• Kevin Bailey (25/25) Jul 02 2021 Sorry if this is too simple of a question, but I'm trying to
• Paul Backus (18/30) Jul 02 2021 The error message here is actually telling you exactly what the
• Kevin Bailey (5/9) Jul 02 2021 Hey Paul,

Kevin Bailey <keraba yahoo.com> writes:

Sorry if this is too simple of a question, but I'm trying to 
simply find
a blank line in a file. I'm doing something like:

```
auto file = File("somefile");
auto r = file.byLine();
auto tup = r.findSplit("");
// tup[0] would contain the range before the line,
// tup[2] would contain the range after, not unlike findSplit
```

Unfortunately, ldc2 is saying:

```
split.d(8): Error: template std.algorithm.searching.findSplit 
cannot deduce function from argument types !()(ByLineImpl!(char, 
char), string), candidates are:
/usr/lib/ldc/x86_64-linux-gnu/include/d/std/algorithm/searching.d(2882):       
findSplit(alias pred = "a == b", R1, R2)(R1 haystack, R2 needle)
   with pred = "a == b",
        R1 = ByLineImpl!(char, char),
        R2 = string
   must satisfy the following constraint:
        isForwardRange!R1
```

I can't tell if findSplit() only works on strings, or if maybe I'm
trying to compare different types of strings, or if I'm overtaxing
the type deduction system.

Paul Backus <snarwin gmail.com> writes:

On Saturday, 3 July 2021 at 00:29:23 UTC, Kevin Bailey wrote:
 Unfortunately, ldc2 is saying:

 ```
 split.d(8): Error: template std.algorithm.searching.findSplit 
 cannot deduce function from argument types 
 !()(ByLineImpl!(char, char), string), candidates are:
 /usr/lib/ldc/x86_64-linux-gnu/include/d/std/algorithm/searching.d(2882):      
 findSplit(alias pred = "a == b", R1, R2)(R1 haystack, R2 needle)
   with pred = "a == b",
        R1 = ByLineImpl!(char, char),
        R2 = string
   must satisfy the following constraint:
        isForwardRange!R1
 ```

The error message here is actually telling you exactly what the 
problem is: `findSplit` requires a forward range, but the range 
returned by `File.byLine` is not a forward range, just an input 
range.

     import std.range;
     pragma(msg, 
isForwardRange!(typeof(File("somefile").byLine())));
     // prints "false"

The difference between an forward range and an input range is 
that forward ranges implement `.save`, a method that allows you 
to save your place in the range and return to it later. The range 
returned by `File.byLine` doesn't have this method because it's 
not always possible to save your place in a file (for example, if 
you're reading from a pipe).

To work around this, you can either (a) read the whole file into 
memory before using `findSplit` on it, or (b) redesign your 
algorithm to work in a single pass over the input file.

Kevin Bailey <keraba yahoo.com> writes:

On Saturday, 3 July 2021 at 01:15:20 UTC, Paul Backus wrote:
 The error message here is actually telling you exactly what the 
 problem is: `findSplit` requires a forward range, but the range 
 returned by `File.byLine` is not a forward range, just an input 
 range.

Hey Paul,

Thanks for the pointer. I doubt I would have figured out that
surprising detail. Nevertheless, now I know where to look for
the exact error. :-)