implicit parameters

[Jeffrey R Lewis]

On Thursday, April 17, 2003, at 10:35 AM, Dean Herington wrote:

> There's apparently something very basic that I don't understand about
> implicit parameters.  In the following program, compilation fails if
> either of the type declarations is omitted.  What is "implicit" about
> implicit parameters, then?
>
> {-# OPTIONS -fglasgow-exts #-}
>
> main = print f1
>
> f1 = let ?f = \ x -> x ++ x in f2
>
> f2 :: (?f :: String -> String) => String
> f2 = f3
>
> f3 :: (?f :: String -> String) => String
> f3 = ?f "x"
>
>

The detail that you are missing is that implicit parameters are subject 
to the "monomorphism restriction".  If f2 and f3 were functions, you 
wouldn't get errors when omitting signatures.

And yes, the decision to have them obey the monomorphism restriction is 
questionable, but then so is the monomorphism restriction ;-)

--Jeff