digitalmars.D.learn - reduce a BitArray[]

Alex <sascha.orlov gmail.com> writes:
```Hi there,
a silly question from me for the turn of the year... I apparently
missing the forest through the trees.
The first part of the code works as expected:

[code]
int[] arr8 = [1,2,3,4,5];

int sum = 0;
foreach(int summand; arr8)
{
sum += summand;
}
writeln("sum1: ", sum);

writeln("sum2: ", reduce!((a,b) => a + b)(arr8));
[/code]

The second one does not:

[code]
import std.bitmanip;

BitArray[] arr7 = [BitArray([0, 1, 0, 1, 0, 1]), BitArray([0,
1, 0, 0, 0, 0]), BitArray([0, 1, 0, 1, 0, 0]), BitArray([0, 1, 0,
1, 0, 0])];

BitArray common = BitArray([1,1,1,1,1,1]);
foreach(BitArray ba; arr7)
{
common &=  ba;
}
writeln("common2: ", common);

//writeln("common1: ", reduce!((a,b) => a & b)(arr7));
[/code]

The problem is, that the last line with the reduce does not
compile. Why?
```
Dec 29 2015
Alex Parrill <initrd.gz gmail.com> writes:
```On Tuesday, 29 December 2015 at 09:26:31 UTC, Alex wrote:
The problem is, that the last line with the reduce does not
compile. Why?

If you get an error, it is imperative that you tell us what it is.

For the record, this code:

import std.bitmanip;
import std.stdio;
import std.algorithm;

void main() {

BitArray[] arr7 = [BitArray([0, 1, 0, 1, 0, 1]), BitArray([0,
1, 0, 0, 0, 0]), BitArray([0, 1, 0, 1, 0, 0]), BitArray([0, 1, 0,
1, 0, 0])];

BitArray common = BitArray([1,1,1,1,1,1]);
foreach(BitArray ba; arr7)
{
common &=  ba;
}
writeln("common2: ", common);

writeln("common1: ", reduce!((a,b) => a & b)(arr7));
}

Gives me the error:

/opt/compilers/dmd2/include/std/algorithm/iteration.d(2570):
Error: variable f868.main.F!(__lambda1).e cannot be declared to
be a function
/opt/compilers/dmd2/include/std/meta.d(546): Error: template
instance f868.main.F!(__lambda1) error instantiating
/opt/compilers/dmd2/include/std/algorithm/iteration.d(2477):
instantiated from here: staticMap!(ReduceSeedType, __lambda1)
/d486/f868.d(16):        instantiated from here:
reduce!(BitArray[])
/d486/f868.d(16): Error: template std.algorithm.iteration.reduce
cannot deduce function from argument types !((a, b) => a &
b)(BitArray[]), candidates are:
/opt/compilers/dmd2/include/std/algorithm/iteration.d(2451):
std.algorithm.iteration.reduce(fun...) if (fun.length >= 1)

On 2.069.1 (dpaste: http://dpaste.dzfl.pl/8d7652e7ebeb). I don't
have time ATM to diagnose it further. It definitely shouldn't be
that cryptic of an error message.
```
Dec 29 2015
Daniel Kozak via Digitalmars-d-learn <digitalmars-d-learn puremagic.com> writes:
```V Tue, 29 Dec 2015 17:42:26 +0000
Alex Parrill via Digitalmars-d-learn
<digitalmars-d-learn puremagic.com> napsáno:

On Tuesday, 29 December 2015 at 09:26:31 UTC, Alex wrote:
The problem is, that the last line with the reduce does not
compile. Why?

If you get an error, it is imperative that you tell us what it is.

For the record, this code:

import std.bitmanip;
import std.stdio;
import std.algorithm;

void main() {

BitArray[] arr7 = [BitArray([0, 1, 0, 1, 0, 1]),
BitArray([0, 1, 0, 0, 0, 0]), BitArray([0, 1, 0, 1, 0, 0]),
BitArray([0, 1, 0, 1, 0, 0])];

BitArray common = BitArray([1,1,1,1,1,1]);
foreach(BitArray ba; arr7)
{
common &=  ba;
}
writeln("common2: ", common);

writeln("common1: ", reduce!((a,b) => a & b)(arr7));
}

Gives me the error:

/opt/compilers/dmd2/include/std/algorithm/iteration.d(2570):
Error: variable f868.main.F!(__lambda1).e cannot be declared to
be a function
/opt/compilers/dmd2/include/std/meta.d(546): Error: template
instance f868.main.F!(__lambda1) error instantiating
/opt/compilers/dmd2/include/std/algorithm/iteration.d(2477):
instantiated from here: staticMap!(ReduceSeedType, __lambda1)
/d486/f868.d(16):        instantiated from here:
reduce!(BitArray[])
/d486/f868.d(16): Error: template std.algorithm.iteration.reduce
cannot deduce function from argument types !((a, b) => a &
b)(BitArray[]), candidates are:
/opt/compilers/dmd2/include/std/algorithm/iteration.d(2451):
std.algorithm.iteration.reduce(fun...) if (fun.length >= 1)

On 2.069.1 (dpaste: http://dpaste.dzfl.pl/8d7652e7ebeb). I don't
have time ATM to diagnose it further. It definitely shouldn't be
that cryptic of an error message.

It is cause by this
https://dlang.org/phobos/std_bitmanip.html#.BitArray.init

So until it will be removed it will does not compile.

Problem is in
https://dlang.org/phobos/std_range_primitives.html#.ElementType
which is implementet as
template ElementType(R)
{
static if (is(typeof(R.init.front.init) T))
alias ElementType = T;
else
alias ElementType = void;
}

maybe rewriting this like:

template ElementType(R)
{
static if (is(typeof(R.init.front) T))
alias ElementType = T;
else
alias ElementType = void;
}

could help too
```
Dec 29 2015