test.rb

require 'psych'

list = ('a'..'z').to_a

# Create a YAML AST
viz = Psych::Visitors::YAMLTree.new {}
viz << list
tree = viz.tree

# Change the Sequence node to be FLOW style
tree.children.first.children.first.style = Psych::Nodes::Sequence::FLOW

# Output as YAML
puts tree.to_yaml


# Or just use JSON style (it defaults to flow)
puts Psych.to_json list