[ME-0005] Let Operator

[illusory0x0]

No description provided.

[bobzhang]

we thought about this and it is not hard to implement, but the thing is that fn is important in MoonBit, many control flows like return needs know explict function boundary, try to hide fn seems not ideal

[illusory0x0]

we thought about this and it is not hard to implement, but the thing is that fn is important in MoonBit, many control flows like return needs know explict function boundary, try to hide fn seems not ideal

This is a let operator so it shouldn't be treated as a function, and the let keyword doesn't handle return.

Although the let keyword can be decoded with lambda abstration and lambda application, it can't to use return.

for example

let foo : Int = {
  let y = 0 
  y + 1 // let can't use return
}

let bar : Int = (fn() {
  (fn(y){
    return y + 1 // here we can use return
  })(0)
})()