Difference between function's no return type and Promise(Never, Void)

[jiggneshhgohel]

Hello,

In the following example shown at https://www.mint-lang.com/

component Counter {
  state counter = 0

  fun increment {
    next { counter = counter + 1 }
  }

  fun decrement {
    next { counter = counter - 1 }
  }

  fun render {
    <div>
      <button onClick={decrement}>
        "Decrement"
      </button>

      <span>
        <{ Number.toString(counter) }>
      </span>

      <button onClick={increment}>
        "Increment"
      </button>
    </div>
  }
}

the functions to be invoked on onClick event i.e. decrement and increment have function type Function() (that's based on my understanding of the documentation under section Type of a function on page https://www.mint-lang.com/guide/reference/functions) however looking at the functions defined in mint-realworld Forms.SignIn I can see the functions defined to have return type Promise(Never, Void).

Based on the documentation at https://www.mint-lang.com/guide/reference/built-in-types Promise(Never, String) is a task that can never fail. So I guess replacing String with Void in Promise(Never, String) to Promise(Never, Void) should imply a Promise that never fails i.e. no error as per Mint's Promise(error, result) first parameter and 2nd parameter result is void implying no value. So I was trying to understand why that return type Promise(Never, Void) is specified for those functions for e.g. see below

  fun setPassword (value : String) : Promise(Never, Void) {
     next { password = value }
  }
      
 fun setEmail (value : String) : Promise(Never, Void) {
    next { email = value }
 }

These functions just sets the defined state using the next expression. I don't think anything can go wrong with that assignment and the body of the above functions and the the ones for increment and decrement functions shown before are similar in implementation then why in latter's case no return type is specified however in former's case return type is specified and that too explicitly Promise(Never, Void)?

Note: I am new to functional programming paradigm so please bear with me if I have asked any silly question.

Thanks.

[gdotdesign]

Hi 👋

I believe these two points can help you better understand what is going on:

• The return type definition can be omitted and in that case it's inferred from the body of the function (since a function can only have one expression)
• The return type of a next { } expression is Promise(Never, Void) because it mutates the state asynchronously.

the functions to be invoked on onClick event i.e. decrement and increment have function type Function() (that's based on my understanding of the documentation under section Type of a function on page mint-lang.com/guide/reference/functions) however looking at the functions defined in mint-realworld Forms.SignIn I can see the functions defined to have return type Promise(Never, Void).

A type of a function must have at least one type parameter which is the return type so it cannot be Function() instead it is Function(Promise(Never, Void) because the return type is inferred from the body of the function.

Based on the documentation at mint-lang.com/guide/reference/built-in-types Promise(Never, String) is a task that can never fail. So I guess replacing String with Void in Promise(Never, String) to Promise(Never, Void) should imply a Promise that never fails i.e. no error as per Mint's Promise(error, result) first parameter and 2nd parameter result is void implying no value.

Yes, that is correct.

Note: I am new to functional programming paradigm so please bear with me if I have asked any silly question.

There are no silly questions 🙂 let me know if I can help with anything else 👍

[jiggneshhgohel]

@gdotdesign Thanks for the elaborate answer. It definitely helped in clearing the confusions I had. So my final understanding is that in case of functions defined in Form.SignIn store (I referenced in my original comment) the return type could be omitted but for clarity for the reader the return type is specified. Is that correct?

BTW about following

The return type definition can be omitted and in that case it's inferred from the body of the function (since a function can only have one expression)

in the documentation at https://www.mint-lang.com/guide/reference/functions I can only see

the return type annotation is optional

but it gets inferred if not specified that is not documented. I think updating the documentation about it should help in clarifying things to the reader reading it.

Thanks.

[gdotdesign]

@gdotdesign Thanks for the elaborate answer. It definitely helped in clearing the confusions I had. So my final understanding is that in case of functions defined in Form.SignIn store (I referenced in my original comment) the return type could be omitted but for clarity for the reader the return type is specified. Is that correct?

Yes, it's mostly the clarity of the reader. There is a check for the return type to match the body type so it can be used for that as well.

I think updating the documentation about it should help in clarifying things to the reader reading it.

Will do 🙏 👍