3. Distinguishing between types and constructors
  • 11.10.4. Promoted list and tuple types
  • 11.10.5. Promoting existential data constructors
• 11.11. Kind polymorphism and Type-in-Type
  • 11.11.1. The difference between TypeInType and PolyKinds
  • 11.11.2. Overview of kind polymorphism
  • 11.11.3. Overview of Type-in-Type
  • 11.11.4. Principles of kind inference
  • 11.11.5. Complete user-supplied kind signatures and polymorphic recursion
  • 11.11.6. Kind inference in closed type families
  • 11.11.7. Kind inference in class instance declarations
  • 11.11.8. Kind inference in type signatures
  • 11.11.9. Explicit kind quantification
  • 11.11.10. Kind-indexed GADTs
  • 11.11.11. Higher-rank kinds
  • 11.11.12. Constraints in kinds
  • 11.11.13. The kind *
  • 11.11.14. Inferring dependency in datatype declarations
  • 11.11.15. Kind defaulting without PolyKinds
  • 11.11.16. Pretty-printing in the presence of kind polymorphism
• 11.12. Levity polymorphism
  • 11.12.1. No levity-polymorphic variables or arguments
  • 11.12.2. Levity-polymorphic bottoms
  • 11.12.3. Printing levity-polymorphic types
• 11.13. Type-Level Literals
  • 11.13.1. Runtime Values for Type-Level Literals
  • 11.13.2. Computing With Type-Level Naturals
• 11.14. Constraints in types
  • 11.14.1. Equality constraints
  • 11.14.2. Heterogeneous equality
  • 11.14.3. Unlifted heterogeneous equality
  • 11.14.4. The Coercible constraint
  • 11.14.5. The Constraint kind
• 11.15. Extensions to type signatures
  • 11.15.1. Explicit universal quantification (forall)
  • 11.15.2. The context of a type signature
  • 11.15.3. Ambiguous types and the ambiguity check
  • 11.15.4. Explicitly-kinded quantification
• 11.16. Lexically scoped type variables
  • 11.16.1. Overview
  • 11.16.2. Declaration type signatures
  • 11.16.3. Expression type signatures
  • 11.16.4. Pattern type signatures
  • 11.16.5. Class and instance declarations
• 11.17. Bindings and generalisation
  • 11.17.1. Switching off the dreaded Monomorphism Restriction
  • 11.17.2. Let-generalisation
  • 11.17.3. Kind generalisation
• 11.18. Visible type application
• 11.19. Implicit parameters
  • 11.19.1. Implicit-parameter type constraints
  • 11.19.2. Implicit-parameter bindings
  • 11.19.3. Implicit parameters and polymorphic recursion
  • 11.19.4. Implicit parameters and monomorphism
• 11.20. Arbitrary-rank polymorphism
  • 11.20.1. Examples
  • 11.20.2. Type inference
  • 11.20.3. Implicit quantification
• 11.21. Impredicative polymorphism
• 11.22. Typed Holes
• 11.23. Partial Type Signatures
  • 11.23.1. Syntax
    • 11.23.1.1. Type Wildcards
    • 11.23.1.2. Named Wildcards
    • 11.23.1.3. Extra-Constraints Wildcard
  • 11.23.2. Where can they occur?
• 11.24. Custom compile-time errors
• 11.25. Deferring type errors to runtime
  • 11.25.1. Enabling deferring of type errors
  • 11.25.2. Deferred type errors in GHCi
• 11.26. Template Haskell
  • 11.26.1. Syntax
  • 11.26.2. Using Template Haskell
  • 11.26.3. Viewing Template Haskell generated code
  • 11.26.4. A Template Haskell Worked Example
  • 11.26.5. Using Template Haskell with Profiling
  • 11.26.6. Template Haskell Quasi-quotation
• 11.27. Arrow notation
  • 11.27.1. do-notation for commands
  • 11.27.2. Conditional commands
  • 11.27.3. Defining your own control structures
  • 11.27.4. Primitive constructs
  • 11.27.5. Differences with the paper
  • 11.27.6. Portability
• 11.28. Bang patterns and Strict Haskell
  • 11.28.1. Bang patterns
  • 11.28.2. Strict-by-default data types
  • 11.28.3. Strict-by-default pattern bindings
  • 11.28.4. Modularity
  • 11.28.5. Dynamic semantics of bang patterns
• 11.29. Assertions
• 11.30. Static pointers
  • 11.30.1. Using static pointers
  • 11.30.2. Static semantics of static pointers
• 11.31. Pragmas
  • 11.31.1. LANGUAGE pragma
  • 11.31.2. OPTIONS_GHC pragma
  • 11.31.3. INCLUDE pragma
  • 11.31.4. WARNING and DEPRECATED pragmas
  • 11.31.5. MINIMAL pragma
  • 11.31.6. INLINE and NOINLINE pragmas
    • 11.31.6.1. INLINE pragma
    • 11.31.6.2. INLINABLE pragma
    • 11.31.6.3. NOINLINE pragma
    • 11.31.6.4. CONLIKE modifier
    • 11.31.6.5. Phase control
  • 11.31.7. LINE pragma
  • 11.31.8. COLUMN pragma
  • 11.31.9. RULES pragma
  • 11.31.10. SPECIALIZE pragma
    • 11.31.10.1. SPECIALIZE INLINE
    • 11.31.10.2. SPECIALIZE for imported functions
    • 11.31.10.3. Obsolete SPECIALIZE syntax
  • 11.31.11. SPECIALIZE instance pragma
  • 11.31.12. UNPACK pragma
  • 11.31.13. NOUNPACK pragma
  • 11.31.14. SOURCE pragma
  • 11.31.15. COMPLETE pragmas
  • 11.31.16. Disambiguating between multiple COMPLETE pragmas
  • 11.31.17. OVERLAPPING, OVERLAPPABLE, OVERLAPS, and INCOHERENT pragmas
• 11.32. Rewrite rules
  • 11.32.1. Syntax
  • 11.32.2. Semantics
  • 11.32.3. How rules interact with INLINE/NOINLINE pragmas
  • 11.32.4. How rules interact with CONLIKE pragmas
  • 11.32.5. How rules interact with class methods
  • 11.32.6. List fusion
  • 11.32.7. Specialisation
  • 11.32.8. Controlling what's going on in rewrite rules
• 11.33. Special built-in functions
• 11.34. Generic classes
• 11.35. Generic programming
  • 11.35.1. Deriving representations
  • 11.35.2. Writing generic functions
  • 11.35.3. Unlifted representation types
  • 11.35.4. Generic defaults
  • 11.35.5. More information
• 11.36. Roles
  • 11.36.1. Nominal, Representational, and Phantom
  • 11.36.2. Role inference
  • 11.36.3. Role annotations
• 11.37. HasCallStack
  • 11.37.1. Compared with other sources of stack traces
• 11.38. Concurrent and Parallel Haskell
  • 11.38.1. Concurrent Haskell
  • 11.38.2. Software Transactional Memory
  • 11.38.3. Parallel Haskell
  • 11.38.4. Annotating pure code for parallelism
  • 11.38.5. Data Parallel Haskell
• 11.39. Safe Haskell
  • 11.39.1. Uses of Safe Haskell
    • 11.39.1.1. Strict type-safety (good style)
    • 11.39.1.2. Building secure systems (restricted IO Monads)
  • 11.39.2. Safe Language
    • 11.39.2.1. Safe Overlapping Instances
  • 11.39.3. Safe Imports
  • 11.39.4. Trust and Safe Haskell Modes
    • 11.39.4.1. Trust check (-fpackage-trust disabled)
    • 11.39.4.2. Trust check (-fpackage-trust enabled)
    • 11.39.4.3. Example
    • 11.39.4.4. Trustworthy Requirements
    • 11.39.4.5. Package Trust
  • 11.39.5. Safe Haskell Inference
  • 11.39.6. Safe Haskell Flag Summary
  • 11.39.7. Safe Compilation