Julia: Underscore not allowed with optional parameters

[cstjean]

julia> f(_, a) = 10
f (generic function with 1 method)

julia> f(_, a=1) = 10
ERROR: syntax: all-underscore identifier used as rvalue around REPL[0]:1

The second form seems legitimate, no? The same issue arises with keyword arguments (f(_; x=1) = ...)

[StefanKarpinski]

It does seem like a good way to indicate that you don't care about an argument, similar to ::Any but shorter.

[JeffBezanson]

Ah, naturally this is due to lowering converting the definition to

f(_) = f(_, 1)
f(_, a) = 10

[quinnj]

Could be nice to lower _ as a function argument to @nospecialize(_)

[schlichtanders]

how does the lowering for keyword arguments look like? is it the same reason there?