On Monday, 11 March 2019 at 15:23:53 UTC, HaraldZealot wrote:
```d
File[] files;
foreach(filename; args[1 .. $])
{
files ~= File(filename, "r");
}
auto ranges = files.map!(a => a.byLineCopy);
writeln(ranges[0].front);
writeln(ranges[0].front);
writeln(ranges[0].front);
```
produces
```
1
2
3
```
[...]
What I'm doing wrong with `map`? Or is this a bug?
`map` is lazy in the sense that it (re-)evaluates the given
function whenever you access an element. That means you're
calling `byLineCopy` three times on the same file. Your code
effectively does this:
writeln(files[0].byLineCopy.front);
writeln(files[0].byLineCopy.front);
writeln(files[0].byLineCopy.front);
The range created by `byLineCopy` immediately reads a line from
the file to populate its `front`. So you're reading three lines
from the file.
Strictly speaking, I don't think any of this qualifies as a bug.
`map`'s behavior might be surprising, but it's deliberate, as far
as I know.
To avoid the re-evaluation, assign `ranges[0]` to a variable
before using it:
auto lines = ranges[0];
writeln(lines.front);
writeln(lines.front);
writeln(lines.front);
That should print the same line three times.
