std.conv.to purity

[Gary Willoughby via Digitalmars-d-learn]

On Saturday, 14 February 2015 at 12:24:51 UTC, ketmar wrote:
> On Sat, 14 Feb 2015 11:29:28 +0000, Jack Applegame wrote:
>
>> why std.conv.to is not pure?
>> 
>> string foo(real v) pure { return v.to!string; }
>> // Error: pure function 'foo' cannot call impure function
>> 'std.conv.to!string.to!(real).to'
>
> 'cause float->string conversion is damned hard task. to perform 
> this
> conversion `to!` falls back to `formatValue` from "std.format", 
> which in
> turn using `snprintf()` from libc.

Forgive me being a bit dense but that doesn't necessarily mean 
it's impure though, right?