Nesting displayed when mapping collection in HAML - html

Question

I'm working on a Rails application, using HAML. I needed to present a list to the user, when I stumbled upon this weird behaviour:
This code generates no output, obviously.
- 5.times.map do |n|
- n + 1
This code generates 1 2 3 4 5, as it's outputting the return value for the inner statement.
- 5.times.map do |n|
= n + 1 # Notice the =
This code generates [1, 2, 3, 4, 5], as it's outputting the mapped array.
= 5.times.map do |n| # Notice the =
- n + 1
So far, so good...
This code generates 1 2 3 4 5 [0, 0, 0, 0, 0], as it doesn't like humans.
= 5.times.map do |n| # Notice the =
= n + 1 # on both lines
This code generates 1 2 3 4 5 [1, 1, 1, 1, 1], as it's nested in the <p>.
%p # Here we are nested one <p> deep
= 5.times.map do |n|
= n + 1
This code generates 1 2 3 4 5 [2, 2, 2, 2, 2], as it's double nested (and so on).
%p # Here we are nested two <p> deep
%p
= 5.times.map do |n|
= n + 1
Does anyone have an explanation for what is happening here in the HAML internals? Is = n+1 both adding a string to some output buffer, and then returning it's nesting level?

Answer 1

Actually, what you are asking is very version dependent. I mean it will produce different results in upcoming version of Haml (v4.1.0).
But first let's find out why this output in version < 4.1.0?
Under the hood, Haml translates templates into executable Ruby code. This job is done in Haml::Compiler class and you can easily debug this code:
require 'haml'
puts Haml::Engine.new(%q{
%p
%p
%p
= 5.times.map do |i|
= i
}).render # => produces the output
# you have in last example
to find out what the associated Ruby code looks like.
With some simplifications, it looks like this:
haml_temp = 5.times.map do |i|
_hamlout.push_text(" #{_hamlout.format_script_false_false_false_false_false_true_false(( i ));}\n", 0, false);
end
You can easily see by now, that your values are mapped to the expression:
_hamlout.push_text(...)
_hamlout here is an instance of Haml::Buffer and this is the last line, which returns value from the push_text method:
#real_tabs += tab_change
#real_tabs is indent level. If we use no indent, then it's 0, when one %p is involved, it becomes 1 and so on. tab_change argument is 0 (see debugged code).
So the output for Haml version 4.0.7 is equal to the level of nesting. This is exactly how your output looks like.
But this behavior will be probably "broken" in upcoming version 4.1.0. Compare the last line of the same method from current master branch:
#buffer << text
which will return some textual value.