Freestanding Macro that doesn't return an expression

Development Macros
1

I was trying to implement Assign if different using freestanding macro and I did succeed but kind of cheating using a closure because the freestanding macro is expected to return and expression.

#assign(b, to: a, if: !=) for example is expanded to:

{
    if a != b {
        a = b
    }
}()

Is this the only way to currently do this? Any plans to lift this restriction with another kind of freestanding macro?

@Douglas_Gregor

2

This is the only way to do this today. The early pitches for declaration macros had "code item" macros that could do this, and there's a partial implementation of the feature in the compiler behind an experimental feature flag (CodeItemMacros), but nobody has followed through with pitch/proposal/complete implementation. Personally, I haven't seen that many use cases for code-item macros vs. the other kinds, so I haven't looked into them further.

Doug

2 Likes
3

Hello,

I have a use case for this kind of feature that might be valuable.

It's not possible to possible to wrap os log calls because OSLogMessage cannot be initiliazed from a String variable, it forces us to duplicate log lines to both log into Unified logging system and to Crashlytics for example.

I would like to use a macro to expand the boilerplate code that perform those 2 calls. It seems that only CodeItemMacro can achieve this but I might be wrong...

1 Like
4

I created a macro for this exact same use case, and ended up using the same immediately-executed closure strategy described in the original post here, in order to consolidate the two expressions down to a single one.

1 Like