Haxe3 Features: Variable Substitution in Haxe3 (aka String Interpolation)

One of my favourite features in the upcoming Haxe3 is also one of the simplest: String Interpolation (also called variable substitution).  If you were using Std.format() in Haxe2, you’ll recognise it.  It lets you do this:

var name = "Jason";
var age = 25;
trace ('My name is $name, I am $age years old.');
// My name is Jason, I am 25 years old

The first point to make is that it is triggered by using single quotation marks (‘), rather than double (“).  If you use double quotes, everything is treated as a plain string:

trace ("My name is $name, I am $age years old.");
// My name is $name, I am $age years old

The next point to make is that this all happens at compile time.  So the trace we made above, in Javascript, would output as:

console.log("My name is " + name + ", I am " + age + " years old");

So you can only do this with strings you have at compile time, as you’re writing the code.  If you want Strings that are given at runtime to have variable interpolation, you should use a templating library like erazor.

Now a few other things to note:

1. If you want to put in a normal dollars sign, you can use two $, like this:
   

   trace ('The item costs $20');
   // "20" is not a valid variable name, so it is ignored.
   // Same as ("The " + item + " costs $20");
   
   trace ('That price is in $USD');
   // Error: Unknown Identifier "USD"
   trace ('That price is in $$USD');
   // Same as ("That price is in $" + "USD");
   
   var cost = 25;
   trace ('That item costs $$$cost');
   // The first two "$" are for the literal sign, the third is part of the variable.
   // Same as ("That item costs $" + cost);

2. If you want to access anything more than a straight variable, use curly brackets:

   // Property access
   trace ('My name is ${user.name} and I am ${user.age} years old.');
   // Same as: "My name is " + user.name + " and I am " + user.age + " years old.";
   // Outputs: My name is jason and I am 25 years old.
   
   // A simple haxe expression
   trace ('$x + $y = ${x + y}');
   // Same as: "" + x + " + " + y + " = " + (x + y);
   // Outputs: 1 + 2 = 3
   
   // A function call 
   trace ('The closest Int to Pi is ${Math.round(3.14159)}');
   // Same as: "The closest Int to Pi is " + Math.round(3.14159);
   // Outputs: The closest Int to Pi is 3

3. There is no HTML escaping, so be careful:

   var bold = "<b>Bold</b>";
   trace ('<i>Italic</i> $bold');
   // <i>Italic</i> <b>Bold</b>
   
   var safeBold = StringTools.htmlEscape("<b>Bold</b>");
   trace ('<i>Italic</i> $safeBold');
   // <i>Italic</i> &lt;b&gt;Bold&lt;/b&gt;
   
   var safeEverything = StringTools.htmlEscape('<i>Italic</i> $bold');
   trace (safeEverything);
   // &lt;i&gt;Italic&lt;/i&gt; &lt;b&gt;Bold&lt;/b&gt;

There you have it: String Interpolation, one of the most helpful (and easy to comprehend) features of the upcoming Haxe3 release.

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