Interactive Session #1: variables

The best way to start with F# is to launch the interactive session and enter some of the idioms to get a feeling how F# looks like.

~> fsi

Microsoft (R) F# 2.0 Interactive build 2.0.0.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

For help type #help;;

>

To assign values to variables you need the ‘let’ keyword; variables in F# are similar to variables in mathematics, you may assign any value you like (therefore the term ‘variable’), but once assigned you can no longer change its value.

> let a = 0;;

val a : int = 0

However, you may be surprised when trying out the following in your interactive session:

> let b = 1;;

val b : int = 1

> let b = 2;;

val b : int = 2

> b;; (* asking the interactive for the value of b *)
val it : int = 2
>

It looks like you actually can change the value of variables, however, this is not true, every new line in the interactive session creates a new scope. Inside the new scope you may define again a variable with the same name as already defined in the previous scope.

Now, enter two definitions of the same variable name by entereing the second line using Shift-Return instead of Return.

> let c = 1  
- let c = 2;;

 let c = 2;;
 ----^

~/stdin(19,5): error FS0037: Duplicate definition of value 'c'
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