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.
Other Haskell-related tools are welcome to add their own messages to the index. Please contact the Haskell Foundation or open an issue to begin the process.
So far, 110 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 |
|---|---|---|---|
| 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-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-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-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-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-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-24180 | Too few arguments to infix type operator | A type operator was not provided with both arguments. | 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-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-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-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-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-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-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-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-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-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-88933 | Non-numeric type in default declaration | Default declarations may only mention types which implement the Num type class | 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-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-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 |