I have a module that has some functions that run some optimization problems modeling using the JuMP package and the Gurobi solver.
a very generic structure of the code is as follows:

module TestModule
using JuMP
using Gurobi
const GRB_ENV = Ref{Gurobi.Env}()
function __init__()
GRB_ENV[] = Gurobi.Env()
return
end
function run_optimization()
model = Model(() -> Gurobi.Optimizer(GRB_ENV))
set_optimizer_attribute(model, "TimeLimit", 3600.0)
#some constraints, variables, objective modeled using JuMP
optimize!(model)
end
export run_optimization
end

When I call the run_optimization function, I get the following error:

using TestModule
run_optimization() #exported from the module
#Error
ERROR: LoadError: MethodError: no method matching Gurobi.Optimizer(::Base.RefValue{Gurobi.Env})
Closest candidates are:
Gurobi.Optimizer() at ~/.julia/packages/Gurobi/vStLK/src/MOI_wrapper/MOI_wrapper.jl:325
Gurobi.Optimizer(::Union{Nothing, Gurobi.Env}; enable_interrupts) at ~/.julia/packages/Gurobi/vStLK/src/MOI_wrapper/MOI_wrapper.jl:325

I am probably doing something silly here. If someone could point out the correct way to call it, that would be great. Thanks!

This is a different error. The Gurobi Reference Manual says this is:

10002 NULL input value provided for a required argument

But I canâ€™t reproduce that one. This works for me in the REPL:

module TestModule
using JuMP
using Gurobi
const GRB_ENV = Ref{Gurobi.Env}()
function __init__()
const GRB_ENV[] = Gurobi.Env()
return
end
function run_optimization()
model = Model(() -> Gurobi.Optimizer(GRB_ENV[])) # <-- Added []
set_optimizer_attribute(model, "TimeLimit", 3600.0)
#some constraints, variables, objective modeled using JuMP
optimize!(model)
end
export run_optimization
end
using .TestModule
run_optimization()

Totally my fault. I changed GRB_ENV to GRB_ENV[] at all places but one (I have multiple functions solving optimization problems). It works now; however, I am just curious why does it work now and not before. Also, the person who made the post here didnâ€™t seem to have any issue with using just GRB_ENV. Further, as @odow mentioned in this comment, any calls to GRB_ENV should be GRB_ENV[]. Any leads to why it works now would be helpful. Thanks again!!

The [] is needed to get or set the element inside a Ref object.

The __init__ is needed so that a new environment is created each time you start Julia. Otherwise, the C pointer will be precompiled and it wonâ€™t exist the next time you load the model.