What's the reason of using ; in F# lists instead of ,?

Question

This might be a strange question but if I want to define a list of integers from:

1, 2, 3, 4, 5, 6, 7, 8, 9

Do I need to do it using the ; character?

[ 1; 2; 3; 4; 5; 6; 7; 8; 9 ]

instead of?:

[ 1, 2, 3, 4, 5, 6, 7, 8, 9 ]

It just seems to me , is more natural and easy on the eyes. Just wondering the idea behind using ;? (Not criticizing)

Answer 1

[1,2,3,4,5] is a list of 1 element of type int * int * int * int * int

[1;2;3;4;5] is a list of 5 elements of type int

also, list comprehensions and ranges are your friends

let bar = [1..9], 1..9 is a range so it get's unfolded into 1;2;3;...9;

let blort = [for i in 1..9 -> i] is a comprehension that does the same thing -- a little more power for some performance issues.

Edit: for completeness sake, you can also do

let foo = [1
           2
           3]

and get a list of [1;2;3]

Answer 2

Other answers have pointed out the main reason.

As an aside it is worth noting that, like most other places that semicolon is used in the language, an alternative is a newline. For example

[ "foo"; "bar"; "baz" ]

can also be written as

[ "foo"
  "bar"
  "baz" ]

or a variety of other layouts where semicolons are replaced by newlines.

Answer 3

Yes you must. The reason why is that in F# the comma operator is used for tuple expressions. So the expression 1,2,...9 is interpreted as a single tuple with 9 values.

[1,2] // List with 1 element that is a tuple value - (int*int) list
[1;2] // List with 2 elements that are integer values - int list

Answer 4

Just wondering the idea behind using ;?

Tuples are more common in ML and using ; lets you write lists of tuples as:

[1, 2; 3, 4]

Historically, F# inherited this syntax from OCaml which created its syntax by trying to remove more superfluous parentheses from Standard ML syntax.