rewrite show as writemime for Julia v0.4

[dancasimiro]

writemime was deprecated in JuliaLang/julia#16563 in julia commit
ae62bf0b813afbf32402874451e55d16de909bd4. This compatibility change
allows code that is written in the Julia v0.5 style to continue working
on Julia v0.4. I have not tested this on Julia v0.3.

The compat macro must be used on the import statement and the function
definition. For example:

@compat import Base.show
@compat show(io::IO, ::MIME"text/plain", ::TestCustomShowType) = print(io, "MyTestCustomShowType")

[TotalVerb]

Could you also support the syntax @compat Base.show([...]) = [...], which would be rewritten to Base.writemime([...]) = [...]?

[TotalVerb]

Also, might it be easier simply to define a generic writemime method that delegates to show? I don't know if this will cause problems; maybe ambiguity?

Base.writemime{M}(io::IO, ::Type{MIME{M}}, x) = Base.show(io, MIME{M}, x)

[stevengj]

In 0.5, you are supposed to define show(io::IO, ::TestCustomShowType), not show(io::IO, ::MIME"text/plain", ::TestCustomShowType), for the text/plain type , so @compat should support the two-argument form too.

[stevengj]

@TotalVerb, Base no longer calls writemime at all, so adding another writemime method won't help.

[TotalVerb]

@stevengj I meant that it may be possible for that code to be included in Compat for older versions. Then one can just define a method of show and the fallback writemime will call it.

There don't seem to be any three-argument shows in Julia v0.3 (haven't checked 0.4), so packages that extend Base.show with three-argument methods should be able to do so with no problems. Then when code calls writemime, and no writemime is defined, it can fall back to show, for which three-argument methods have been defined.

[tkelman]

this should be version dependent otherwise it's calling a deprecated function on 0.5

[TotalVerb]

The below code snippet seems to work in some situations, but sometimes more code is needed for it to work cleanly. It might be better to just to go along with the proposal here, though @compat in front of Base.show extensions look ugly to me.

# Provide writemime ⇒ show fallback
if VERSION < v"0.5.0-dev+4356"
    Base.show(x::IO, y::AbstractString, z) = show(x, MIME(y), z)
    Base.writemime(x::IO, y, z) = show(x, y, z)
end

[dancasimiro]

@TotalVerb: I tried defining the writemime to show fallback that you suggested, but I couldn't get it working. The compiler never dispatched to my custom implementation. As a result, the string comparison in the new tests failed.

[TotalVerb]

@dancasimiro Do you have code that demonstrates the problem? I have a similar solution in place on Currencies.jl's master branch, and it works for my (very limited) use case.

[stevengj]

@TotalVerb, that will (a) break mimewritable (which is used e.g. by IJulia to detect which mime formats are available for a given type); (b) not work if there is an more specific writemime method (such as the one for AbstractVector) if you want to override it for an even more specific type (e.g. your own vector type); and (c) does not seem like it should work even for overriding the default writemime(io::IO, ::MIME"text/plain", x) = show(io, x) that is already in Base in 0.4, though if you define a 2-argument show for text/plain it will continue to work (unless there is already a more specific writemime, as for AbstractVector).

I really think we need a @compat show(io, ...) = .... rewriting macro to cover all of the bases here.

[TotalVerb]

Point taken. I think the solution in this PR is best then.

[dancasimiro]

The new tests that I wrote are passing on 0.3, 0.4, and 0.5. Is there anything else that I need to address before squashing and rebasing?

[TotalVerb]

I still would like a Base.show rewriting, but having something is good. 👍

[nalimilan]

Could you add a short explanation to README.md?

[timholy]

Thanks @dancasimiro!

[stevengj]

Note that we should also have a show(io::IO, m, x) = writemime(io, m, x) method on 0.4 for people that want to call the three-argument show, not to define new methods. You shouldn't be forced to call @compat for this.