[jw-]

Really interesting stuff! I tried out some higher-order examples:

INPUT >>> function add(f, b) { return f(b); };

let a = add(function(x) {return x + 1}, 3);

OUTPUT >>> function add(f: function, b: number): number { return f(b); };

let a: number = add(function(x: number): number {return x + 1}, 3);

and I noticed that the flow type annotations are added to the call site, rather than on the argument f in the definition of add. Are all the types that you generate first order?