TL,
If you want to be a language lawyer then you must cite the law (The Go
Language Specification) to support your argument. For example, perhaps
something like this:
>From the law:
Method expressions
Consider a struct type T with two methods, Mv, whose receiver is of type T,
and Mp, whose receiver is of type *T.
type T struct {
a int
}
func (tv T) Mv(a int) int { return 0 } // value receiver
func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
var t T
For a method with a value receiver, one can derive a function with an
explicit pointer receiver, so
(*T).Mv
yields a function value representing Mv with signature
func(tv *T, a int) int
Such a function indirects through the receiver to create a value to pass as
the receiver to the underlying method; the method does not overwrite the
value whose address is passed in the function call.
In your case:
type Age int
func (age Age) CanDrink() bool {
age++
return age >= 18
}
(*Age).CanDrink
gives
func(age *Age) bool
>From the law:
Calls
Given an expression f of function type F,
f(a1, a2, … an)
calls f with arguments a1, a2, … an. Except for one special case, arguments
must be single-valued expressions assignable to the parameter types of F
and are evaluated before the function is called.
Assignability
A value x is assignable to a variable of type T ("x is assignable to T") in
any of these cases:
x's type is identical to T.
x's type V and T have identical underlying types and at least one of V
or T is not a named type.
T is an interface type and x implements T.
x is a bidirectional channel value, T is a channel type, x's type V and
T have identical element types, and at least one of V or T is not a named
type.
x is the predeclared identifier nil and T is a pointer, function,
slice, map, channel, or interface type.
x is an untyped constant representable by a value of type T.
In your case:
You call function
func(age *Age) bool
with
(*Age).CanDrink(age)
By assignment you have types
*Age = Age
which gives an assignability error
cannot use age (type Age) as type *Age in argument to (*Age).CanDrink
Peter
On Saturday, December 10, 2016 at 4:00:17 AM UTC-5, T L wrote:
>
>
>
> On Saturday, December 10, 2016 at 4:11:43 PM UTC+8, Axel Wagner wrote:
>>
>> On Sat, Dec 10, 2016 at 9:00 AM, T L <tapi...@gmail.com> wrote:
>>
>>>
>>>
>>> On Saturday, December 10, 2016 at 3:42:34 PM UTC+8, Axel Wagner wrote:
>>>>
>>>> I don't understand. You are saying, that you want a method on a pointer
>>>> to Age and then find it unreasonable, that you are getting a method on a
>>>> pointer to Age?
>>>>
>>>> If you don't want the argument to be a pointer, use Age.CanDrink
>>>> instead. Both are valid, because the method set of *Age contains all
>>>> methods declared on *Age or on Age (according to the spec
>>>> <https://golang.org/ref/spec#Method_sets>).
>>>>
>>>
>>> That is what it is weird, Age.CanDrink and (*Age).CanDrink should be the
>>> same function.
>>>
>>
>> Why? Age and *Age are different types. Why would they be interchangeable
>> in this case? When the programmer *explicitly* asked for different
>> things? That seems like confusing (and infuriating) behavior to me.
>> You need to come up with a reason why this should be the case, you can't
>> simply state that it should.
>>
>> I'm sorry, you often make controversial but somewhat valid arguments
>> about inconsistencies in go on this list, but this just isn't one of them.
>> This is just an unreasonable complaint.
>>
>
> May Go spec is some vague here.
>
> Maybe I should interpret it as following:
> when a method is defined for a non-interface type and non-pointer named
> type T,
> a method with the same name is also defined for type *T, implicitly.
> The only difference between the signatures of the two methods is they have
> different receiver parameter types,
> one receiver type is T, the other is type *T.
> The implicit method of *T has only one line of code which is a calling of
> the corresponding method of T.
>
> For this example:
>
> type Age int
> func (age Age) CanDrink() bool {
> age++
> return age >= 18
> }
>
> // the following method is defined implicitly
> /*
> func (age *Age) CanDrink() bool {
> return (*age).CanDrink()
> }
> */
>
>
>
>
>>
>>
>>
>>>
>>>
>>>>
>>>> On Sat, Dec 10, 2016 at 8:17 AM, T L <tapi...@gmail.com> wrote:
>>>>
>>>>>
>>>>> package main
>>>>>
>>>>> import "fmt"
>>>>> import "reflect"
>>>>>
>>>>> type Age int
>>>>> func (age Age) CanDrink() bool {
>>>>> age++
>>>>> return age >= 18
>>>>> }
>>>>>
>>>>> func main() {
>>>>> var age Age = 11
>>>>>
>>>>> Age.CanDrink(age)
>>>>> // (*Age).CanDrink(age) // cannot use age (type Age) as type *Age
>>>>> in argument to (*Age).CanDrink
>>>>> (*Age).CanDrink(&age)
>>>>> fmt.Println(age) // 11
>>>>>
>>>>> fmt.Println(reflect.TypeOf(Age.CanDrink)) // func(main.Age) bool
>>>>> fmt.Println(reflect.TypeOf((*Age).CanDrink)) // func(*main.Age)
>>>>> bool
>>>>> }
>>>>>
>>>>> Why is the parameter of (*Age).CanDrink is a pointer? It is not
>>>>> reasonable.
>>>>>
>>>>> --
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>>>>> Groups "golang-nuts" group.
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>>>>>
>>>>
>>>> --
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>>>
>>
>>
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