A variable is a section of the computer's memory used to temporarily hold a value. Declaring a variable means that the variable has been reserved a portion of memory.
The let keyword is used for variable declaration. The type of a value is inferred from the definition. For a primitive type, such as an integral or floating-point number, the type is determined from the literal type.
let a = 17
let b = 10.5
let str = "text"
Console.WriteLine(a)
Console.WriteLine(b)
Console.WriteLine(str)
In the above example, the compiler infers the type of a to be an int, the type of b to be a double, whereas the compiler infers the type of str to be a string.
By default, Variables in F# are immutable, which means once a variable is bound to a value, it can't be changed.
They are compiled as static read-only properties. Let's consider the following example.
let a = 17
let b = 10.5
let str = "text"
Console.WriteLine("a: {0}", a)
Console.WriteLine("b: {0}", b)
Console.WriteLine("str: {0}", str)
b <- 12.5
Console.WriteLine("b: {0}", b)
You will now see the following error.
Error FS0027: This value is not mutable. Consider using the mutable keyword, e.g. 'let mutable b = expression'.
A variable definition tells the compiler where and how much storage for the variable should be created. A variable definition may specify a data type and contains a list of one or more variables of that type, as shown in the following example.
let a:int32 = 17
let b:double = 10.5
let str:string = "text"
Console.WriteLine("a: {0}", a)
Console.WriteLine("b: {0}", b)
Console.WriteLine("str: {0}", str)
The variable can be changed if you provide the mutable keyword for that variable.
You can assign an initial value to a mutable variable, by using the let keyword in the same way as you would define a value. However, the difference is that you can subsequently assign new values to mutable variables by using the <- operator, as in the following example.
let a = 17
let mutable b = 10.5
let str = "text"
Console.WriteLine("a: {0}", a)
Console.WriteLine("b: {0}", b)
Console.WriteLine("str: {0}", str)
b <- 12.5
Console.WriteLine("b: {0}", b)
When you execute the above code you will see the following output.
a: 17
b: 10.5
str: text
b: 12.5