Lists in F#

Lists typically play an important role in functional programming and F# is no different. Lists are simply a typed collection of values. By "typed" I mean that the list can only contain entities of the list's type. Type the following code into the F# Interactive:

> let list = [1;2;3];;
val list : int list

The Interactive shows us that our list value has the type of int. Now lets try and create a string list and concatenate our two lists: using the @ operator:

> let list2 = ["a";"b";"c"];;
val list2 : string list

> let list3 = list @ list2;;
stdin(15,19): error: FS0001: Type mismatch

As you can see we get a type mismatch here. This is to be expected since lists are typed and that operation is trying to concatenate lists of two different types. Here's a successful concatenation with string lists:

> let list1 = ["a";"b";"c"] @ ["d";"e";"f"];;
val list1 : string list
>list1;;
val it : string list = ["a"; "b"; "c"; "d"; "e"; "f"]

Other operations on lists

    
            Example
            Description
        
            > [];;
            Empty List
        
            val it : 'a list = []
        
            > "a" :: "b" :: "c" :: [];;
            Adds an element to the head of a list. Similar to Lisp's "cons"
        
            val it : int list = ["a"; "b"; "c"] 
        
            [1..5];;
            Declares a range of elements in a list
        
            val it : int list = [1; 2; 3; 4; 5]

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Example	Description
> [];;	Empty List
val it : 'a list = []
> "a" :: "b" :: "c" :: [];;	Adds an element to the head of a list. Similar to Lisp's "cons"
val it : int list = ["a"; "b"; "c"]
[1..5];;	Declares a range of elements in a list
val it : int list = [1; 2; 3; 4; 5]