Optional brackets around lambda expression (was: Expected distribution of lambda sizes)

[Jonathan Halterman]

This approach (Redmond) seems the cleanest of the bunch.

Jonathan

On Thu, Jun 16, 2011 at 3:19 AM, Ali Ebrahimi <ali.ebrahimi1781 at gmail.com>wrote:

> Hi, how about this one:
>
> process( x -> x + 1 );
>  process( x -> { return x + 1; } );
>  process(  x -> {
>  if (x == 0) {
>    return 0;
>  }
>  return x + 1;
>  });
>
>
> Ali Ebrahimi
>
> On Thu, Jun 16, 2011 at 1:33 PM, Stephen Colebourne <scolebourne at joda.org
> >wrote:
>
> > I believe I agree with Steven and Reinier that the strawman syntax
> > family would be best with only one paren for expressions:
> >
> >  process( #(x) x + 1 )
> >  process( #(x) { return x + 1; } );
> >  process( #(x) {
> >   if (x == 0) {
> >     return 0;
> >   }
> >   return x + 1;
> >  });
> >
> > The one less paren helps me read the code, compared to this:
> >
> >  process( #(x) (x + 1) )
> >
> > (Of course, users might choose to add the parens themselves, as thats
> > legal for any expression)
> >
> >
> > I have mixed feelings on supporting a lone expression within the
> > block. Thats because I'm unconvinced that the equivalent for a method
> > is as readable as using a "return":
> >
> > process( #(x) {x + 1} );   // reads well enough to me
> > public String getSurname() {surname}  // doesn't read quite as well to me
> >
> >
> > For other syntax families, I also suspect that I prefer that the
> > expression itself does not require being surrounded by parens.
> >
> > Stephen
> >
> >
> > On 16 June 2011 07:55, Reinier Zwitserloot <reinier at zwitserloot.com>
> > wrote:
> > > My gut instinct was: That's a really bad idea. But I decided to play
> > around
> > > with it anyway and it actually resulted in easier to scan code (to me,
> > > anyway). The use cases I've worked on before indeed usually boil down
> to
> > > passing the closure straight into a method as an argument, and in such
> > cases
> > > it's better. As Pavel said, if you treat the closure preamble (the =>
> or
> > the
> > > #() or whatever the syntax calls for) as an operator with low
> precedence,
> > it
> > > works out.
> > >
> > > This way the strawman syntax almost seems to have that best of both
> > worlds
> > > scenario: It looks good both for long and one-liner closures. Passing
> > > closures inline as argument to a method, i.e. the map/filter usecase,
> > looks
> > > a lot cleaner when you lose a closing brace/paren.
> > >
> > > Precedence-wise, if you want to exit the closure scope, you have to
> > resort
> > > to this:
> > >
> > > int ageOfJoe = (#(Person x) x.getAge()).invoke(joe);
> > >
> > > which is different from what traditional strawman wanted you to do:
> > >
> > > int ageOfJoe = #(Person x) (x.getAge()).invoke(joe);
> > >
> > > Personally I found it easier to scan the new form vs. the old one (i.e.
> > its
> > > easier to see in the top version that the toString applies just as much
> > to
> > > the #(Person x) part as the x.getAge() part), so this is not
> necessarily
> > a
> > > bad thing.
> > >
> > > I don't think its necessary but this proposal can be extended by
> allowing
> > > braces for single expressions, i.e:
> > >
> > > int ageOfJoe = #(Person x) {x.getAge()}.invoke(joe);
> > >
> > >  though I'm not sure how easy it is to implement this in an LL(k)
> parser
> > > (you'd have to conflate the first statement/expression as 'its one of
> > those
> > > two', then scan for either a semicolon or the closing brace, at which
> > point
> > > you have to go back and doublecheck that the statement or expression
> you
> > > parsed is actually an expression if there wasn't one, or actually a
> > > statement if there was. That's different than what javac currently
> does;
> > it
> > > always knows if it wants a statement or an expression, it never needs
> to
> > > parse an 'it's one of those'. ecj will have any problem with this as it
> > > already junks statements and expressions into the same pile.
> > >
> > >
> > >  --Reinier Zwitserloot
> > >
> > >
> > >
> > > On Wed, Jun 15, 2011 at 10:42 PM, Pavel Minaev <int19h at gmail.com>
> wrote:
> > >
> > >> Note that this applies also to "Redmond syntax" in general; e.g.:
> > >>
> > >>   new User().use(x => x + "a string");
> > >>
> > >> However, I do not recall it ever being a problem in C# in practice,
> nor
> > did
> > >> I ever hear of complaints about that in C#. In general, lambda syntax
> is
> > >> treated as a sort of "lambda operator" =>, with a certain
> "precedence",
> > and
> > >> said "precedence" is lower than any other operator - in this case,
> lower
> > >> than "+". In that sense, it's not much different than correctly
> parsing
> > >> expressions such as a || b && c.
> > >>
> > >> On Wed, Jun 15, 2011 at 1:35 PM, Yuval Shavit <yshavit at akiban.com>
> > wrote:
> > >>
> > >> > What would happen with something like:
> > >> >
> > >> >  public interface Sam {
> > >> >    String doSomething(String arg);
> > >> >  }
> > >> >  public class User {
> > >> >    public void use(Sam sam) { System.out.println("saw a sam"); }
> > >> >    public void use(String string) { System.out.println("saw a
> > string"); }
> > >> >  }
> > >> >
> > >> >  new User().use( #(x) x + "a string" );
> > >> >
> > >> > In that context, which are we passing?
> > >> >   - lambda that takes a String, concatenates "a string" to it and
> > returns
> > >> > the result
> > >> >   - a string consisting of (lambda x -> x).toString() concatenated
> > with
> > >> "a
> > >> > string"
> > >> >
> > >> > This is just one simple and somewhat contrived example, but the
> point
> > is
> > >> > that without braces or something similar, it's not too hard to come
> up
> > >> with
> > >> > a situation where it's not clear where the lambda ends and the rest
> of
> > >> the
> > >> > expression begins.
> > >> >
> > >> > On Wed, Jun 15, 2011 at 4:18 PM, Steven Simpson <
> ss at comp.lancs.ac.uk>
> > >> > wrote:
> > >> >
> > >> > > On 13/06/11 21:10, Pavel Minaev wrote:
> > >> > > > Statement lambdas would typically be thus wrapped, yes
> > >> (coincidentally,
> > >> > > it's
> > >> > > > also why I'd prefer to have a separate form for expression
> lambdas
> > >> > which
> > >> > > > does not include the {} so as to be visually distinct from
> > statement
> > >> > > > blocks).
> > >> > >
> > >> > > Yes, I think I'd appreciate that visual distinction too.  Is it
> > >> possible
> > >> > > to have an unambiguous expression syntax not requiring any form of
> > >> > > brackets around either the whole lambda or its body?:
> > >> > >
> > >> > >  #() 3
> > >> > >  #() 3 + 4
> > >> > >  #(x) x + 1
> > >> > >  #(x, y) x + y
> > >> > >
> > >> > > When used like this, they will likely be in argument lists, so
> they
> > are
> > >> > > naturally delimited by commas and the list-terminating bracket,
> and
> > >> need
> > >> > > no brackets of their own.  IOW, #() would have quite a low
> > precedence,
> > >> > > and any brackets put around it would be part of existing syntax
> > (e.g.
> > >> > > for normal expressions and argument lists).
> > >> > >
> > >> > > If you had one lambda in the body of another, it will likely have
> to
> > be
> > >> > > inside an enclosed call, whose brackets will naturally delimit it:
> > >> > >
> > >> > >  #(a) a + process(#(b) b * b) + 10
> > >> > >
> > >> > > If it wasn't so embedded, you could always put normal expression
> > >> > > brackets around it:
> > >> > >
> > >> > >  #(a) a + (#(b) b * b).invoke(a)
> > >> > >
> > >> > > (And that's a little contrived.  And possibly quite difficult to
> > >> > > type-infer…?)
> > >> > >
> > >> > > I don't think you could drop the parameter-list brackets too, as
> > that
> > >> > > would make parsing much more complex (e.g. #a + a).  Looking at
> that
> > >> > > more positively, it would keep lambda syntax and appearance
> > obviously
> > >> > > distinct from method literals.  ("#(" => Bam!  I'm a lambda!)  The
> > >> > > presence of braces would then further distinguish lambda
> expressions
> > >> > > from lambda statements.
> > >> > >
> > >> > > In summary, we require brackets around neither the lambda
> expression
> > >> nor
> > >> > > its body.  We are then forced to used some around the parameter
> > list,
> > >> > > but take advantage of this as the constant discriminant of lambda
> vs
> > >> > > method literal.
> > >> > >
> > >> > > Applied to some other expressions seen on the list lately:
> > >> > >
> > >> > >   list.filter( #(t) t.length() > 3 )
> > >> > >       .map( #(t) t.barCount )
> > >> > >       .max();
> > >> > >
> > >> > >   students.filter(#(s) s!=null && "Smith".equals(s.getName()))
> > >> > >
> > >> > >   List men = personList.filter(#(p) p.isMale());
> > >> > >
> > >> > >   List<Double> adjustedPrices =
> > >> > >      prices.map(#(price) price + getShipping());
> > >> > >
> > >> > > Cheers,
> > >> > >
> > >> > > Steven
> > >> > >
> > >> > >
> > >> > >
> > >> >
> > >> >
> > >>
> > >>
> > >
> > >
> >
> >
>
>