On 04/16/2012 01:20 PM, Russel Winder wrote:
On Wed, 2012-04-11 at 18:32 +0200, Jakob Ovrum wrote:
[...]
Ranges are iterated in-place. You can't mutate a const range,
hence you cannot advance it by one (`popFront`), which is
required by the lowering of foreach using the range interface.
[...]
I think I am still trying to recover from the shock of this; it is very
likely to turn anyone interested in any form of declarative expression
away from D. Also the internal activity is being exposed at the API
level. Not to mention the history of iteration being one of working
with mutable object on immutable structures.
Trying to create the smallest possible example:
auto r = iota ( 0 , 10 ) ;
writeln ( map ! ( i => i * i ) ( r ) ) ;
works fine. Let us try:
const r = iota ( 0 , 10 ) ;
writeln ( map ! ( i => i * i ) ( r ) ) ;
works fine. Great. Now
auto r = iota ( 0 , 10 ) ;
writeln ( reduce ! ( ( a, b ) => a + b ) ( 10 , r ) ) ;
works fine. Let's try:
const r = iota ( 0 , 10 ) ;
writeln ( reduce ! ( ( a, b ) => a + b ) ( 10 , r ) ) ;
Oh dear:
/home/users/russel/lib.Linux.x86_64/DMD2/bin64/../../src/phobos/std/algorithm.d(725):
Error: function std.range.iota!(int,int).iota.Result.popFront () is not
callable using argument types ()
/home/users/russel/lib.Linux.x86_64/DMD2/bin64/../../src/phobos/std/algorithm.d(725):
Error: function std.range.iota!(int,int).iota.Result.front () is not callable
using argument types ()
example.d(7): Error: template instance
example.main.reduce!(__lambda3).reduce!(int,const(Result)) error instantiating
Not only is the error message incomprehensible to someone who doesn't
know the details of the internal workings (OK not a killer blow), but
the very inconsistency of behaviour between map and reduce is a
violation of the Principle of Least Surprise, and seems like a killer
blow.
Have you filed a bug report against Phobos for this?
