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
msg01816/pgp00000.pgp
Description: PGP signature
