delegate object instead of a literal

[Jonathan M Davis]

On Sunday, August 14, 2011 20:50:03 Joel Christensen wrote:
> Hi,
> 
> This program loops through a string until it finds a number and gives
> the position of it.
> 
> The first assert works, but not the second one.
> 
> import std.algorithm;
> 
> void main() {
> 	static bool isNumber( char input, char dummy ) {
> 		if ( ( input >= '0' && input <= '9' ) || input == '.' )
> 			return true;
> 		else
> 			return false;
> 	}
> 
> 	string str = "abc123";
> 	assert( countUntil!( ( input, b ) {
> 		if ( ( input >= '0' && input <= '9' ) || input == '.' ) return true;
> else return false;
> 		} )(str, 0) == 3 ); // works
> 	assert( countUntil!( isNumber, string, string )( str, 0 ) == 3 );
> }
> 
> This is the error:
> parse.d(15): Error: template instance
> countUntil!(isNumber,string,string) does not match template declaration
> countUntil(alias pred = "a == b",R1,R2) if
> (is(typeof(startsWith!(pred)(haystack,needle))))

Okay. Several things here. One, don't ever declare an individual char. char is 
a UTF-8 code unit, not a code point. And characters are code points. ASCII 
characters only require one UTF-8 code unit, so they can be held in a single 
char, but that's not true of characters in general. When dealing with 
individual characters, always use dchar. So, if you're using foreach for a 
string, use dchar as the iteration type. e.g.

foreach(dchar c; str) {...}

Youre code will be wrong if it iterates over char or wchar, since those aren't 
full characters. And because of that, _all_ strings are ranges of dchar, not 
char or wchar. So, the fact that you're using string instead of dstring with a 
range-based function like countUntil is irrelevant. You're still dealing with 
a range of dchars. So, isNumber should definitely be taking dchars, not chars.

Two, don't pass 0 as a string. 0 is an int. _None_ of those countUntil calls 
shouldn't be compiling at all. It's actually a bit disturbing thath the first 
one compiles. It definitely looks like a bug. countUntil takes two ranges with 
the same element type. 0 is not a range, let alone a range of dchars. 0 
shouldn't even really be used as a dchar, since 0 is an int, not a character.

Third, you really shouldn't be using if statements like that. It looks like 
you don't know what you're doing if you do

if(condition)
    return true;
else
    return false;

The condition is already a bool. You can simply do

    return condition;

Plenty of newbie programmers make that mistake, and I don't know how 
experienced a programmer you are, but I'd advise you not to do that anymore.

In any case, I'm a bit shocked that the first call to countUntil compiles, let 
alone works. That's quite disturbing actually. It looks like countUntil has a 
bug. However, your usage of countUntil here does imply that it could use an 
overload which takes a unary function and only one range.

- Jonathan M Davis