Beginner question. Given

  if (startsWith(input, "0x", "0X"))

How do I turn that into a case-insensitive startsWith? startsWith says it takes a predicate but I can't figure out how to pass it one. The examples all use "a == b" !? These attempts below, and other things I have tried, fail with "cannot deduce function from argument types".

  if (startsWith!"icmp(a, b) == 0"(input, "0x"))
  if (startsWith!"std.uni.icmp(a, b) == 0"(input, "0x"))
  if (startsWith!((a,b) => icmp(a,b) == 0)(input, "0x"))

On Tuesday, 28 April 2015 at 21:45:10 UTC, PhilipDaniels wrote:
> Beginner question. Given
>
>   if (startsWith(input, "0x", "0X"))
>
> How do I turn that into a case-insensitive startsWith? startsWith says it takes a predicate but I can't figure out how to pass it one. The examples all use "a == b" !? These attempts below, and other things I have tried, fail with "cannot deduce function from argument types".
>
>   if (startsWith!"icmp(a, b) == 0"(input, "0x"))
>   if (startsWith!"std.uni.icmp(a, b) == 0"(input, "0x"))
>   if (startsWith!((a,b) => icmp(a,b) == 0)(input, "0x"))

I believe the issue is that the predicate expects a boolean, icmp returns an int. Try a == toLower(b) as your predicate(there's probably a better solution somewhere hidden in phobos though.)

On Tue, 28 Apr 2015 21:45:07 +0000, PhilipDaniels wrote:

> Beginner question. Given
> 
>    if (startsWith(input, "0x", "0X"))
> 
> How do I turn that into a case-insensitive startsWith? startsWith says it takes a predicate but I can't figure out how to pass it one. The examples all use "a == b" !? These attempts below, and other things I have tried, fail with "cannot deduce function from argument types".
> 
>    if (startsWith!"icmp(a, b) == 0"(input, "0x"))
>    if (startsWith!"std.uni.icmp(a, b) == 0"(input, "0x"))
>    if (startsWith!((a,b) => icmp(a,b) == 0)(input, "0x"))

The issue is that those icmp functions take strings as arguments while startsWith expects the predicate to take individual characters.

I believe the best solution here is to use the lazy toLowerCase function
in std.uni and the default predicate:
  if (startsWith(input.toLowerCase, "0x".toLowerCase))

On Tuesday, 28 April 2015 at 22:34:07 UTC, Justin Whear wrote:
>>    if (startsWith!"icmp(a, b) == 0"(input, "0x"))
>>    if (startsWith!"std.uni.icmp(a, b) == 0"(input, "0x"))
>>    if (startsWith!((a,b) => icmp(a,b) == 0)(input, "0x"))
>
> The issue is that those icmp functions take strings as arguments while
> startsWith expects the predicate to take individual characters.
>

Thanks. That seems obvious now that you mention it but honestly I could not tell that from the documentation :-(

> I believe the best solution here is to use the lazy toLowerCase function
> in std.uni and the default predicate:
>   if (startsWith(input.toLowerCase, "0x".toLowerCase))

I think I will have to make a "string idioms" wiki page...

if("0X".std.string.indexOf("0x", CaseSensitive.no) == 0)

should work

Note that my solution relies on the pre-release version of std.uni, those lazy functions aren't in the latest release.