On Sat, Jul 13, 2002 at 07:58:19PM +1000, Bernard James POPE wrote:
... I'm fond of the idea proposed by Marcin 'Qrczak' Kowalczyk:
May I propose an alternative way of specifying an alternative Prelude? Instead of having a command line switch, let's say that 3 always means Prelude.fromInteger 3 - for any *module Prelude* which is in scope!
That is, one could say: import Prelude () import MyPrelude as Prelude IMHO it's very intuitive, contrary to -fno-implicit-prelude flag.
I don't agree with this, since the Haskell 98 standard explicitly contradicts it. I don't see what's wrong with a command line switch that would do this, anyway.
Presuming of course that defaulting would follow this path and refer to the new Prelude.
I never came up with a design that would allow this. Defaulting seems to be the one piece of the Haskell standard for which there is not yet a general solution. Although now that I think about it, if you could just specify which fromInteger you wanted (i.e., give that fromInteger a more specific type) the problem would go away. Perhaps that's really the better solution anyway: how often do people want to default to something that's not the first on the defaulting list? I think that might end up being less surprising to programmers, anyway. It might work as a temporary hack for you, anyway. (That is, add declarations like fromInteger :: Integer -> Int fromRational :: Rational -> Double in your new Prelude. This would work as long as users don't otherwise use fromInteger.) I don't know how you want to transform the types, but there are at least two areas where there are still special types: List and Bool. For List, I don't actually see any problem in principle with allowing other implementations of list comprehensions and whatnot, but Simon Peyton-Jones indicated that it would be difficult to actually implement; with Bool, one would need to define additional functions (like ifThenElse). Best, Dylan