GHC does indeed include the notion of "rebindable syntax". It would be
straightforward to extend it to include if-then-else. In effect, that
would mean that
if e1 then e2 else e3
would behave exactly like
cond e1 e2 e3
including from the point of view of typing. (You could choose a
different name than 'cond'.) Then by importing a 'cond' with (say) type
cond :: MyBool -> b -> b
you could use a different kind of Boolean. You could even overload the
bool:
cond :: Boolean a => a -> b -> b
This could be done with a few hours work. But not a few minutes. Want
to put a feature request in Trac?
Simon
| -----Original Message-----
| From: haskell-cafe-bounces@haskell.org
[mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Niklas
| Broberg
| Sent: 27 July 2006 09:01
| To: Haskell-cafe
| Subject: Re: [Haskell-cafe] Why does Haskell have the if-then-else
syntax?
|
| I often find myself at odds with this choice. The reason is that I use
| Haskell as a host for embedded languages, and these often come with
| their own control flows. So I find myself wanting to write my own
| definition of the if-then-else construct that works on terms of some
| other type, e.g. tests on values of type Exp Bool instead of Bool, and
| at the same time make sure that the user doesn't use the built-in
| if-then-else. Sure, I can (and do) call my own version if_, ifElse or
| something else along those lines, but it's sure to be a constant
| source of programmer errors, writing if-then-else instead of if_ by
| habit.
|
| A thought that has crossed my mind on several occasions is, why not
| make the syntactic if-then-else construct rebindable, like the do
| notation? I think I know the answer already -- the do notation is
| syntactic sugar for >>= and company so it's easy to translate it into
| non-prelude-qualified versions of functions with those names. This is
| not the case for if-then-else. But it could be, the prelude could
| define a function if_ (or whatever) that the if-then-else construct is
| made to be sugar for, and thus also amenable to rebinding by not
| prelude-qualifying.
|
| /Niklas
|
| On 7/27/06, Paul Hudak