Bruno Oliveira wrote:
On Tue, 27 Dec 2005 16:39:34 +0000, Chris Kuklewicz wrote:
I was wondering if it this small snippet of code could be altered to require fewer OPTIONS -fallow-... switches.
Here is a partial solution using only -fglasgow-exts:
...deleted...
The problem of this solution is that, because we are using show in the definition of swim for (a -> t), the strings are beeing printed with extra "\"".
*MyShow> main Hello" ""World #"[17,18,19]"!" I also think [4,5,6]" and "7" are ""cool""."
The extra double quotes being what I am trying to avoid with MyShow
instance MyShow t => MyShow (String -> t) where swim s x = swim (s ++ x)
This is interesting. It does remove the need for -fallow-undecidable-instances. Thanks.
Could this be improved by TypeEq machinery? Is there a Haskell 98 way to make myShow work this way?
The problem of the previous solution is that you do not want, in the general case, use the default instance of Show for Strings. An alternative is to just introduce a newtype S (isomorphic to String) that allows you to handle Strings differently:
... deleted...
By using the newtype S instead of Haskell String, you obtain an Haskell 98 solution. I am not sure if this is good enough for you but it seems a good compromise.
Hope it helps!
Cheers,
Bruno
I did find the newtype solution, and trying to avoid that is what motivated such ugly switch flipping usage of GHC. My intuition was that this example of creating "something like a sub-type-class of Show" which treats String differently ought to be possible. And it is possible, and I would hazard a guess that your solution with "MyShow (String->t)" is safe in spite of the extra switches. The sad thing is that the two -fallow switches infect client code that uses the MyShow module. So allowing those tricks for class MyShow must turn on for classes in code that imports MyShow. If GHC could -fallow those flags only for certain typeclasses then it would be confinable. In principle, could GHC ever be extended to confine -fallow this way? -- Chris