Hello everyone,

I'm trying to write a command-line application that can detect whether the input file is gzipped or not. Sounds simple enough, but I can't seem to do it properly in D (this is my first foray to the language).

After checking whether the file is gzipped or not, I try to create a new instance of a class that is basically an InputRange returning each line of the input file:

```
File f = File("input_file")
// detect gzip ...
if (isGzip)
    auto fileIter = new MyFileReader!GzipIterator(f)
else
    auto fileIter = new MyFileReader!NormalIterator(f)

foreach(string line; fileIter) {
// do things
}
```

However I always get the compiler error `undefined identifier lineIter`.

Now, my questions are:

1. What is causing the error? Is this caused by the compiler not being able to figure out what fileIter is at compile time?

2. I realize the current way of creating file handles may not be the best way to handle gzipped or non-gzipped file. Is there a better way to detect and create such file handles based on run time values?

Thanks in advance :).

On Monday, 4 August 2014 at 22:00:18 UTC, splatterdash wrote:
> ```
> File f = File("input_file")
> // detect gzip ...
> if (isGzip)
{
>     auto fileIter = new MyFileReader!GzipIterator(f)
}
> else
{
>     auto fileIter = new MyFileReader!NormalIterator(f)
}
>
> foreach(string line; fileIter) {
> // do things
> }
> ```

(Added braces for clarity.)

> However I always get the compiler error `undefined identifier lineIter`.

You have two independent variables called fileIter. Both are only
available in their respective scopes. Declare the variable before
the `if` to get one variable with a wider scope:

SomeCommonBaseTypeOfThoseMyFileReaderVariants fileIter;
if (isGzip)
     fileIter = new MyFileReader!GzipIterator(f);
else
     fileIter = new MyFileReader!NormalIterator(f);

On Monday, 4 August 2014 at 22:09:49 UTC, anonymous wrote:
> On Monday, 4 August 2014 at 22:00:18 UTC, splatterdash wrote:
>> ```
>> File f = File("input_file")
>> // detect gzip ...
>> if (isGzip)
> {
>>    auto fileIter = new MyFileReader!GzipIterator(f)
> }
>> else
> {
>>    auto fileIter = new MyFileReader!NormalIterator(f)
> }
>>
>> foreach(string line; fileIter) {
>> // do things
>> }
>> ```
>
> (Added braces for clarity.)
>
>> However I always get the compiler error `undefined identifier lineIter`.
>
> You have two independent variables called fileIter. Both are only
> available in their respective scopes. Declare the variable before
> the `if` to get one variable with a wider scope:
>
> SomeCommonBaseTypeOfThoseMyFileReaderVariants fileIter;
> if (isGzip)
>      fileIter = new MyFileReader!GzipIterator(f);
> else
>      fileIter = new MyFileReader!NormalIterator(f);

Indeed I do. I'm not sure which type I should use for the common base type, though. MyFileReader is a templated class, so using it plainly did not work. I also tried `InputRange!string` to no avail despite `MyFileReader` implementing the three InputRange requirement (popFront(), front, and empty).

Any ideas on what I should as the class?

On Monday, 4 August 2014 at 22:18:24 UTC, splatterdash wrote:
> Indeed I do. I'm not sure which type I should use for the common base type, though. MyFileReader is a templated class, so using it plainly did not work. I also tried `InputRange!string` to no avail despite `MyFileReader` implementing the three InputRange requirement (popFront(), front, and empty).
>
> Any ideas on what I should as the class?

Let MyFileReader implement an interface that has the operations
you need. That interface can be std.range.InputRange!string, or
you can define your own.

Note that a type is an input range when it has the input range
primitives (front, popFront, empty), but it's only a
std.range.InputRange!T when it implements the interface in the
OOP sense: class C : InputRange!E {...}.

Phobos generally doesn't use InputRange, but templatizes
everything. You can go that way, too, and move the foreach loop
to a templated function:

void main()
{
     File f = File("input_file")
     // detect gzip ...
     if (isGzip)
         doThings(new MyFileReader!GzipIterator(f));
     else
         doThings(new MyFileReader!NormalIterator(f));
}
void doThings(I)(I fileIter)
{
     foreach(string line; fileIter) {
     // do things
     }
}

On Monday, 4 August 2014 at 22:45:15 UTC, anonymous wrote:
> On Monday, 4 August 2014 at 22:18:24 UTC, splatterdash wrote:
>> Indeed I do. I'm not sure which type I should use for the common base type, though. MyFileReader is a templated class, so using it plainly did not work. I also tried `InputRange!string` to no avail despite `MyFileReader` implementing the three InputRange requirement (popFront(), front, and empty).
>>
>> Any ideas on what I should as the class?
>
> Let MyFileReader implement an interface that has the operations
> you need. That interface can be std.range.InputRange!string, or
> you can define your own.
>
> Note that a type is an input range when it has the input range
> primitives (front, popFront, empty), but it's only a
> std.range.InputRange!T when it implements the interface in the
> OOP sense: class C : InputRange!E {...}.
>
> Phobos generally doesn't use InputRange, but templatizes
> everything. You can go that way, too, and move the foreach loop
> to a templated function:
>
> void main()
> {
>      File f = File("input_file")
>      // detect gzip ...
>      if (isGzip)
>          doThings(new MyFileReader!GzipIterator(f));
>      else
>          doThings(new MyFileReader!NormalIterator(f));
> }
> void doThings(I)(I fileIter)
> {
>      foreach(string line; fileIter) {
>      // do things
>      }
> }

That does it, thanks :)!