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

[Bertram Felgenhauer]

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.

I have little preference between these options.

Bertram