[Haskell-cafe] if-then-else as rebindable syntax (was Re: Why
does Haskell have the if-then-else syntax?)
dmhouse at gmail.com
Thu Jul 27 11:48:52 EDT 2006
(Apologies to Niklas for multiple copies, it was a Reply/Reply to all mixup.)
On 27/07/06, Niklas Broberg <niklas.broberg at gmail.com> wrote:
> First of all, programs that import names from the Prelude explicitly
> would no longer be able to use if-then-else unless they also added
> 'cond' to that input list (or redefined it of course). This shouldn't
> really be a problem, since the rebindable syntax is turned on by
> adding some flag anyway, and if you add that flag you know you're no
> longer H98. Still, it's going to break a lot of existing programs.
> The second problem is that it would require the addition of the cond
> function to the Prelude. This will probably not break too many
> existing programs, but still it is a more serious problem since it
> will have effect even without any flags to GHC. Or is it possible to
> govern the contents of the Prelude based on flags?
How about we drop the idea of an auxilary cond function, and instead
just use a Boolean typeclass?
class Boolean b where
isTrue :: b -> Bool
isFalse :: b -> Bool
Then the semantics of if-then-else would change to something like this:
if b then t1 else t2
b is required to be of a type which instantiates Boolean
If isTrue b is True, then t1 is executed, otherwise if isFalse b is
True, then t2 is executed, otherwise _|_ is returned.
Then you get the benefit of being able to use arbitrary 'boolean-like'
types in actual if statements, without messing around with
-fno-implicit-prelude and rebindable syntax.
-David House, dmhouse at gmail.com
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