static array is not a range

[Alexibu]

On Tuesday, 9 January 2024 at 13:22:24 UTC, bachmeier wrote:
> On Tuesday, 9 January 2024 at 10:11:35 UTC, Alexibu wrote:
>> It looks like isInputRange is false for arrays with fixed 
>> length by design.
>>
>> I can do:
>>
>> ```d
>> float[4] arr;
>> foreach(x;arr)
>>    writefln("%s",x)
>> ```
>> but not :
>>
>> ```d
>> arr.each!(a => a.writefln("%s",a));
>> ```
>> Is there a good reason for this ?
>> It took my a long time to figure out.
>
> Jonathan's been giving you good general information about this. 
> I'm curious about your partial example. If I fix the writefln 
> call, it works.
>
> ```
> import std;
> float[4] arr;
> void main() {
>   arr[0] = 1;
>   arr[1] = 2;
>   arr[2] = 3;
>   arr[3] = 4;
>   arr.each!(a => "%s".writefln(a));
> }
> ```

You're right. My original problem was with the map algorithm, and 
I changed the example to make it simpler thinking all range 
algorithms would be the same.
If each works, I can't see why map filter etc can't work 
consistently where they only need an input range.
```d
auto line = arr.filter!(a > 0).map!(a => 
a.to!string).joiner("\t").text;
```
Should be fine because each result range is passed on the stack 
to the next algorithm, and then at the end the text (or array) 
algorithm doesn't return a range.
Also this should be fine because the ranges are all used on the 
stack.
```d
float[6] array;
string[] result;
auto line = arr.filter!(a > 0).map!(a => a.to!string).each(a => 
result ~= a);
return result;
```

If someone keeps ranges around longer than the static array then 
there are the problems Jonathan is describing.
```d
float[6] array;
auto filtered = arr.filter!(a > 0);
return filtered;
```
I wonder if the compiler could tell if you are only using the 
range as a temporary argument as opposed to assigning it to a 
variable ? Is there that rvalue lvalue distinction in D ?

Obviously anything that adds or removes values won't work, and as 
Jonathan points out returning a slice from find etc would be a 
slice to a static array which could cause problems.

I don't actually use static arrays very often but they can be 
convienient, especially if you were trying to convert maths or 
scientific code from something like matlab or numpy.