GHC accepts invalid Haskell: `class Eq (a Int) => C a where`
From the Haskell 2010 report chapter 4,
- Class and instance declarations:
| class [scontext =>] ...
| instance [scontext =>] ...- Normal type signatures:
vars :: [context =>] ...Notice the difference between scontext (//with// s) and context (without s).
scontext → simpleclass
| ( simpleclass1 , … , simpleclassn ) (n ≥ 0)
simpleclass → qtycls tyvarcontext → class
| ( class1 , … , classn ) (n ≥ 0)
class → qtycls tyvar
| qtycls ( tyvar atype1 … atypen ) (n ≥ 1)
GHC seems to ignore this difference, and happily accepts class Eq (a Int) => C a where. Hugs (Version: September 2006) reports for that same example:
Illegal Haskell 98 class constraint in class declaration
*** Constraint : Eq (a Int)Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.0.1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | lowest |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |