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

Answer 1

+8 A:

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

JaredPar 2010-09-20 17:51:10

Thanks Jared, then the reason , is used for tuples is because defining literal tuples is more common than defining literal lists, right?

Joan Venge 2010-09-20 17:54:45

@Joan I don't know the history of this choice other than it's a hold over from OCaml.

JaredPar 2010-09-20 17:55:56

Thanks Jared. I got it now.

Joan Venge 2010-09-20 17:57:14

Note that in OCaml, calling `f(x,y)` means calling `f` with a single argument that is a tuple `(x,y)`. Parameter lists probably are more common that list literals, if one wants to posit the 'most common should use commas' rationale.

Brian 2010-09-20 18:07:44

Thanks Brian, can you please tell me what parameter lists are? Lists like [1;2;3]?

Joan Venge 2010-09-20 18:14:58

I think Brian meant by parameter lists the list of parameters (arguments) passed to a function. They are called list in almost all languages so you call a function `f` of two parameters using the syntax `f(x,y)`. However in OCaml and F# such a call is passing a tuple to a function of a single parameter. This makes the use of tuples more common. In short, tuples (used as parameters) are more common than list literals in F#/OCaml.

Muhammad Alkarouri 2010-09-20 18:58:34

Note that if F# had tuple syntax like Python where tuples have parens around them, you could have commas between list items.

Gabe 2010-09-20 19:05:40

@Gabe: It does; there is parens around them. I have always wondered about the point that you made.

Muhammad Alkarouri 2010-09-20 19:30:15

Answer 2

+6 A:

[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]

2010-09-20 17:51:18

Thanks, why is there perf penalty for [1..9]? Isn't the compiler just unfolds it to [1;2;3...9]?

Joan Venge 2010-09-20 17:55:52

in the case of [1..9] there shouldn't be, but list comprehensions can get quite complicated i.e. [for i in 1 .. 9 -> i * i], I have tried to use nested for loops in list comprehensions and had some substantial problemsSequences on the other hand worked fine.

2010-09-20 17:57:08

Thanks, by "substantial problems" you mean perf problems? Also so [1..9] unfolds at compile time but [for i in 1 .. 9 -> i * i] is unfolded at runtime? Or are they all executed at runtime?

Joan Venge 2010-09-20 18:13:47

Oh wow, thanks. Can you really do [1 2 3] to get [1;2;3]? Then it's even better. Way better. Thanks again.

Joan Venge 2010-09-20 18:18:22

sorry about the formatting, that should get the point across, as above you can use new lines to delimit a list

2010-09-20 18:50:46

Answer 3

+8 A:

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.

Brian 2010-09-20 18:05:21

Thanks the newline trick is useful.

Joan Venge 2010-09-20 18:16:45

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

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