Why does F# warn me for ignoring a result

Question

I'm playing with F# and the compiler warns me if I don't use some result (same problem described here). Since F# even has the function "Ignore" for that, it seems that it's somewhat important, but I don't really understand why - why doesn't C# care about it, but F# does?

Answer 1

One fundamental difference between C# and F# is that in F# everything is an expression (as opposed to a mix of expressions and statements). This includes things that in C-style languages are statements, like control flow constructs.

When programming in a functional way, you want to have small pieces of referentially transparent code that you can compose together. The fact that everything is an expression plays right into that.

On the other hand, when you do something that gives you a value, and you just leave it there, you are going against that mindset. You are either doing it for some side-effect or you simply have a piece of left-over code somewhere. In either case it's fair game to warn you that you're doing something atypical.

F# discourages, but doesn't disallow side-effects, and lets you have (potentially side-effecting) expressions executed in a sequence, as long as the intermediate ones are of type unit. And this is what ignore does - takes an argument and returns unit.