Add support for preserve_single_line_if::Bool

[alexandergunnarson]

Before formatting (or with preserve_single_line_if = true):

function f()
    if rand() < 0.5 return 2 end
    0
end

After formatting with preserve_single_line_if = false (default):

function f()
    if rand() < 0.5
        return 2
    end
    0
end

Resolves #516.

[domluna]

please add some test cases

looks good otherwise.

[penelopeysm]

This patch no longer works on the current version:

julia> using JuliaFormatter: format_text

julia> s = "if rand() < 0.5 return 2 end"
"if rand() < 0.5 return 2 end"

julia> format_text(s; preserve_single_line_if=true) |> println
if rand() < 0.5
    return 2
end

@alexandergunnarson Are you still interested in working on this? No worries if not as I can pick it up (at some point in time....).

[alexandergunnarson]

@domluna Sorry — added test cases in the most recent commit.

@penelopeysm — Thanks for alerting me! Patch now works with latest version.

[penelopeysm]

Thanks 😊 This looks generally great. I have one question about what the intended behaviour should be when overflowing the max width. Currently we have this:

julia> using JuliaFormatter: format_text

julia> format_text("if x > 0 return 1 end"; preserve_single_line_if=true, margin=7) |> println
if x >
   0 return 1 end

julia> format_text("if x > 0 return 1 end"; preserve_single_line_if=true, margin=8) |> println
if x > 0 return 1 end

The issue is that the x > 0 gets nested if it goes over the margin. While the result parses fine, I think it's a bit unprincipled for the if-condition to determine whether there's a line break. I think there are two possible options for preserve_single_line_if = true:

1. If the entire if ... end exceeds the margin, then we split it up.

2. Never split it up.

I would lean towards (2) Actually, I'm kind of undecided :/ -- what do you think?

[penelopeysm]

(If you agree with (2), then I believe this is the appropriate fix:)

Suggested change

	add_node!(t, pretty(style, c, s, ctx, lineage), s; join_lines = true)
	ctx2 = preserve_single_line_if ? newctx(ctx; nonest=true) : ctx
	add_node!(t, pretty(style, c, s, ctx2, lineage), s; join_lines = true)

[penelopeysm]

(1) looks trickier to accomplish to me. I haven't attempted to figure out how that would be done.