Do you have to declare implicit type?

In this instance it's not necessary to declare that it's a String as Swift will infer the correct type because you're initialising the variable str with a string literal ("Hello").

You can use literals to get Swift to infer the type without having to declare it.

So we can do the following;

// Use string literals to get Swift to infer as String
var str = "hello"

// Use boolean literals to get Swift to infer as Boolean
var goodWeather = false

// Use floating-point literals to get Swift to infer as Double
var pi = 3.14159265359

// etc

A quick thing to note on using floating-point literals;

Swift always chooses Double (rather than Float) when inferring the type of floating-point numbers.

So if you specifically need a float you'll have to declare the type.

If you're not setting any values with literals, then you will need to define the type. Otherwise you'll get the following error;

"Type annotation missing in pattern"

As for best practices, Apple tend not to explicitly declare types when literals are used. A good read is the Swift language guide from Apple.

You should specifically check out the Type Safety and Type Inference section within The Basics chapter

Hope this helps a bit more.