Sum type experiment

[Sina Siadat]

Hi golang-dev :)

I was wondering what would be an idiomatic Go way

to implement a basic sum type in Go. After several

iterations I came up with 2 approaches I will share here.

(1) Group types in an interface

The first approach does not require any new tools

and is statically checked by Go1 compilers.  In

this approach, I group several types by having

them implement a common interface whose sole

purpose is to group those types. Here is an

example:

        type IPv4 [4]byte

        type IPv6 [16]byte

        

        type IP interface {

                GroupTypes(IPv4, IPv6)

        }

        

        func (IPv4) GroupTypes(IPv4, IPv6) {}

        func (IPv6) GroupTypes(IPv4, IPv6) {}

That's it. The GroupTypes method implementations

are no-op.  Their purpose is only to implement the

IP interface.

Now we can leverage Go's static type checking to

make sure we never assert an IP to anything other

than IPv4 and IPv6.  For example, this code will

error:

        var ip IP

        switch ip.(type) {

        case string: // string does not implement IP

        }

I wrote a simple generator and pushed in here:

http://github.com/siadat/implement-group-interface

(2) Type list

The second design is basically the same as the Sum

type (defined in dev.go2go). However, I wanted it

to work with Go1 code, so I decided to specify the

type list as a special comment (eg // #type T1, T2)

and forked dev.go2go to convert that commented type

list to an actual go2go type list.  The code is

available in

https://github.com/siadat/interface-type-check

Let me know what you think :)

sina

[Ian Davis]

On Tue, 11 Aug 2020, at 3:38 PM, Sina Siadat wrote:

Hi golang-dev :)

I was wondering what would be an idiomatic Go way

to implement a basic sum type in Go. After several

iterations I came up with 2 approaches I will share here.

You may be interested in the discussions on https://github.com/golang/go/issues/19412 if you haven't already seen them.

Ian

[Sina Siadat]

Thanks for the link, Ian. I will have a look at it :)