What's the difference between these two F# sequences?

Question

> seq { for i in 0..3 do yield float i };;
val it : seq<float> = seq [0.0; 1.0; 2.0; 3.0]
> seq [ for i in 0..3 do yield float i ];;
val it : seq<float> = [0.0; 1.0; 2.0; 3.0]

P.S. why did F# originally expect sequences without the "seq" prefix, but now they want the prefix?

Answer 1

More clarification on the two forms you posted:

The first one uses a direct sequence comprehension, "seq { ... }". The "seq" part was optional before, but that's not going to be supported in the future. I guess it makes things like "async { ... }" and the workflow syntax more difficult or something.

Now, the second one is a pure list:

> let x = [ for i in 0..3 do yield float i ];;

val x : float list

"seq" is also a function:

> seq;;
val it : (seq<'a> -> seq<'a>) = <fun:clo@0_1>

And since any list is also a sequence, doing "seq [1;2;3;]" is just applying the seq function to the list. It's sorta like casting it to seq from the list type.

> x;;
val it : float list = [0.0; 1.0; 2.0; 3.0]

> seq x;;
val it : seq<float> = [0.0; 1.0; 2.0; 3.0]

Edit: The source for the seq function is:

let seq (x : seq<_>) = (x :> seq<_>)

So, "sorta like casting it" should read "casting it via a helper function". As for the printing of sequences inside brackets, I think that's just a pretty printing artifact.

Answer 2

The first is a seq (IEnumerable).

The second is weird; get rid of the 'seq' and it's a list comprehension (list). There's no reason to write it as is; either use seq{ } or [ ] or [| |] to make a seq or a list or an array.

The curlies-not-prefixed-by-seq is a form that will be deprecated, as it makes some other parts of the language potentially ambiguous.