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Comment by hardwaresofton | Hacker News Reader

Type aliasing!

  type T1 = T2

I'm currently writing a pretty large project in Haskell and am thoroughly enjoying it -- as everyone and their mother says, the type system is superb, and even simple aliases like:

  type UserID = Int

Make it so much easier to read and understand functions.

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I'm not sure if type aliasing is for that use case. I think you're more thinking about things like

  type Celsius int

that in theory prevent you from passing a bare int and mixing up Celsius/Fahrenheit.

In go this almost works. If you define

  type Celsius int

  func isBoiling(t Celsius) bool {
      return t >= 100
  }

then You need to call it by doing something like

  t := Celsius(100)
  isBoiling(t)

and just

  t := 212
  isBoiling(t)

will not compile with

  cannot use t (type int) as type Celsius in argument to isBoiling

But unfortunately you can get away with

  isBoiling(212)

However, If you just used a type alias then you'd be able to do either.

Right. You could always define a new type with an existing underlying type, like Celsius with the underlying type int in your example. See the spec here: https://golang.org/ref/spec#Types

This would define a new, non-interchangeable type, even though it's just an int behind the scenes. The new type alias on the other hand define a new name for an existing type; you still have just one type. In the user ID example above that's not what you want, it would allow using an int where a UserID is required.

As I understand it, there have been a couple of compiler-blessed aliases for a long time (byte -> uint8 and rune -> int32), making this a similar thing but now at user level.

> But unfortunately ...

It is not due to some unkind turn of fortune that the Go type system drops the ball here and there.

> However, If you just used a type alias then you'd be able to do either.

The important distinction between a type and an alias is that you can define methods for a type and you can not do the same for an alias.

Thanks for the clarification! You're absolutely right -- Go's regular types would have actually helped with that.

I don't know that I would build the same kind of type machinery though (I might only use the alias, not the actual type), since a call like

  isBoiling(212)

would fall apart.

In particular, I think the type aliasing will make function signatures a lot clearer -- if I to write a `Celsius` type class, I would likely just make a small, hopefully simple `Temperature` type. Unfortunately last time I wrote any Go code, I wasn't anywhere near that careful/purposeful with the types (I mostly left things as `int`s, etc)

Go constants are untyped, which means you can pass a number as argument to a function that takes a type whose underlying type is an integer (e.g. time.Duration) without having to explicitly mention the type. That's how time.Sleep(5 * time.Minute) works

> But unfortunately ...

You can't get away with:

    isBoiling(Fahrenheight(212))

    cannot use 212 (type Fahrenheight) as type Celsius in argument to isBoiling

You can if

    func Fahrenheight(f int) Celsius {
      return Celsius(5*(f-32)/9)
    }

Well, sure, but I meant that in the context of:

    type (
        Celsius    float64
        Fahrenheit float64
    )

You should Newtype it though. Type aliases aren't substitute for semantics. They are for to simplify naming of complicated types.

In the Go world, they also serve as a way to gradually shift code around. Large code bases tend to have parts which are on transition from an "old" way to a "new" way, and type aliases allows you to maintain a backwards compatible stub type for a newly changed system.

This then avoids the patch of 40k lines changed as you can gradually go to different teams and get a checkmark that the changes you made to the teams code is still going to work as expected. This is usually better coordination-wise than a 40k line patch everyone has to sign off on.

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