Call to arms: lambda-case is stuck and needs your help
Dan Doel
dan.doel at gmail.com
Sat Jul 7 18:16:51 CEST 2012
If we're voting....
I think \of is all right, and multi-argument case could be handy,
which rules out using 'case of' for lambda case, because it's the
syntax for a 0-argument case:
case of
| guard1 -> ...
| guard2 -> ...
Then multi-argument lambda case could use the comma syntax of
multi-argument case.
One thing I don't think makes sense in combination is \of with
0-arguments, since any desugaring of that is not going to involve and
actual lambda expression.
-- Dan
On Thu, Jul 5, 2012 at 5:04 PM, Edward Kmett <ekmett at gmail.com> wrote:
> I really like the \of proposal!
>
> It is a clean elision with \x -> case x of becoming \of
>
> I still don't like it directly for multiple arguments.
>
> One possible approach to multiple arguments is what we use for multi-argument case/alt here in our little haskell-like language, Ermine, here at S&P CapitalIQ, we allow for ',' separated patterns, but without surrounding parens to be treated as a multi argument case and alt pair. Internally we desugar our usual top level bindings directly to this representation. When mixed with the \of extension, this would give you:
>
> foo :: Num a => Maybe a -> Maybe a -> Maybe a
> foo = \of
> Just x, Just y -> Just (x*y)
> _, _ -> Nothing
>
> but it wouldn't incur parens for the usual constructor pattern matches and it sits cleanly in another syntactic hole.
>
> A similar generalization can be applied to the expression between case and of to permit a , separated list of expressions so this becomes applicable to the usual case construct. A naked unparenthesized , is illegal there currently as well. That would effectively be constructing then matching on an unboxed tuple without the (#, #) noise, but that can be viewed as a separate proposal' then the above is just the elision of the case component of:
>
> foo mx my = case mx, my of
> Just x, Just y -> Just (x*y)
> _, _ -> Nothing
>
> On Jul 5, 2012, at 2:49 PM, wagnerdm at seas.upenn.edu wrote:
>
>> Quoting wagnerdm at seas.upenn.edu:
>>
>>> Well, for what it's worth, my vote goes for a multi-argument \case. I
>>
>> Just saw a proposal for \of on the reddit post about this. That's even better, since:
>>
>> 1. it doesn't change the list of block heralds
>> 2. it doesn't mention case, and therefore multi-arg \of is perhaps a bit less objectionable to those who expect "case" to be single-argument
>> 3. 40% less typing!
>>
>> Can I change my vote? =)
>> ~d
>>
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