> 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?
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.
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.