Please help me understand this function signature: std.stdio.File.byLine

[jfondren]

On Thursday, 15 July 2021 at 18:08:45 UTC, Scotpip wrote:
> The relevant function appears to be 
> [std.stdio.File.byLine](https://dlang.org/library/std/stdio/file.by_line.html).
>
> The default isn't breaking the lines properly, so I have to 
> pass in the line ending. But the signature has me baffled:
>
> ```
> auto byLine(Terminator, Char) (
>   KeepTerminator keepTerminator = No.keepTerminator,
>   Terminator terminator = '\x0a'
> )
> if (isScalarType!Terminator);
>
> auto byLine(Terminator, Char) (
>   KeepTerminator keepTerminator,
>   Terminator terminator
> )
> if (is(immutable(ElementEncodingType!Terminator) == 
> immutable(Char)));
> ```
>
> To specify the line ending, it appears to be asking for a type 
> "Terminator" which I can't find in the library.

Terminator appears a few times in the signature. The first 
appearance, in `byLine(Terminator, ...)(...)`, is a template 
parameter. So Terminator is whatever you want that otherwise 
works.

> Doing the obvious doesn't work:
>
> ```
> // Err: cannot deduce function from argument types
> myFile.byLine(`\r\n`);
> ```

The first parameter is the keepTerminator flag. That wysiwyg 
string is four bytes long and includes the literal slashes.
https://dlang.org/spec/lex.html#wysiwyg

```
$ rdmd --eval '`\r\n`.representation.writeln'
[92, 114, 92, 110]
```

Here's a simple dos2unix:

```d
import std.stdio, std.typecons;

void main() {
     foreach (line; stdin.byLine!(string, char)(No.keepTerminator, 
"\r\n"))
         writeln(line);
}
```

> Also, how would you specify the Char type as a parameter?

The !(string, char) isn't necessary in the last example, but 
that's how you'd provide it.

Usage:

```
$ echo -ne "hi\r\nthere\r\n" | dmd -run dosin|od -c
0000000   h   i  \n   t   h   e   r   e  \n
0000011
```

>
>
> The examples are unenlightening as they only show the default 
> case, so any help would be much appreciated. Any wider tips on 
> how to read these cryptic signatures would be a bonus!