[Haskell] Re: Using implicit parameter constraints in data decl

[Benjamin Franksen]

On Sunday 07 November 2004 23:19, Ben Rudiak-Gould wrote:
> Benjamin Franksen wrote:
>  >data (?val::Bool) => Test = Test { name :: String }
>  >
>  >is rejected by the compiler:
>  >
>  >TestBug.hs:3:
>  >    Illegal constraint ?val :: Bool
>  >    In the context: (?val :: Bool)
>  >    While checking the context of the data type declaration for `Test'
>  >    In the data type declaration for `Test'
>  >
>  >which is unfortunate since it means that you cannot put a function that
>  >depends on an implicit parameter into a data structure.
>
> Does this do what you want?:
>
>     data Test = Test { name :: (?val::Bool) => String }

Thanks for the hint, but no:

TestBug.hs:4:
    Illegal constraint ?val :: Bool
    In the type: ({?val :: Bool} => String) -> Test
    While checking the type of constructor `Test'
    In the data type declaration for `Test'

Ben