What does @. do in Julia?

Question

I am trying to use the LsqFit package in Julia, and the documentation makes use of a macro @. without really explaining how it works. It is not clear to me from the package documentation whether this is a standard Julia macro or something peculiar to the package, and I can't find a reference to this macro elsewhere. How does @. work in Julia?

Answer 1

You can check documentation for any Julia function by pressing ? to go to the help mode, see the output below:

help?> @.
  @. expr

  Convert every function call or operator in expr into a "dot call" (e.g. convert f(x) to f.(x)), and convert
  every assignment in expr to a "dot assignment" (e.g. convert += to .+=).

  If you want to avoid adding dots for selected function calls in expr, splice those function calls in with $.
  For example, @. sqrt(abs($sort(x))) is equivalent to sqrt.(abs.(sort(x))) (no dot for sort).

Describing the documentation above in other words the @. allows you to vectorize all function calls in the expression following a macro.

If you are unsure how any Julia macro works I recommend using the @macroexpand macro, for example:

julia> @macroexpand @. [1, 2, 3] + [4, 5, 6]
:((+).([1, 2, 3], [4, 5, 6]))