Different combinations of let[-in] expression in the F# interactive

let-in-interactive.fsx

let a = 3
a + 4
;;
(* =>
val a : int = 3
val it : int = 7
*)

a + 5
;;
(* => val it : int = 8 *)

let b = 3 in
b + 4
;;
(* =>
val b : int = 3
val it : int = 7
*)

b + 5
;;
(* => val it : int = 8 *)

let c = 3 in c + 4
;;
(* => val it : int = 7 *)

c + 5
;;
(* => error FS0039: The value or constructor 'c' is not defined *)

let d = 3 in d + 4
d + 5
;;
(* =>
warning FS0020: This expression should have type 'unit', but has type 'int'. Use 'ignore' to discard the result of the expression, or 'let' to bind the result to a name.
val it : int = 8
*)

let e = 3 in
e + 4
e + 5
;;
(* =>
val e : int = 3
val it : int = 8
*)