strict type assignability to prevent arbitrary values

[bsr]

Hello,

I am a long time user of go, but I always had the impression that below code would not work as string and Status are different type.

I thought I need to explicitly convert as ```exec(Status("abc"))``` it to work.

I think, this part of the spec may be the reason https://golang.org/ref/spec#Assignability

• x is an untyped constant representable by a value of type T.

Is there a way I can prevent this behavior.

I am using Status like an enum, and only predefined status values should be allowed.

https://play.golang.org/p/4zsb7KtPBC6

package main

import (
    "fmt"
)

type Status string

func main() {
    exec("abc")
}

func exec(s Status) {
    fmt.Printf("Hello, %s", s)
}

[Jan Mercl]

On Thu, Nov 7, 2019 at 11:58 AM bsr <bsr...@gmail.com> wrote:

> Is there a way I can prevent this behavior.

Yes, if the rules for untyped literals are not wanted, make the
literal typed: https://play.golang.org/p/-f75y0Gb1Is

[speter]

Hi bsr,

I'd suggest to use a struct type with a single string field. It will prevent conversion from untyped string constant "by mistake".

Moreover, if you make the string field unexported, you can limit new instance creation to the declaring package, allowing to enforce predefined values.

Unlike with some other languages, there is no memory or runtime overhead due to encapsulating the string within a struct.

HTH,

Peter

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[bsr]

Thanks Jan & Peter. I would use one of the approaches.

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[Mohamed Yousif]

Hi Speter,

Can you elaborate more on this point please?

> there is no memory or runtime overhead due to encapsulating the string within a struct.

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[Jake Montgomery]

The inability to have a type safe enum in Go has bothered me as well.

While using a struct, as Peter suggests, does prevent accidental use of literals, it also prevents you from making your enum items constant.This means that the values can be changed, accidentally, or intentionally, in another package. For example:

package enum

import "fmt"

type Status struct {
    s string
}

var One = Status{"one"}
var Two = Status{"two"}

func PrintIt(stat Status) {
    fmt.Println("Status is", stat.s)
}

But a client of your package can then do:

func Foo() {

    enum.One = enum.Two
    enum.PrintIt(enum.One)

}




Which will print "two", and perminantly alter the meaning of enum.One.



On Thursday, November 7, 2019 at 8:15:20 AM UTC-5, speter wrote:

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[Michael Jones]

...job for a getter function.

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[speter]

Hi Mohamed,

The memory layout of "string" is the same as that of "struct { string }", so there is no extra allocation or indirection involved on creation and access, hence no runtime overhead.

Peter

[Wojciech S. Czarnecki]

On Thu, 7 Nov 2019 02:58:17 -0800 (PST)
bsr <bsr...@gmail.com> wrote:

> Is there a way I can prevent this behavior.
> I am using Status like an enum, and only predefined status values should be allowed.

You cannot statically prevent someone from willfully assigning
to a variable of accessible type (eg. a function call parameter).
If you're just after sanity check there are many ways.

At runtime you can eg. use a map: https://play.golang.org/p/5rwJLFpU3a4

For compile time - you can use stringer tool to generate numerical constants mapped to strings.

hope this helps,

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