Thanks --- I understand the difference now... I had expected the compiler
to handle it, on the basis of being able to use an implementation of an
interface type wherever the interface type is used, but it looks like it's
a mixture of static and dynamic typing and this is where they don't quite
join up. I'll try the solution you suggest.
On Thursday, June 11, 2020 at 6:03:59 PM UTC+1, burak serdar wrote:
>
> On Thu, Jun 11, 2020 at 10:50 AM John Sturdy <jcg....@gmail.com
> <javascript:>> wrote:
> >
> > I've defined an interface type, and a concrete type that I think
> implements that interface, but when I use it in a list, I get an error from
> the compiler that makes it look like it doesn't.
> >
> > Here's my interface type, in anything/anything.go:
> >
> > package anything
> >
> > import (
> > "fmt"
> > )
> >
> > type AnyThing interface {
> > Textual() string
> > Numeric(name string) int
> > }
> >
> > func OneCallee(thing AnyThing) {
> > fmt.Printf("textual result is %s and numeric result is
> %d\n", thing.Textual(), thing.Numeric("default"))
> > }
> >
> > func ListCallee(things []AnyThing) {
> > fmt.Printf("textual result is %s and numeric result is
> %d\n", things[0].Textual(), things[0].Numeric("default"))
> > }
> >
> >
> > and here's a concrete type that defines the methods required by the
> interface, in particularthing/particularthing.go:
> >
> > package particularthing
> >
> > type ParticularThing struct {
> > Name string
> > Number int
> > }
> >
> > func (n ParticularThing) Textual() string {
> > return n.Name
> > }
> >
> > func (n ParticularThing) Numeric(name string) int {
> > return n.Number
> > }
> >
> > and here's the caller:
> >
> > package main
> >
> > import (
> > "example.com/user/scratch/anything"
> > "example.com/user/scratch/particularthing"
> > )
> >
> > func main() {
> > onething := particularthing.ParticularThing{"this", 1}
> > anything.OneCallee(onething)
> > thinglist := make([]particularthing.ParticularThing, 1)
> > anything.ListCallee(thinglist)
> > }
> >
> > When I compile it, I get:
> >
> > ./caller.go:12:21: cannot use thinglist (type
> []particularthing.ParticularThing) as type []anything.AnyThing in argument
> to anything.ListCallee
> >
> > Could some explain what the problem with this is?
>
> An array of values implementing an interface cannot be used in place
> of an array of those interfaces. Go's type system simply compares the
> type required with the type provided, and []someInterface is not
> []someType. Note that the structure of the two objects passed to the
> function is quite different. The function expects a []someInterface
> where each element of the array is a pair containing the pointer to
> the value and the type, but you are sending []someType where each
> element is a value. So you have to convert:
>
> intfList:=make([]AnyThing,0)
> for _,x:=range thingList {
> intfList=append(intfList,x)
> }
> anything.ListCallee(intfList)
>
> You can argue that the compiler can do this conversion. Such
> potentially expensive operations are explicit in Go, which creates
> some clutter but overall not a bad thing.
>
>
>
> >
> > I've attached a tarball of the files to make experimental changes
> easier.
> >
> > --
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>
>
>
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