预期的布尔但有不同的类型

Question

在以下方法中，我在if表达式中尝试调用isSample时收到以下错误：error FS0001: This expression was expected to have type bool but here has type Outcome list * ProbTree -> bool。我不知道为什么收到这条消息。

isSample（os，t）目前属于'a * 'b -> Outcome list * ProbTree -> bool类型。有任何想法吗？我不明白为什么它突然期望bool类型。

let rec isSample(os, t) = function
| ([], Branch(_, p, b1, b2))                         -> false
| (os, Branch(_, p, b1, b2)) when List.length os = 1 -> true 
| (h::t, Branch(_, p, b1, b2)) ->
    if h = S then isSample(t, b1) else isSample(t, b2) //This is where I get the error
| _ -> false


type Outcome  = | S | F 
type Sample   = Outcome list
type ProbTree = | Branch of string * float * ProbTree * ProbTree
                | Leaf of string