Error says to fix incorrect return type
Defining a constructor that returns the wrong type (non-parent type).
If the constructor is not fully-applied or over-saturated GHC reports an error, instead of telling the developer to change the return type:
Prelude> data A
Prelude> data B a where MkB :: A ()
<interactive>:2:23: error:
• Expecting one fewer argument to ‘A’
Expected kind ‘* -> *’, but ‘A’ has kind ‘*’
• In the type ‘A ()’
In the definition of data constructor ‘MkB’
In the data declaration for ‘B’Expected:
- Suggest GADTs:
• Illegal generalised algebraic data declaration for ‘B’
(Use GADTs to allow GADTs)
• In the data declaration for ‘B’- Complain about incorrect return type:
• Data constructor ‘MkB’ returns type ‘A’
instead of an instance of its parent type ‘B a’
• In the definition of data constructor ‘MkB’
In the data type declaration for ‘B’It shouldn't suggest the user fix the incorrect return type
Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |