F#, assignment operator vs set operator

Question

I have started with F# and some code structure wonder me. For example:

I have next code:

let mutable s = 10
s <- 1 + s
printf \"%i\" s


        

Answer 1

The code

s=1+s

will does not modify s - the equivalent C# is

s==1+s

which just returns false

In fact, the compiler should have issued a warning on that line about a value being unused.

The = operator just does an equality check - it does not assign any values.

Answer 2

To answer your second question -

Why didn't I get a error? Is s = 1 + s just ignored? Why? I didn't get any error in the output.

The reason for this is that F# distinguishes between top-level scope in a script file and local scope. When you are in a script file (*.fsx file in Visual Studio or inside Try F#), then you probably want to write a sequence of expressions (that may return a value) and execute them interactively one by one. This means that when you write s = 1 + s on one line, this is not an error because you can evaluate the line (select it and hit Ctrl+Enter or Alt+Enter) and see the result in the interactive output.

However, when this appears inside a function (or in other local scope where you are not expected to evaluate it interactively), you get a warning. The easiest way to see is to create a local scope using do (and this works in Try F# too):

do
  let mutable s = 10
  s = 1 + s           // Warning: This expression should have type 'unit', 
                      //    but has type 'bool'. If assigning to a property 
                      //    use the syntax 'obj.Prop <- expr'.
  printf "%i" s