朱莉娅：以循环宏为条件

Question

Is there an easy way to change a "for loop header" depending on the packages a user has? For example, @progress for is good for adding a progress bar in Juno/Atom (just found out!), while we also have things like @simd, @acc, and @parallel. So what I want to have on this loop is to put a bunch of these macros conditionally given a boolean from the user or depending on availability. However, if I do a simple if isdefined(@progress) @progress for ... elseif accelerate @acc for ... elseif @parallel for ... end or something of that sort, I would have to keep pasting around the same for loop code. Is there some more elegant way of doing this? Also, I may want to combine some, and so once you start looking at the viable combinations that ends up being a lot of code!

Answer 1

The Pkg.installed method will error if the package isn't installed. It takes a string, and returning the decorated expression after that line with the other possibility in the catch block is effective for this sort of thing:

macro optional_something(pkg, expr)
    try
        Pkg.installed(string(pkg)) == nothing && return expr
        esc(quote
            @time $expr
        end)
    catch
        expr
    end
end

# this won't add the macro @time
@optional_something XXX rand(1000)    

# this will
@optional_something Plots rand(1000)