如何在 Haml 中写 if 条件？

In haml, use the - (dash) to indicate a line is Ruby code. Furthermore, indention level indicates block level. Combine the two for if/else statements.

- if signed_in?
%li= link_to "Sign out", sign_out_path
- else
%li= link_to "Sign in", sign_in_path

is the same as the following code in ERB:

<% if signed_in? %>
<li><%= link_to "Sign out", sign_out_path %></li>
<% else %>
<li><%= link_to "Sign in", sign_in_path %></li>
<% end %>

HAML is indentation based , and the parser can be tricky.You don't need to use "- end" in Haml. Use indentation instead.In Haml,a block begins whenever the indentation is increased after a Ruby evaluation command. It ends when the indentation decreases.Sample if else block as follows.

- if condition
= something
- else
= something_else

A practical example

- if current_user
= link_to 'Logout', logout_path
- else
= link_to 'Login', login_path

Edit : If you just want to use if condition then

 - if current_user
= link_to 'Logout', logout_path

In haml two operators are used for ruby code.

• = is used for ruby code which is evaluated and gets inserted into document.

Example:

= form_for @user

• - is used for ruby code which is evaluated and NOT get inserted into document.

Example:

- if @user.signed_in?
= "Hi"
- else
= "Please sign in!"

If you want to put condition inside your tag

%section{:class => "#{'new-class' if controller.action_name == 'index'}"}

UPDATE

Here is another variation

%nav(class="navbar"){class: content_for?(:navbar_class) ? yield(:navbar_class) : nil}