what's `@fastmath` doing here · helpdesk (published)

Stream: helpdesk (published)

Topic: what's `@fastmath` doing here

wheeheee (Oct 02 2025 at 15:30):

I found that @fastmath has some problems with Base.Cartesian.@nexprs, like so:

julia> Base.@nexprs 5 j -> y_j = rand()
0.13646394987531396

julia> @fastmath Base.@nexprs 4 j -> x_j = y_{j + 1}
ERROR: UndefVarError: `y_` not defined in `Main`
Suggestion: check for spelling errors or missing imports.
Stacktrace:
 [1] top-level scope
   @ REPL[2]:1

julia> @macroexpand @fastmath Base.@nexprs 4 j -> x_j = y_{j + 1}
quote
    x_1 = y_{Base.FastMath.add_fast(1, 1)}
    x_2 = y_{Base.FastMath.add_fast(2, 1)}
    x_3 = y_{Base.FastMath.add_fast(3, 1)}
    x_4 = y_{Base.FastMath.add_fast(4, 1)}
end

julia> @macroexpand Base.@nexprs 4 j -> x_j = y_{j + 1}
quote
    x_1 = y_2
    x_2 = y_3
    x_3 = y_4
    x_4 = y_5
end

It looks like it's basically @fastmath(@nexprs(4, ...)), so evaluating from inside to outside, shouldn't @fastmath just enable fastmath for the whole block after getting the output from the @nexprs macro? Or does macro evaluation follow a different order, like from outside in?

Mason Protter (Oct 02 2025 at 15:51):

Yes, macros evaluate outside in, because they operate on chunks of code.

wheeheee (Oct 02 2025 at 16:26):

Can I sort of make them evaluate inside out?

wheeheee (Oct 02 2025 at 16:26):

Or achieve that kind of effect, basically.

Mason Protter (Oct 02 2025 at 20:58):

yes, you just have to make the outer macro look for macros inside of it and then macroexpand them

Daniel Wennberg (Oct 02 2025 at 23:32):

I suppose the most general way would be to define a separate macro @expandafter that rewrites

@expandafter @outer @inner expr

@outer inner_expr_expanded

In TeX, which is a pure macro expansion language, this macro is built in.

Mosè Giordano (Oct 03 2025 at 05:56):

https://jkrumbiegel.com/pages/2022-08-09-composing-macros/

Last updated: Oct 18 2025 at 04:39 UTC