If you define a While method of the builder-object, you can use while-loops in your computation expressions. The signature of the While method is:

member b.While (predicate:unit->bool, body:M<'a>) : M<'a>

For comparison, the signature of the For method is:

member b.For (items:seq<'a>, body:unit->M<'a>) : M<'a>

You should notice that, in the While-method, the body is a simple type, and not a function as in the For method.

You can embed some other statements, like let and function-calls inside your computation-expressions, but those can impossibly execute in a while-loop more than once.

builder {
    while foo() do
      printfn "step"
      yield bar()
}

Why is the while-loop not executed more than once, but merely repeated? Why the significant difference from for-loops? Better yet, is there some intended strategy for using while-loops in computation-expressions?