The new Map syntax in Haxe 3

I was using the Haxe3 Release Candidate for well over a month before I realised that the new Map data structures could be created with a nice syntax.  The basic gist is this:

var map1 = [ 1=>"one", 2=>"two", 3=>"three" ];

Like Arrays, but you use the “=>” operator to define both the key and the value.  And Haxe’s type inference is as strong as ever.  Here’s what the compiler picks up:

var map1 = [ 1=>"one", 2=>"two", 3=>"three" ];
$type (map1); // Map<Int, String>

var map2 = [ "one"=>1, "two"=>2, "three"=>3 ];
$type (map2); // Map<String, Int>

var map3 = [
    Date.now() => "Today",
    Date.now().delta(24*60*60*1000) => "Tomorrow",
    Date.now().delta(-24*60*60*1000) => "Yesterday"
];
$type (map3); // Map<Date, String>

var map4 = [
    { name: "Tony Stark" } => "Iron Man",
    { name: "Peter Parker" } => "Spider Man"
];
$type (map4); // Map<{ name : String }, String>

var map5 = [
    [1,2] => ["one","two"],
    [3,4] => ["three","four"],
];
$type (map5); // Map<Array<Int>, Array<String>>

So it’s pretty clever.  If for some reason you need to type explicitly to StringMap, IntMap, or ObjectMap, you can:

var stringMap:StringMap<Int> = [
    "One" => 1,
    "Two" => 2
];
$type(stringMap); // haxe.ds.StringMap<Int>

var intMap:IntMap<String> = [
    1 => "One",
    2 => "Two"
];
$type(intMap); // haxe.ds.IntMap<String>

var objectMap:ObjectMap<{ name:String }, String> = [
    { name: "Tony Stark" } => "Iron Man",
    { name: "Peter Parker" } => "Spider Man"
];
$type(objectMap); // haxe.ds.ObjectMap<{ name : String }, String>

But if you try to do anything too funky with types, the compiler will complain.  Haxe likes to keep things strictly typed:

var funkyMap = [
    { name: "Tony Stark" } => "Iron Man",
    { value: "Age" } => 25
]; // Error: { value : String } has no field name ... 
   // you are a bad person, and your items are not comprehensible 
   // to Haxe's typing system

Finally, Haxe won’t let you do define duplicate keys using this syntax:

var mapWithDuplicates = [
    1 => "One",
    2 => "Two",
    1 => "uno"
]; // Error: Duplicate Key ... previously defined (somewhere)

If you’re using an object map, it’s only a duplicate if you’re dealing with the exact same object, not a similar one.  For example, this is allowed:

var similarObjectKeys:ObjectMap<Array<Int>, String> = [
    [0] => "First Array object",
    [0] => "Second Array object"
]; // Works, you now have 2 items in your map.

But if you use the exact same object, Haxe will pick it up:

var key = [0];
var sameObjectKey = [
    key => "First Array object",
    key => "Second Array object"
]; // Error: Duplicate Key ...

So there you have it.  A nice feature that I didn’t see mentioned anywhere else.  Thanks Haxe team!

….

Update:

It’s worth mentioning that once you have created your map, you can use array access (“[” and “]”) to read or modify entries in your map.

var map = [ 1=>"one", 2=>"two", 3=>"three" ];

// Reading a value
map[1];   "one"
var i = 2;
map[i];   "two"
map[++i]; "three"

// Setting a value
map[4] = "four";
map[1] = "uno";
map[i] = "THREE";
map;   // [ 1=>"uno", 2=>"two", 3=>"THREE", 4=>"four" ]

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.