nothrow and std.exception.ifThrown

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On Thursday, 29 April 2021 at 16:02:20 UTC, novice2 wrote:
> Hello.
> I need use std.format.format() in nothrow function.
> format() can throw.
> For this case i have special default string.
> I don't want embrace format into try..catch block,
> and i found elegant std.exception.ifThrown.
> But DMD say "ifThrown not nothrow"
>
> https://run.dlang.io/is/kXtt5q
> ```d
> nothrow string foo(int x, string def) {
>     import std.format: format;
>     import std.exception: ifThrown;
>
>     return format("%d", x).ifThrown(def);
> }
>
> Error: function std.exception.ifThrown!(Exception, string, 
> string).ifThrown is not nothrow
> ```
>
> What i can use instead of ifThrown, or how it can be changed to 
> nothrow?
> Thanks.

The reason for this, apparently, is in the definition of 
`ifThrown`:
```
CommonType!(T1, T2) ifThrown(E : Throwable = Exception, T1, 
T2)(lazy scope T1 expression, lazy scope T2 errorHandler) nothrow
```

It's not marked as `nothrow` in the function's definition, so 
even if the delegate passed to ifThrown _is_ nothrow, the 
compiler can't tell. There's no easy way around this that I can 
think of OTOH that doesn't involve some effort on your part. One 
thing you can do is wrap ifThrown with 
`std.exception.assumeWontThrow`:
```
import std.exception: ifThrown, assumeWontThrow;
import std.functional: pipe;

alias ifThrown = pipe!(std.exception.ifThrown, assumeWontThrow);

nothrow string foo(int x, string def) nothrow {
     import std.format: format;

     return format("%d", x).ifThrown(def);
}

```