var vs let in Swift

Question

I'm new to Swift programming, and I've met the var and let types. I know that let is a constant and I know what that means, but I never used a constant mainly because I didn't need to. So why should I use var instead of let, at what situation should I use it?

Answer 1

Rather than constant and variable, the correct terminology in swift is immutable and mutable.

You use let when you know that once you assign a value to a variable, it doesn't change - i.e. it is immutable. If you declare the id of a table view cell, most likely it won't change during its lifetime, so by declaring it as immutable there's no risk that you can mistakenly change it - the compiler will inform you about that.

Typical use cases:

• A constant (the timeout of a timer, or the width of a fixed sized label, the max number of login attempts, etc.). In this scenario the constant is a replacement for the literal value spread over the code (think of #define)
• the return value of a function used as input for another function
• the intermediate result of an expression, to be used as input for another expression
• a container for an unwrapped value in optional binding
• the data returned by a REST API call, deserialized from JSON into a struct, which must be stored in a database

and a lot more. Every time I write var, I ask myself: can this variable change?. If the answer is no, I replace var with let. Sometimes I also use a more protective approach: I declare everything as immutable, then the compiler will let me know when I try to modify one of them, and for each case I can proceed accordingly.

Some considerations:

For reference types (classes), immutable means that once you assign an instance to the immutable variable, you cannot assign another instance to the same variable.

For value types (numbers, strings, arrays, dictionaries, structs, enums) immutable means that that once you assign a value, you cannot change the value itself. For simple data types (Int, Float, String) it means you cannot assign another value of the same type. For composite data types (structs, arrays, dictionaries) it means you cannot assign a new value (such as a new instance of a struct) and you cannot change any of their stored properties.

Also an immutable variable has a semantic meaning for the developer and whoever reading the code - it clearly states that the variable won't change.

Last, but maybe less important from a pure development point of view, immutables can be subject to optimizations by the compiler.

Answer 2

Generally speaking, mutable state is to avoid as much as possible.

Immutable values help in reasoning about code, because you can easily track them down and clearly identify the value from the start to the end.

Mutable variables, on the other hand, make difficult to follow your data flow, since anyone can modify them at any time. Especially when dealing with concurrent applications, reasoning about mutable state can quickly become an incredibly hard task.

So, as a design principle, try to use let whenever possible and if you need to modify an object, simply produce a new instance.

Whenever you need to use a var, perhaps because using it makes the code clearer, try to limit their scope as much as possible and not to expose any mutable state. As an example, if you declare a var inside a function, it's safe to do so as long as you don't expose that mutability to the caller, i.e. from the caller point of view, it must not matter whether you used a var or a val in the implementation.

Answer 3

In general, if you know a variable's value is not going to change, declare it as a constant. Immutable variables will make your code more readable as you know for sure a particular variable is never being changed. This might also be better for the compiler as it can take advantage of the fact that a variable is constant and perform some optimisations.

This doesn't only apply to Swift. Even in C when the value of a variable is not be changed after being initialised, it's good practise to make sure it's const.

So, the way you think about "I didn't need to" should change. You don't need a constant only for values like TIMEOUT etc. You should have a constant variable anywhere you know the value of a variable doesn't need to be changed after initialisation.

Note: This is more of a general "programming as a whole" answer and not specific to Swift. @Antonio's answer has more of a focus on Swift.