Stream: helpdesk (published)

Topic: Break/continue statements error inside a do block

Moorits Muru (Mar 11 2025 at 06:50):

I found out that using break/continue statements inside a do block (for example, open(file) do f ... end) produces an error. Is there a work-around or should I just avoid do blocks when I want to use these jump statements?

A MWE:

for i in 1:10
  map(1:5) do j
    if i == 2
      continue
    end
    2j
  end
end

which produces

ERROR: syntax: break or continue outside loop
Stacktrace:
 [1] top-level scope
   @ REPL[1]:1

Júlio Hoffimann (Mar 11 2025 at 08:30):

The do syntax is sugar to convert map(f, xs) into

map(xs) do x
  f(x)
end

Your f is not a valid Julia function.

Moorits Muru (Mar 11 2025 at 09:18):

This is just an example to show the behaviour of the continue statement, and it does seem like a valid function to me.

julia> map(1:5) do f
       2f
       end
5-element Vector{Int64}:
  2
  4
  6
  8
 10

Mason Protter (Mar 11 2025 at 09:28):

What would you expect continue to do inside the map body?

Mason Protter (Mar 11 2025 at 09:36):

and it does seem like a valid function to me.

Júlio was referring specifically to your use of continue in the function body. To be clear, when you write

map(1:5) do j
    if i == 2
        continue
    end
    2j
end

that is basically syntax sugar for

function _f(j)
    if i == 2
        continue
    end
    2j
end

map(f, 1:5)

The continue belongs lexically to the function body, not to the outer for loop.

Gunnar Farnebäck (Mar 11 2025 at 09:41):

This gives me a dejavu to https://discourse.julialang.org/t/use-break-or-continue-statement-with-pmap/126425

Moorits Muru (Mar 11 2025 at 09:41):

I was hoping that in a loop the i is also in the scope of the do block. My real life case is that I have loop over file paths, open them using a do block and check whether the file contains certain info, if it doesn't, continue with the next file.

Gunnar Farnebäck (Mar 11 2025 at 09:42):

If you just want to continue, a return from the implicit anonymous function does just that.

Moorits Muru (Mar 11 2025 at 09:44):

I could try that, thanks!

Mason Protter (Mar 11 2025 at 09:48):

yeah, map was a confusing example here because the mapping function is meant to be called multiple times, and the results are all supposed to be collected in order, so it wasn't clear to me what you expected to happen in the iterations where you continue. (i.e. should the output vector be shorter, should it contain nothings, etc?)

Last updated: Apr 04 2025 at 04:42 UTC