F#. Expecting a type supporting the operator '-' but given a function type

I'm new to F# and have some compilation problems in this code fragment:

let rec mywhile p f s =
    if p s then s
    else
       let s1 = f s
       mywhile p f s1

let eps = 0.001

let dichotomy f a b = 
    let out (a, b) = a
    let c a b = (a + b) / 2.
    out (mywhile (fun (a, b) -> a - b < eps)
        (fun (a, b) -> if f c * f a < 0 then (a, c) else (c, b))
        (a, b))

In particular, here: a - b < eps, then (a, c)

Expecting a type supporting the operator '-' but given a function type. You may be missing an argument to a function.