Concatenation strings in freemarker

Question

How can I concatenate strings in freemarker?

This doesn't work.

    <#function foo input>
<#local str="Hello ">
${str} = ${str} + ${" world"}

<#return str>
</#function>

${foo("a")}

Here is online evaluator:
http://freemarker-online.kenshoo.com/

Edit: To make it clear I need to use it with variables, to be able write something like this.

public String sayHello() {return "Hello";}
public String sayWorld() {return "world"};
public String sayPeople() {return "people";}

public void main() { 
 String str = "";
 str += sayHello();
 str += "";
 str += sayWorld();
 str += "";
 str += sayPeople();

 return str;
}

Answer 1

Like <#return "Hello " + input + "!">, or <#return "Hello ${input}!">. If you try to print to the output inside a #function (as opposed to inside a #macro), it will be ignored.

Edit: Analogously with the Java example added:

<#function concatDemo>
 <#local str = "">
 <#local str += sayHello()>
 <#local str += " ">
 <#local str += sayWorld()>
 <#local str += "!">
 <#return str>
</#function>

<#function sayHello><#return "Hello"></#function>
<#function sayWorld><#return "World"></#function>

${concatDemo()}