问F#值限制

EN

type tree<'a> = 
    | Nil
    | Node of (tree<'a> * 'a * tree<'a>)

let rec flatten = function
    | Nil -> []
    | Node ( Nil, b, Nil ) -> [b]
    | Node ( l, h, p ) -> List.concat [(flatten l);[h];(flatten p)]

error FS0030: Value restriction. The value 'it' has been inferred to have generic type
    val it : '_a list    
Either define 'it' as a simple data term, make it a function with explicit arguments or, if you do not intend for it to be generic, add a type annotation.