conditional expressions that eagerly evaluate the condition at top-level?

[simonster]

It would be great to do things like:

@cond VERSION >= v"0.3-" ? eigvecs(A, ev) : eigvecs(A, ev)[1]
@cond isdefined(:setfield!) ? setfield!(x, y) : setfield(x,y)
@cond WINDOWS ? 1 : 2

This would fix #4233 as well as give us an approach to writing code that is compatible with multiple Julia or package versions that isn't as awkward as #5029. (It would be great to have something like this for 0.3, so that package authors can avoid separate branches for different Julia versions provided the differences aren't too big.) I'm not sure how to do this, though. It seems like @cond would have to be a macro that calls eval. Ignoring that this is considered bad practice, the argument to eval seems to be executed in the macro's module and not the calling module, which could be confusing. Any thoughts?

[StefanKarpinski]

It would be nice to call this @if, but you can't use a keyword (currently).

[ihnorton]

crossref #7449 which covers one of the use-cases.

[MikeInnes]

I implemented this for Lazy.jl so that you can do things like

@cond if WINDOWS
  foo()
elseif OSX
  bar()
else
  error()
end

It supports chains of both block and ternary if syntax (you could even mix them I guess, yay recursive macros). Base is welcome to it.

[tkelman]

@static implemented in #16219