Common mistake

mistake3.fs

let plusOne (s : seq<_>) =
    let e = s.GetEnumerator()
    seq {
        while e.MoveNext() do
            yield e.Current + 1
    }

let r = plusOne [1; 2; 3]
printfn "%A" r // prints [2; 3; 4]
printfn "%A" r // prints []

// The variable e is immutable but contains mutable state. 
// The above function is mutating captured state 
// while the below function is mutating state that wasn't captured.

let plusOneCorrect (s : seq<_>) =
    seq {
        let e = s.GetEnumerator()
        while e.MoveNext() do
            yield e.Current + 1
    }

let r = plusOneCorrect [1; 2; 3]
printfn "%A" r // prints [2; 3; 4]
printfn "%A" r // prints [2; 3; 4]