The Haskell Error Index
Welcome
This site describes the various messages that can be returned by Haskell-related tools, including both errors and warnings.
For example GHC, the most-commonly-used Haskell implementation, started emiting a code with the format [GHC-12345] for its
messages in version 9.6.1.
These codes can be looked up below for further information.
So far, 141 errors and warnings are documented here. If you encounter a message that is not yet documented, please open an issue or submit a pull request with documentation. Pull requests with additional examples and improved explanations for existing messages are also very welcome, as are improvements to the generated site.
This site is a project of The Haskell Foundation. Please get in touch if you have feedback or if you'd like to get involved!
The Messages
| Nr. | Message | Summary | Since |
|---|---|---|---|
| Cabal-7070 | Run whole executable | Cabal runs executables but not modules | 3.12.1.0 |
| GHC-00158 | Cannot derive instance for non-stock-deriveable class | An instance cannot be derived because the class is not stock deriveable. | 9.6.1 |
| GHC-00482 | Lambda syntax in pattern | Lambda syntax appears in a pattern match. | 9.6.1 |
| GHC-00711 | Duplicate warning declarations | Duplicate deprecation warnings are illegal | 9.6.1 |
| GHC-01239 | Unexpected if expression in function application | If expression used as function argument | 9.6.1 |
| GHC-01629 | Too many type arguments in constructor pattern | Too many type applications to a constructor in a pattern. | 9.6.1 |
| GHC-02256 | Ambiguous record update | Record update with duplicate field names is ambiguous | 9.6.1 |
| GHC-02550 | Quantified predicate must have a class or type variable head | The head of a quantified constraint must be either a class or a type variable | 9.6.1 |
| GHC-03272 | Bidirectional formatting warning | The source file contains Unicode bidirectional formatting instructions | 9.6.1 |
| GHC-04584 | Expression syntax in pattern | There has been an attempt to pattern match on expression syntax. | 9.6.1 |
| GHC-04924 | Unknown flag in GHC options pragma | Unknown flag in {-# OPTIONS_GHC #-} pragma. | 9.6.1 |
| GHC-04956 | Can't derive instances of nullary classes | It is not possible to derive instances of argumentless type classes without the DeriveAnyClass extension. | 9.6.1 |
| GHC-05380 | Multiple newtype constructors | More than 1 constructor for a newtype | 9.6.1 |
| GHC-05641 | Multiple Haddock comments | Multiple Haddock comments for a single entity are not allowed | 9.6.1 |
| GHC-05661 | Cannot use qualified both before and after the module | A module cannot be imported using the qualified modifier both before and after the name of the module | 9.6.1 |
| GHC-05989 | Too many type binders | A type constructor is declared with more arguments than its kind annotation specifies. | 9.6.1 |
| GHC-06201 | Missing method in type class instance | A required method is missing from the instance declaration. | 9.6.1 |
| GHC-06202 | Illegal type signature in instance declaration | Method in class instance has type signature, but extension InstanceSigs is not enabled | 9.6.1 |
| GHC-06446 | Do notation in pattern match | Do notation in pattern match. | 9.6.1 |
| GHC-07626 | Parse error in pattern | Compiler not able to parse pattern. | 9.6.1 |
| GHC-08838 | Wrong do bind | Returning a monadic computation in a do block and not binding it will not run it | 9.6.1 |
| GHC-09009 | Illegal role name. | Type roles should be one of representational, nominal, or phantom. | 9.6.1 |
| GHC-09646 | Tuple section in pattern | There has been an attempt to pattern match on a tuple section. | 9.6.1 |
| GHC-10190 | Empty enumeration | An enumeration would be empty. | 9.6.1 |
| GHC-10333 | Generalized newtype deriving doesn't work on non-newtypes | Generalized newtype deriving works only for types declared with the newtype keyword. | 9.6.1 |
| GHC-10498 | Conflicting definitions | The same entity has more than one definition | 9.8.1 |
| GHC-11861 | Empty single quotes | No character literal provided within single quotes. | 9.6.1 |
| GHC-11913 | Illegal deriving item | Something other than a type class appears in a deriving statement. | 9.6.1 |
| GHC-12003 | Type equality not in scope | The type equality operator has not been imported into the current module. | 9.6.1 |
| GHC-13218 | Illegal linear function in kind | A linear function was used for a higher kind, which is not allowed | 9.6.1 |
| GHC-15708 | Missing FunctionalDependencies | Missing FunctionalDependencies language extension | 9.6.1 |
| GHC-15843 | Type constructor has conflicting definitions in the module and its hs-boot/hsig file | The type constructor in the module and the signature in the boot module differ | 9.6.1 |
| GHC-16437 | Bad Constructor used with deriving clause | Standard deriving fails on GADTs and higher-rank types | 9.6.1 |
| GHC-17335 | Incomplete record selector application | A record selector application may not be successful | 9.10.1 |
| GHC-19244 | Module is annotated as trustworthy, but is inferred as safe | The module is annotated to be trustworthy, but it could be annotated as safe | 9.6.1 |
| GHC-20125 | Missing field(s) | Initialization of record with missing field(s). | 9.6.1 |
| GHC-20825 | Empty record update | Record update syntax requires that at least one field be specified. | 9.6.1 |
| GHC-21231 | Numeric escape sequence out of range | The numeric escape sequence represents a number that is too large | 9.6.1 |
| GHC-21926 | Version number or non-alphanumeric character in package name | Package imports may only consist of alphanumeric characters, and must omit the version. | 9.6.1 |
| GHC-24180 | Too few arguments to infix type operator | A type operator was not provided with both arguments. | 9.6.1 |
| GHC-24922 | Cannot reify type because name is not in scope | A Template Haskell error indicating that the given name is not in scope and therefore cannot be reified. | 9.6.1 |
| GHC-25078 | Precedence out of range | An invalid operator precedence was provided. | 9.6.1 |
| GHC-25897 | GADT pattern match must have a known result type | A pattern match on a GADT cannot succeed unless GHC knows the result type of the pattern match. | 9.6.1 |
| GHC-27207 | Missing space after tilde `~` | Lazy pattern in expression context. | 9.6.1 |
| GHC-28007 | Misplaced LANGUAGE pragma | LANGUAGE pragmas should come before the module declaration. | 9.6.1 |
| GHC-30606 | Redundant constraints | A binding has constraints that are redundant. | 9.6.1 |
| GHC-31574 | Illegal use of linear functions | The linear function type is used, but LinearTypes are not enabled | 9.6.1 |
| GHC-38520 | Redundant Bang Patterns | Used a bang pattern that has no effect | 9.6.1 |
| GHC-38856 | A file that doesn't use everything imported from a given module | One or more explicit imports are unused. | 9.6.1 |
| GHC-39999 | No instance arising | An expression requires a type class instance which is not provided by the context. | 9.6.1 |
| GHC-40798 | Operator whitespace | An optional warning for detecting usage of infix, suffix or prefix operators that could be parsed differently in future due to whitespace. | 9.6.1 |
| GHC-42044 | Unrecognised pragma | GHC didn't recognize pragma and will thus ignore it | 9.6.1 |
| GHC-43085 | Overlapping Instances | Multiple type class instances are resolved for a single type class method call. | 9.6.1 |
| GHC-44360 | Cannot import unsafe modules in Safe Haskell | Only safe modules can be imported in modules which are declared safe | 9.6.1 |
| GHC-44432 | Type signature lacks an accompanying binding | A type signature was provided, but no binding was given. | 9.6.1 |
| GHC-45696 | If-Then-Else in pattern match | If-Then-Else expression in pattern match. | 9.6.1 |
| GHC-45906 | Illegal standalone kind signature | Program contains a standalone kind signature, but the active extensions do not allow it. | 9.6.1 |
| GHC-46537 | Unsupported extension | GHC failed to recognize name of a language extension | 9.6.1 |
| GHC-46956 | Kind variable would escape its scope | A local kind variable was used to classify a type from a scope in which the kind variable is not available. | 9.6.1 |
| GHC-47535 | Identifier is not a record selector | An identifier does not refer to a record selector but is used as such. | 9.6.1 |
| GHC-47854 | Duplicate Exports | An identifier appears twice in an export list. | 9.6.1 |
| GHC-48099 | Top-level strict or unlifted binds not allowed | Top-level bindings may not be strict, and they may not have unlifted types. | 9.6.1 |
| GHC-48361 | Binding type variables is not allowed in pattern bindings | You can only bind value-level variables in a pattern, not type variables. | 9.6.1 |
| GHC-49957 | Unticked promoted constructors | A promoted data constructor was used as a type without it being indicated with a tick mark. | 9.6.1 |
| GHC-50928 | Noncanonical `mappend` definition | A Monoid instance uses a custom `mappend` implementation rather than re-using the default `(<>)` operation from its Semigroup instance. | 9.8.1 |
| GHC-51179 | Missing LambdaCase | Missing LambdaCase language extension | 9.6.1 |
| GHC-53633 | Redundant patterns | A pattern is impossible to reach due to earlier patterns | 9.6.1 |
| GHC-53786 | Case expression included in pattern | A pattern contains case-of syntax. | 9.6.1 |
| GHC-54540 | Cannot derive instance without constructors in scope | It is not possible to derive a typeclass instance if the constructors of the type are not in scope | 9.6.1 |
| GHC-55666 | Strictness annotation on unlifted type | Using a strictness annotation (bang) on an unlifted type is redudant as unlifted values are strict by definition | 9.6.1 |
| GHC-56147 | Rewrite rules are ignored in Safe Haskell | Rewrite rules are not allowed in Safe Haskell and are therefore ignored | 9.6.1 |
| GHC-56538 | Instance head is not headed by a class | A type class instance declaration is declared for something that is not a type class. | 9.6.1 |
| GHC-57396 | Linear types are not supported in FFI | Linear types are not supported when using GHC's FFI functionality | 9.6.1 |
| GHC-58008 | Pattern matching on GADTs without MonoLocalBinds is fragile | Pattern matching on GADTs without MonoLocalBinds is fragile | 9.6.1 |
| GHC-58481 | Parse error on input | Generic parsing error. | 9.6.1 |
| GHC-58656 | Module is inferred to be safe | If a module is inferred as safe, then it should be annotated as safe | 9.6.1 |
| GHC-59155 | Illegal tuple section | A tuple section is used, but the TupleSections extension is not enabled | 9.6.1 |
| GHC-59692 | Duplicate Instances | Multiple instances defined for the same type class and type. | 9.6.1 |
| GHC-59738 | Scoped type variables only appears non-injectively in declaration header | A data declaration has a kind signature, where the implictly bound type variables cannot be matched up unambiguosly with the ones from the signature itself | 9.6.4 |
| GHC-59840 | GHC does not support GADTs or type families which witness equality of multiplicities | GHC does not support GADTs or type families which witness equality of multiplicities | 9.6.1 |
| GHC-61689 | Bad import not exported | Module does not export the imported symbol. | 9.8.1 |
| GHC-62016 | Cannot derive well-kinded instance | An instance cannot be derived because the kinds cannot be made to match. | 9.6.1 |
| GHC-62161 | Incomplete patterns | Pattern match(es) are non-exhaustive. | 9.6.1 |
| GHC-62330 | Underscores not allowed in float and integer literals | Float and integer literals cannot contain underscores. | 9.6.1 |
| GHC-63394 | Messages from WARNING and DEPRECATED pragmas | Warning or deprecation message attached to a function, class, type, or module in a library | 9.6.1 |
| GHC-64088 | Use of "forall" as an identifier | Use of "forall" as an identifier is discouraged | 9.6.1 |
| GHC-64725 | User-defined type error | An invalid constraint or type family reduces to a custom type error. | 9.6.1 |
| GHC-66228 | View pattern in expression context | A view pattern was used in an expression, rather than a pattern. | 9.6.1 |
| GHC-67120 | Missing main | The IO action ‘main’ is not defined in module ‘Main’. | 9.8.1 |
| GHC-68686 | Cannot parse LANGUAGE pragma | The arguments to the LANGUAGE pragma could not be parsed | 9.6.1 |
| GHC-69158 | Conflicting exports | Different identifiers with the same name are (re-)exported from the same module. | 9.6.1 |
| GHC-69925 | Illegal unboxed string literal in pattern | Illegal unboxed string literal in pattern. | 9.6.1 |
| GHC-70712 | Double dots in record update | Double-dot syntax is not allowed in a record update. | 9.6.1 |
| GHC-71614 | Lambda requires at least one parameter | A lambda expression must have at least one parameter. | 9.6.1 |
| GHC-72771 | Mismatched default method signature | The provided default method implementation's type signature does not match the corresponding method type signature. | 9.8.1 |
| GHC-75356 | Export item suggests constructors/methods | An export item suggests that (in-scope) constructors or class methods exist when they do not. | 9.6.1 |
| GHC-76037 | Not in scope | An identifier is not in scope. | 9.6.1 |
| GHC-77037 | No explicit import list | Items brought into scope are not listed explicitly. | 9.6.1 |
| GHC-77539 | Illegal tuple constraint | A tuple of constraints was used without enabling the ConstraintKinds extension | 9.6.1 |
| GHC-78892 | let-syntax in pattern | An attempt has been made to use a let expression whilst pattern matching. | 9.6.1 |
| GHC-80768 | Class kind signatures need to be constraints | It is an error to use type families in the return kind of a class | 9.6.1 |
| GHC-81995 | Unused "do" bind | A return value of a monadic action is ignored | 9.6.1. |
| GHC-83475 | Unused Record Wildcard | Pattern contains record wildcard that is not used | 9.6.1 |
| GHC-83865 | Type Mismatch | You provided a value of a given type, whereas GHC expected a different type. | 9.6.1 |
| GHC-84077 | Type application without space | A type application with @ does not have a space before. | 9.6.1 |
| GHC-85401 | Missing Export List | The module export list is missing | 9.6.1 |
| GHC-87139 | Illegal deriving strategy | Use of a deriving strategy without enabling the corresponding language extension | 9.8.1 |
| GHC-87429 | Illegal datatype context | Constraints present in datatype declaration without DatatypeContexts. | 9.6.1 |
| GHC-87491 | Found 'qualified' after the module | qualified after the module requires the ImportQualifiedPost extension. | 9.6.1 |
| GHC-88464 | Variable not in scope | An unknown variable name was referenced. | 9.6.1 |
| GHC-88747 | Precedence parsing error | Cannot determine the order of operators in an expression | 9.6.1 |
| GHC-88933 | Non-numeric type in default declaration | Default declarations may only mention types which implement the Num type class | 9.6.1 |
| GHC-89347 | Constraint tuple arity too large | A constraint cannot be defined on a tuple with a big arity | 9.6.1 |
| GHC-90177 | Orphan instance | An instance was defined separately from its type or class. | 9.6.1 |
| GHC-90584 | Deriving `Typeable` has no effect | The type class `Typeable` does not need to be derived. | 9.6.1 |
| GHC-90973 | Exported module not imported | Export list contains a module that is not imported | 9.6.1 |
| GHC-91028 | Cannot equate polymorphic types with type variables | During type inference, GHC can't instantiate a type variable with a type that is itself polymorphic. | 9.6.1 |
| GHC-91938 | Equations have different number of arguments | Each equation in a function definition must have the same number of arguments. | 9.6.1 |
| GHC-92994 | Unexpected nested forall in foreign declaration | A foreign function import uses a higher-rank type. | 9.6.1 |
| GHC-93557 | Illegal typeclass instance | Illegal typeclass instance | 9.8.1 |
| GHC-94458 | Illegal position of Haddock comment | A Haddock comment appears in an illegal position | 9.6.1 |
| GHC-94817 | Tab character | A tab character occurred in the input file. | 9.6.1 |
| GHC-95644 | Missing space after exclamation mark `!` | Bang pattern in expression context. | 9.6.1 |
| GHC-95781 | Invalid type application | The expression cannot be applied to the given type argument. | 9.6.1 |
| GHC-95909 | Missing strict fields | Constructor was not instantiated with required strict field(s). | 9.6.1 |
| GHC-97044 | User-specified instance is not allowed | Type class does not allow user-specified instances | 9.6.1 |
| GHC-97170 | Duplicate role annotations | A type declaration has more than one accompanying role annotation | 9.8.1 |
| GHC-97441 | Overflowed Literals | Literal overflowing range of supported values | 9.6.1 |
| GHC-97739 | Kind and type arguments out of dependency order | Kind arguments must occur prior to the types that they classify. | 9.6.1 |
| GHC-98980 | Command syntax in pattern | Arrow command syntax was used in a pattern. | 9.6.1 |
| GHC-99565 | Multiple default declarations | Multiple default declarations are not allowed | 9.6.1 |
| GHC-99623 | Import item suggests constructors/methods | An import item suggests that (in-scope) constructors or class methods exist when they do not. | 9.6.1 |
| GHCup-00010 | Unable to find a download for the requested version/distro | GHCup wasn't able to find a binary distribution for the requested tool for the current platform | 0.1.19.0 |
| GHCup-00110 | The tool is already installed with that version | GHCup has already installed the requested version of the tool | 0.1.19.0 |
| GHCup-00160 | JSON decoding failed | GHCup was unable to decode a JSON or YAML document (e.g. metadata file) | 0.1.19.0 |
| GHCup-00200 | File digest verification failed | GHCup tried to verify the integrity/authenticity of a downloaded file, but the verification failed | 0.1.19.0 |
| GHCup-00210 | GPG verify failed | GHCup tried to verify the authenticity of a software channel using GPG, but failed | 0.1.19.0 |
| GHCup-00841 | A process exited prematurely | GHCup executed a subprocess that did not complete successfully | 0.1.19.0 |
| GHCup-05841 | A download failed | GHCup tried to download a tool, but the download failed with a subprocess exiting prematurely | 0.1.19.0 |
| S-4804 | Stack failed to construct a build plan | The package with the given name or version could not be found in the snapshot | 2.9.3 |
| S-6602 | Could not parse YAML configuration file | A YAML configuration file was either not well-formed YAML or didn't contain the expected data | 2.9.3 |