fix @manipulate hygiene and add a test

[rdeits]

@manipulate was escaping too many things, which meant that it relied on the user having done using Interact and using Reactive. This change removes that requirement, and will also prevent weird errors in the case that a user has some local function called signal or preserve.

Fixes #184
Also fixes #156

[rdeits]

Dang, I think I'm running into https://discourse.julialang.org/t/problem-with-escaping-anonymous-function-expressions-and-let-blocks/1314 and thus JuliaLang/julia#16096 (comment)

I'll try a different approach...

Edit: nope, not those issues

[rdeits]

Ok, instead of escaping less, we can interpolate more. Hopefully this will work on 0.5-0.7

Edit: no, that wasn't actually the problem

[rdeits]

Ah, never mind. That "invalid let syntax" shows up with Interact.jl master, so I won't worry about it for this PR.

Edit: the invalid let syntax was actually JuliaLang/julia#21774 . I've added an explicit version check to get the tests working in all supported versions.

[rdeits]

Actually, the let syntax issue was pretty easy to work around. Tests are now passing on Julia v0.5, v0.6, and v0.7. I haven't actually got a working IJulia in v0.7, so I don't know if the widgets work, but this is at least an improvement over master

[JobJob]

Thanks! Just one question, when I run this in IJulia:

using Interact: @manipulate

# Dummy implementations that should never be called:
signal(x...) = error("I (signal) should not be called")
preserve(x...) = error("I (preserve) should not be called")

@manipulate for i in 1:10, j in ["x", "y", "z"]
    2 * i, j * " hello"
end

It errors with "I (preserve) should not be called", is that expected?

[rdeits]

Uh, I see that too. That's...baffling. Running macroexpand shows:

macroexpand(:(@manipulate for i in 1:10, j in ["x", "y", "z"]
    2 * i, j * " hello"
            end))

:(let i = (Interact.widget)(1:10, "i"), j = (Interact.widget)(["x", "y", "z"], "j")
        display(i)
        display(j)
        (Interact.preserve)((Interact.map)(((i, j)->begin  # In[3], line 8:
                        (2i, j * " hello")
                    end), (Interact.signal)(i), (Interact.signal)(j), typ=Interact.Any))
    end)

which seems to clearly show that Interact.preserve will be called. I'm so confused...

[rdeits]

Oh now I see. The error isn't coming from the call to preserve inside manipulate, but instead from a different call to preserve in Interact/src/IJulia/setup.jl, line 81. I don't know much about that file, but the way it's written it will just call whatever preserve() is defined in the user's workspace. That's probably worth fixing too :)

[JobJob]

Nice, yeah that makes sense.