Specifying seq<int> as return type in F#

Question

I want to make a function that takes a string and returns a sequence of numbers. It should return a sequence containing only 0 if it passed a empty string. I tried doing the following:

let mapToInt (s: string) :seq<int> = 
    if s.Length = 0 then
        seq {0} 
    else
        s.Split ' ' 
        |> Seq.map int

This however gives the following error message:

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.

What's wrong with my code?

Answer 1

Your sequence expression needs to use yield to yield a value:

if s.Length = 0 then
    seq { yield 0 }