Re: Support Iterable in for-each

[Per Bothner <per@bothner.com>]

On 02/27/2015 09:35 AM, Jamison Hope wrote:
> On Feb 26, 2015, at 9:25 PM, Per Bothner <per@bothner.com> wrote:
>> I've checked in code to generalize map, for-each, and vector-map
>> so the sequence arguments can be "generalized sequences":

> Very cool.  I see that map always returns a list, regardless of the
> type(s) of the sequence argument(s).

Yes - the alternative seems too complicated.  Scala has a complicated
framework for picking a result type based on the argument type, but it
seems very complicated and fragile.  What if you map sqrt over an int[]?
Should the result be a double[]?  What when there are multiple sequence
arguments?  Note the rules have to work sanely and consistently both
with compile-time typing and run-time-typing.  It's a lot easier when
the compiler just knows the result type.  And as you show there is an
easy work-around with constructors and splices.

> So if I've got an ArrayList of numbers
> #|kawa:1|# (define arraylist (java.util.ArrayList 1 2 3 4 5))
> and I want to produce another ArrayList containing the squares of
> those numbers, I can do.
> #|kawa:2|# (java.util.ArrayList @(map square arraylist))
> [1, 4, 9, 16, 25]
  
> Which is more idiomatic, (apply type list) or (type @list) ?

I'd say the latter.  Currently apply isn't well optimized, Though splicing isn't
as well optimized as I'd like, it's better than apply.  For example calling
a varargs method with a splice is complied pretty well.

My plan is to have the compiler rewrite: (apply f x y lst)
to: (f x y @lst)
When that is done they'll have the same performance,

Further out, the plan is to allow this syntax:
   (java.util.ArrayList (square (each arraylist)) ...)
This uses the same '...' as in syntax patterns, and 'each' is a magic
function inside the context of a '...'.  It would also work in patterns:
   (! [x ...] arraylist) ;; x is 'each' element of arraylist
   (java.util.ArrayList (* x x) ...)

I expect this feature before the next release.

> BTW, I had to define a remove() method for CharacterIterator in
> gnu/lists/Sequences.java to build with Java 1.7, which doesn't have
> these newfangled "default" methods.

Oops - fixed.

> I also had to add
>        <include name="kawa/Source*.java"/>
> to the java-classes target in build.xml.

It doesn't seem like it's strictly needed, but I fixed this too.
-- 
	--Per Bothner
per@bothner.com   http://per.bothner.com/