A Strange but Effective Debugging Technique in Julia

Often, most of the work during debugging is to reconstruct the intermediate states in an algorithm. A debugger can often facilitate this need, but it could be rather cumbersome to use if the breakpoint condition is complicated.

In Julia, we usually develop a package with a Julia REPL open on the side. All the code executed in the REPL resides in the Main module. Hence, one can access REPL variables via Main.a_variable_name in the package one is developing. While debugging, we need to send the intermediate states of a function to Julia REPL. Unfortunately, Julia forbids rebinding of a variable from another module. One can bypass this limitation by defining an unused variable like _a = Ref{Any}() in the REPL. Then, modify this variable via Main._a[] = a_state_of_interest_1, a_state_of_interest_2 in some function of a package.

An Example

julia> function foo(n)
           a = b = 1
           for i in 1:n
               a, b = b, a + b
               if a <= 0
                   Main._a[] = a, b
               end
           end
           return a
       end
foo (generic function with 1 method)

julia> _a = Ref{Any}()
Base.RefValue{Any}(#undef)

julia> foo(100)
1298777728820984005

julia> _a
Base.RefValue{Any}((-2437933049959450366, 3736710778780434371))