Optional import mechanism

[Júlio Hoffimann]

Dear all,

Consider the following code:

try

    require("ImageView")

    global view = ImageView.view

catch err

    @show err

    # fallback to nothing

    global view = view(args...; kargs...) = (nothing, nothing)

end

I have ImageView installed, but the catch block is executed with err => UndefVarError(:ImageView). The symbol ImageView isn't visible somehow. How to fix the error?

Best,

Júlio.

[Kevin Squire]

What happens if you use import instead is require?

[Júlio Hoffimann]

Hi Kevin,

What happens if you use import instead is require?

ERROR: error compiling anonymous: unsupported or misplaced expression import in function anonymous

Júlio.

[Joey Huchette]

It’s a bit ugly, but this should work:

try
    eval(Expr(:import, :ImageView))
    global view = ImageView.view
catch err
    @show err
    # fallback to nothing
    global view = view(args...; kargs...) = (nothing, nothing)
end

[Júlio Hoffimann]

Hi Joey,

It’s a bit ugly, but this should work:

try
    eval(Expr(:import, :ImageView))
    global view = ImageView.view
catch err
    @show err
    # fallback to nothing
    global view = view(args...; kargs...) = (nothing, nothing)
end

Unfortunately it doesn't work:

err => UndefVarError(:ImageView)

err => ErrorException("invalid redefinition of constant view")

Júlio.

[Miles Lubin]

This may do what you want (snippet from JuMP):

if isdir(Pkg.dir("ArrayViews"))
    eval(Expr(:import,:ArrayViews))
    const subarr = ArrayViews.view
else
    const subarr = Base.sub
end

[Tim Holy]

Doesn't just adding Main in front solve the problem?

view = Main.ImageView.view

--Tim

[Júlio Hoffimann]

Doesn't just adding Main in front solve the problem?

    view = Main.ImageView.view

--Tim

Thanks Tim! It did the trick. The correct solution: https://github.com/juliohm/ImageQuilting.jl/blob/master/src/imquilt.jl#L22-L28

It's not ideal, but it works. It would be much nicer if `using ImageView` raised an ImportError exception or something rather than relying on the require() function.

I invite everyone to give it a try with a fresh Julia build: https://github.com/juliohm/ImageQuilting.jl

Best,

Júlio.

[Iain Dunning]

Soon you'll be able to use https://github.com/JuliaLang/julia/pull/6884

and everything can be much nicer :)

[David van Leeuwen]

Hello, 

I've read some of the discussion related to conditional / optional module loading, and I am not sure what the status of https://github.com/JuliaLang/julia/pull/6884---I guess it still is open for merge.  I'm also not sure if it would address the following.  

I'm in the situation where a (small) part of a package would depend on a plotting package, e.g., Winston, but I would like to be able to use most of the functionality on a run-time system without interactive graphics capabilities.   

A top level `using Winston` now bails out in a command-line-only environment, because there is no DISPLAY environment variable that Tk needs that is required by Winston. 

A load-at-runtime by including the `using Winston` in a clever `eval`-way inside a plotting function, such that an attempt to load the package is made only after first call to my plotting function is further complicated by the fact that my package needs to import the `plot` function from Winston if I want to call this plotting function `plot(::MyType)`. 

Any ideas?

Thanks, 

---david

[Tim Holy]

For this specific case, if you're on Linux, from the command line try
xvfb-run julia -e 'include("my_plotting_script")'
You'll need xvfb installed, of course.

--Tim

[David van Leeuwen]

Thanks, Tim, this indeed solves my immediate problem!

---david