Glasgow Haskell Compiler User's Guide

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11. Advice on: sooner, faster, smaller, thriftier

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12.1. Language options

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12. GHC Language Features¶

12.1. Language options
12.2. Unboxed types and primitive operations
- 12.2.1. Unboxed types
- 12.2.2. Unboxed type kinds
- 12.2.3. Unboxed tuples
- 12.2.4. Unboxed sums
12.3. Syntactic extensions
- 12.3.1. Unicode syntax
- 12.3.2. The magic hash
- 12.3.3. Negative literals
- 12.3.4. Fractional looking integer literals
- 12.3.5. Binary integer literals
- 12.3.6. Hexadecimal floating point literals
- 12.3.7. Numeric underscores
- 12.3.8. Pattern guards
- 12.3.9. View patterns
- 12.3.10. n+k patterns
- 12.3.11. The recursive do-notation
  - 12.3.11.1. Recursive binding groups
  - 12.3.11.2. The mdo notation
- 12.3.12. Applicative do-notation
  - 12.3.12.1. Strict patterns
  - 12.3.12.2. Things to watch out for
- 12.3.13. Parallel List Comprehensions
- 12.3.14. Generalised (SQL-like) List Comprehensions
- 12.3.15. Monad comprehensions
- 12.3.16. New monadic failure desugaring mechanism
- 12.3.17. Rebindable syntax and the implicit Prelude import
  - 12.3.17.1. Things unaffected by RebindableSyntax
- 12.3.18. Postfix operators
- 12.3.19. Tuple sections
- 12.3.20. Lambda-case
- 12.3.21. Empty case alternatives
- 12.3.22. Multi-way if-expressions
- 12.3.23. Local Fixity Declarations
- 12.3.24. Import and export extensions
  - 12.3.24.1. Hiding things the imported module doesn’t export
  - 12.3.24.2. Package-qualified imports
  - 12.3.24.3. Safe imports
  - 12.3.24.4. Explicit namespaces in import/export
- 12.3.25. More liberal syntax for function arguments
  - 12.3.25.1. Changes to the grammar
- 12.3.26. Summary of stolen syntax
12.4. Extensions to data types and type synonyms
- 12.4.1. Data types with no constructors
- 12.4.2. Data type contexts
- 12.4.3. Infix type constructors, classes, and type variables
- 12.4.4. Type operators
- 12.4.5. Liberalised type synonyms
- 12.4.6. Existentially quantified data constructors
- 12.4.7. Declaring data types with explicit constructor signatures
- 12.4.8. Generalised Algebraic Data Types (GADTs)
12.5. Extensions to the record system
- 12.5.1. Traditional record syntax
- 12.5.2. Record field disambiguation
- 12.5.3. Duplicate record fields
- 12.5.4. Record puns
- 12.5.5. Record wildcards
- 12.5.6. Record field selector polymorphism
  - 12.5.6.1. Solving HasField constraints
  - 12.5.6.2. Virtual record fields
12.6. Extensions to the “deriving” mechanism
- 12.6.1. Deriving instances for empty data types
- 12.6.2. Inferred context for deriving clauses
- 12.6.3. Stand-alone deriving declarations
- 12.6.4. Deriving instances of extra classes (Data, etc.)
- 12.6.5. Generalised derived instances for newtypes
- 12.6.6. Deriving any other class
- 12.6.7. Deriving strategies
  - 12.6.7.1. Default deriving strategy
- 12.6.8. Deriving via
12.7. Pattern synonyms
- 12.7.1. Record Pattern Synonyms
- 12.7.2. Syntax and scoping of pattern synonyms
- 12.7.3. Import and export of pattern synonyms
- 12.7.4. Typing of pattern synonyms
- 12.7.5. Matching of pattern synonyms
12.8. Class and instances declarations
- 12.8.1. Class declarations
- 12.8.2. Functional dependencies
  - 12.8.2.1. Rules for functional dependencies
  - 12.8.2.2. Background on functional dependencies
    - 12.8.2.2.1. An attempt to use constructor classes
    - 12.8.2.2.2. Adding functional dependencies
- 12.8.3. Instance declarations
- 12.8.4. Overloaded string literals
- 12.8.5. Overloaded labels
- 12.8.6. Overloaded lists
- 12.8.7. Undecidable (or recursive) superclasses
12.9. Type families
- 12.9.1. Data families
- 12.9.2. Synonym families
- 12.9.3. Wildcards on the LHS of data and type family instances
- 12.9.4. Associated data and type families
- 12.9.5. Import and export
  - 12.9.5.1. Examples
  - 12.9.5.2. Instances
- 12.9.6. Type families and instance declarations
- 12.9.7. Injective type families
  - 12.9.7.1. Syntax of injectivity annotation
  - 12.9.7.2. Verifying injectivity annotation against type family equations
12.10. Datatype promotion
- 12.10.1. Motivation
- 12.10.2. Overview
- 12.10.3. Distinguishing between types and constructors
- 12.10.4. Promoted list and tuple types
- 12.10.5. Promoting existential data constructors
12.11. Kind polymorphism
- 12.11.1. Overview of kind polymorphism
- 12.11.2. Overview of Type-in-Type
- 12.11.3. Principles of kind inference
- 12.11.4. Complete user-supplied kind signatures and polymorphic recursion
- 12.11.5. Kind inference in closed type families
- 12.11.6. Kind inference in class instance declarations
- 12.11.7. Kind inference in type signatures
- 12.11.8. Explicit kind quantification
- 12.11.9. Kind-indexed GADTs
- 12.11.10. Higher-rank kinds
- 12.11.11. Constraints in kinds
- 12.11.12. The kind Type
- 12.11.13. Inferring dependency in datatype declarations
- 12.11.14. Inferring dependency in user-written foralls
- 12.11.15. Kind defaulting without PolyKinds
- 12.11.16. Pretty-printing in the presence of kind polymorphism
12.12. Levity polymorphism
- 12.12.1. No levity-polymorphic variables or arguments
- 12.12.2. Levity-polymorphic bottoms
- 12.12.3. Printing levity-polymorphic types
12.13. Type-Level Literals
- 12.13.1. Runtime Values for Type-Level Literals
- 12.13.2. Computing With Type-Level Naturals
12.14. Equality constraints, Coercible, and the kind Constraint
- 12.14.1. Equality constraints
- 12.14.2. Heterogeneous equality
- 12.14.3. Unlifted heterogeneous equality
- 12.14.4. The Coercible constraint
- 12.14.5. The Constraint kind
12.15. Quantified constraints
- 12.15.1. Motivation
- 12.15.2. Syntax changes
- 12.15.3. Typing changes
- 12.15.4. Superclasses
- 12.15.5. Overlap
- 12.15.6. Instance lookup
- 12.15.7. Termination
- 12.15.8. Coherence
12.16. Extensions to type signatures
- 12.16.1. Explicit universal quantification (forall)
- 12.16.2. The context of a type signature
- 12.16.3. Ambiguous types and the ambiguity check
- 12.16.4. Explicitly-kinded quantification
12.17. Lexically scoped type variables
- 12.17.1. Overview
- 12.17.2. Declaration type signatures
- 12.17.3. Expression type signatures
- 12.17.4. Pattern type signatures
- 12.17.5. Class and instance declarations
12.18. Bindings and generalisation
- 12.18.1. Switching off the dreaded Monomorphism Restriction
- 12.18.2. Let-generalisation
- 12.18.3. Kind generalisation
12.19. Visible type application
12.20. Implicit parameters
- 12.20.1. Implicit-parameter type constraints
- 12.20.2. Implicit-parameter bindings
- 12.20.3. Implicit parameters and polymorphic recursion
- 12.20.4. Implicit parameters and monomorphism
12.21. Arbitrary-rank polymorphism
- 12.21.1. Examples
- 12.21.2. Type inference
- 12.21.3. Implicit quantification
12.22. Impredicative polymorphism
12.23. Typed Holes
- 12.23.1. Valid Hole Fits
  - 12.23.1.1. Refinement Hole Fits
  - 12.23.1.2. Sorting Valid Hole Fits
12.24. Partial Type Signatures
- 12.24.1. Syntax
- 12.24.2. Where can they occur?
12.25. Custom compile-time errors
12.26. Deferring type errors to runtime
- 12.26.1. Enabling deferring of type errors
- 12.26.2. Deferred type errors in GHCi
- 12.26.3. Limitations of deferred type errors
12.27. Template Haskell
- 12.27.1. Syntax
- 12.27.2. Using Template Haskell
- 12.27.3. Viewing Template Haskell generated code
- 12.27.4. A Template Haskell Worked Example
- 12.27.5. Using Template Haskell with Profiling
- 12.27.6. Template Haskell Quasi-quotation
12.28. Arrow notation
- 12.28.1. do-notation for commands
- 12.28.2. Conditional commands
- 12.28.3. Defining your own control structures
- 12.28.4. Primitive constructs
- 12.28.5. Differences with the paper
- 12.28.6. Portability
12.29. Bang patterns and Strict Haskell
- 12.29.1. Bang patterns
- 12.29.2. Strict-by-default data types
- 12.29.3. Strict-by-default pattern bindings
- 12.29.4. Modularity
- 12.29.5. Dynamic semantics of bang patterns
12.30. Assertions
12.31. Static pointers
- 12.31.1. Using static pointers
- 12.31.2. Static semantics of static pointers
12.32. Pragmas
- 12.32.1. LANGUAGE pragma
- 12.32.2. OPTIONS_GHC pragma
- 12.32.3. INCLUDE pragma
- 12.32.4. WARNING and DEPRECATED pragmas
- 12.32.5. MINIMAL pragma
- 12.32.6. INLINE and NOINLINE pragmas
- 12.32.7. LINE pragma
- 12.32.8. COLUMN pragma
- 12.32.9. RULES pragma
- 12.32.10. SPECIALIZE pragma
- 12.32.11. SPECIALIZE instance pragma
- 12.32.12. UNPACK pragma
- 12.32.13. NOUNPACK pragma
- 12.32.14. SOURCE pragma
- 12.32.15. COMPLETE pragmas
- 12.32.16. Disambiguating between multiple COMPLETE pragmas
- 12.32.17. OVERLAPPING, OVERLAPPABLE, OVERLAPS, and INCOHERENT pragmas
12.33. Rewrite rules
- 12.33.1. Syntax
- 12.33.2. Semantics
- 12.33.3. How rules interact with INLINE/NOINLINE pragmas
- 12.33.4. How rules interact with CONLIKE pragmas
- 12.33.5. How rules interact with class methods
- 12.33.6. List fusion
- 12.33.7. Specialisation
- 12.33.8. Controlling what’s going on in rewrite rules
12.34. Special built-in functions
12.35. Generic classes
12.36. Generic programming
- 12.36.1. Deriving representations
- 12.36.2. Writing generic functions
- 12.36.3. Unlifted representation types
- 12.36.4. Generic defaults
- 12.36.5. More information
12.37. Roles
- 12.37.1. Nominal, Representational, and Phantom
- 12.37.2. Role inference
- 12.37.3. Role annotations
12.38. HasCallStack
- 12.38.1. Compared with other sources of stack traces
12.39. Concurrent and Parallel Haskell
- 12.39.1. Concurrent Haskell
- 12.39.2. Software Transactional Memory
- 12.39.3. Parallel Haskell
- 12.39.4. Annotating pure code for parallelism
12.40. Safe Haskell
- 12.40.1. Uses of Safe Haskell
  - 12.40.1.1. Strict type-safety (good style)
  - 12.40.1.2. Building secure systems (restricted IO Monads)
- 12.40.2. Safe Language
  - 12.40.2.1. Safe Overlapping Instances
- 12.40.3. Safe Imports
- 12.40.4. Trust and Safe Haskell Modes
- 12.40.5. Safe Haskell Inference
- 12.40.6. Safe Haskell Flag Summary
- 12.40.7. Safe Compilation