[Haskell-cafe] Syntax for lambda case proposal could be "\of"

[Thomas Schilling]

On 16 aug 2007, at 13.46, Bertram Felgenhauer wrote:

> Duncan Coutts wrote:
>> On Wed, 2007-08-15 at 11:06 -0700, Stefan O'Rear wrote:
>>
>>> foo = getSomethingCPS $ \ arg ->
>>>       moreStuff
>>>
>>> is now a syntax error (\ { varid -> } matches no productions).
>>
>> I'm not sure I follow.
>>
>> The patterns would have to match up in a column, so
>>
>> foo = getSomethingCPS $ \ arg ->
>>       moreStuff
>>
>> should be fine, to add another alternative it'd have to be:
>>
>> foo = getSomethingCPS $ \ Pat1 ->
>>       moreStuff
>>                           Pat2 ->
>>       evenMoreStuff
>
> I don't like this - it's not in the spirit of the existing layout
> rule at all. You should have to indent 'moreStuff' deeper than
> Pat1 I think;
>
>> foo = getSomethingCPS $ \ Pat1 ->
>>                               moreStuff
>>                           Pat2 ->
>>                               evenMoreStuff
>
> would be (visually) ok. But then Stefan's point is valid.
>
> So there should be two productions, I think, one for non-case
> lambdas and one for case-lambdas, like this:
>
> non-case-lambda:
>   '\' apat+ '->' expr
>
> case-lambda:
>   '\' '{' ( apat+ '->' expr ';' )* '}'
>
> Unfortunately this naive approach would add another point of
> arbitrarily large look-ahead to the grammar. The easiest way to
> resolve this is to add some keyword before or after '\',
> bringing is back to the previous proposals.
>
> I like both
>
>    \of x y z -> ...
>        a b c -> ...
>
> and
>
>    case \ of x y z -> ...
>              a b c -> ...
>
> (I'd add the space here, viewing \ as a pseudo-expression)
>
> In the spirit of the \\ proposal, while \\ itself is an operator,
>
>   \ \ x y z -> ...
>       a b c -> ...
>
> is still available as a syntax, but may be confusing.

   .\ x y z -> ...
      a b c -> ...

or just require unicode \lambda ;)