簡單來說，expression是表達式，不是程序。 如 3 + 5, (let b = 3) in b + 5
而statement，可以理解為最短的程序。 如 a = 3 + 5 ( let a = 3 + 5)
https://stackoverflow.com/questions/4728073/what-is-the-difference-between-an-expression-and-a-statement-in-python#:~:text=An%20expression%20evaluates%20to%20a,A%20statement%20does%20something.&text=Statements%20represent%20an%20action%20or%20command%20e.g%20print%20statements%2C%20assignment%20statements.&text=Expression%20is%20a%20combination%20of,that%20yields%20a%20result%20value.