Is there a standard way to handle errors in a chain of range transformations?

Let's say I want to read some comma separated numbers from a file.

auto myArray = file.byLine().splitter().map!(to!int).array();

Now, besides fatal errors (like I/O), let's suppose I want to handle some errors in a silent way:
- skip unicode decoding errors;
- assume blank records with 0;
- skip the line entirely if the conversion to int is not possible;

I can catch UTFException, ConvException or ConvOverflowException for the entire syntax chain but there are some disadvantages:
- I don't know exactly which of the chain members thrown the exception;
- I cannot skip/handle the error and continue;

The only solution I thought about is to break the nice chain syntax and handle errors on each of the chain members, but I wonder if there is probably another way.

Thanks.

On Thursday, 5 April 2018 at 17:06:04 UTC, rumbu wrote:
> Is there a standard way to handle errors in a chain of range transformations?
>
> Let's say I want to read some comma separated numbers from a file.
>
> auto myArray = file.byLine().splitter().map!(to!int).array();
>
> Now, besides fatal errors (like I/O), let's suppose I want to handle some errors in a silent way:
> - skip unicode decoding errors;
> - assume blank records with 0;
> - skip the line entirely if the conversion to int is not possible;
>
> I can catch UTFException, ConvException or ConvOverflowException for the entire syntax chain but there are some disadvantages:
> - I don't know exactly which of the chain members thrown the exception;
> - I cannot skip/handle the error and continue;
>
> The only solution I thought about is to break the nice chain syntax and handle errors on each of the chain members, but I wonder if there is probably another way.
>
> Thanks.

You could use predicates:

```
    string list = "3, 5, 1, , not a number, 100";

    int[] numbers = list.split(",").filter!((entry) {
        if (!isNumeric(entry.strip))
            return false;
        else // ... even more cases?
            return true;
    	})
        .map!((e) => e.strip.to!int).array;

    assert(numbers == [3, 5, 1, 100]);
```
https://run.dlang.io/gist/413282d9726dbac137bf5f35033a8eea

On Thursday, 5 April 2018 at 17:06:04 UTC, rumbu wrote:
> Is there a standard way to handle errors in a chain of range transformations?
>
> [...]

Are you aware of ifThrown?

https://dlang.org/phobos/std_exception.html#ifThrown

It's not perfect, but imho a nice start and one of the places where lazy really shines.

On Thursday, 5 April 2018 at 17:36:56 UTC, Seb wrote:
> On Thursday, 5 April 2018 at 17:06:04 UTC, rumbu wrote:
>> Is there a standard way to handle errors in a chain of range transformations?
>>
>> [...]
>
> Are you aware of ifThrown?
>
> https://dlang.org/phobos/std_exception.html#ifThrown
>
> It's not perfect, but imho a nice start and one of the places where lazy really shines.

Thanks, ifThrown is perfect.