markup.txt

{? trollLocation="cave" }
There is a vicious troll glaring at you.
{|}
The air smells bad here, like rotting meat.
{.}

{? canteenLocation="cave" }
There is a canteen on the ground.
{.}

SimpleParse.rb

class SafeTemplate < Parslet::Parser
  # ...
  rule(:cond) do
    test.as(:test) >> markup.as(:out)
    >> (elif.as(:test) >> markup.as(:out)).repeat.as(:elifs)
    >> (ells >> markup.as(:elseout)).maybe
    >> stop
  end
  # ...
end
  
class Transform < Parslet::Transform
  rule(test:simple(:test), out:simple(:out), elifs:subtree(:elifs)) do
    if test
      out
    elsif valid = elifs.find{ |h| h[:test] }
      valid[:out]
    end.to_s
  end
  
  rule(test:simple(:test), out:simple(:out), elifs:subtree(:elifs), elseout:simple(:elseout)) do
      if test
        out
      elsif valid = elifs.find{ |h| h[:test] }
        valid[:out]
      else
        elseout
      end.to_s
    end
  end
end