Making implicit parameters explicit

[Tomasz Zielonka]

On Wed, Nov 05, 2003 at 09:13:12PM +0100, Stefan Reich wrote:
> Hi,
> 
> I discovered implicit parameters today and I'm very excited about them 
> because they allow to express certain code constructs more elegantly. 
> However, I stumbled upon a problem. Suppose I have these definitions 
> (the definition of Request is irrelevant):
> 
> type Controller = (?req :: Request) => String
> 
> controller1 :: Controller
> controller2 :: Controller
> 
> controllerList = [controller1, controller2]
> 
> GHC complains with:
> 
>     Unbound implicit parameter (?req :: Request)
>       arising from use of `controller1'
>     In the list element: controller1
>     In the definition of `controllerList':
>         controllerList = [controller1, controller2]

You have just been bitten by monomorphism restriction. Write a type
signature for controllerList and it should work:

controllerList :: (?req :: Request) => [String]

Best regards,
Tom

-- 
.signature: Too many levels of symbolic links