you are correct when you say that the value of three in

let three = (1+3; printfn "OK"; printfn "Thank You");;

will ultimately be unit.

the warning that you get may be related to the following write-up in

[link:research.microsoft.com]

the relevant passage is this:

...

Sequence expressions. e1; e2 is a sequence expression and evaluates e1 and returns the result of evaluating e2 and has the same type as e2. A warning may be reported if e1 does not have type unit.

...

when you typed

let two = (printfn "OK"; printfn "Thank You");;

and fsi displayed the following in the console:

val two : unit

OK

Thank You

fsi is telling you that whatever the contents of the strings that are printed out, the value assigned to two (the identifier to the left hand side of the = sign) will not be any of the strings ("OK" or "Thank You") printed out; rather, it will always get assigned the value of the last expression evaluated; since the last expression evaluated in your example is a printfn expression, and a printfn expression always yields as a result the special value called unit, the identifier two will also get assigned this special value called unit.

when you then typed:

two;;

and fsi displayed the line below

val it : unit = ()

fsi is telling you -- really just confirming for you -- that the value of two is indeed the special value called unit [which is also sometimes represented as the () symbol, thus, unit = () and two = () also].

does this help?

Hi Joel,

Sorry. I should have been clearer. Type two;; in a line by itself.

> let two = (printfn "Hello World";1+1);;

val two : int

Hello World

> two;;

val it : int = 2

One of the primitive constructs of F# is to write sequential code on a single line.

e.g.,
let two = (printfn "Hello World"; 1+1);;

val two : int

Hello World

But I can't put my fingers on what exactly means by the following words - "The first expression is evaluated (usually for its side effects), its result is discarded, and the overall expression evaluates to the result of the second." (from Don's Expert F#)

Is there anyone who would like to get the meanings across?

There are two sub-expressions on the right-side of the let statement separated by a semi-colon and surrounded by parentheses. These two sub-expressions make up one expression, whose result is bound to the name, two. Here are the two sub-expressions:

1

printfn "Hello World"

and

1

1 + 1

The first expression returns unit. That's unit, not uint. If you're coming from C#, it's often convenient to think of unit like void. However, that's a bit misleading because unit is a value (unlike void). So, this is completely legal:

1

let p = printfn "Hello World"

When executed in F# interactive, the above line of code outputs the following:

val p : unit

Hello World

Expressions returning unit generally perform some sort side effect. In this case, the side effect is printing to the console.

Putting those pieces of information together should clarify the statement from Expert F#:

"The first expression is evaluated (usually for its side effects), its result is discarded"

The first expression, printfn "Hello World", is evaluated. It performs the side effect of writing to the screen. It's unit result is discarded.

"and the overall expression evaluates to the result of the second."

The second expression, 1 + 1, is evaluated and its result (the int, 2) is used as the result of the entire expression enclosed in parentheses.

I hope that helps.

in your example, printfn "Hello World" is the first expression, and 1+1 is the second expression. if you had followed the let with

two;;

fsi would have indicated that the identifier two has been assigned a value of 2

val it: int = 2