Enable contracts for functions?

[mlhaufe]

demands and ensures maybe, but what does rescue look like? invariant seems pointless.

Does anything apply to closures specifically?

Something like:

const squareContract = new FunctionContract<typeof square>({
  demands(x) { return typeof x == 'number' },
  ensures(result, [x]) { return result >= x } 
})

@Contracted(squareContract)
function square(x){ return x**2 }