Can I pass the method as parameter to another function ?

[davy zhang]

here is the code

http://play.golang.org/p/Ol5smHpBLS

prog.go:50: method t.Year is not an expression, must be called

[David DENG]

1. If you want to reference that method as a function, use time.Time.Year
   
2. Its type is func(time.Time) int, not func() int

David

[Kevin Gillette]

The language may eventually allow curried method expressions, but for right now, they're not legal. You may pass a normal method expression, such as shown at: http://play.golang.org/p/URL115raiJ

In this case, the method expression yields a func signature with the receiver type as the explicit first argument.

[davy zhang]

Yep, you'r right, I updated my code as you said, but still has compiler complains, thanks

http://play.golang.org/p/dJbJAtLboA

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[Rémy Oudompheng]

On 2013/3/5 davy zhang <davy...@gmail.com>:

> Yep, you'r right, I updated my code as you said, but still has compiler
> complains, thanks
>
> http://play.golang.org/p/dJbJAtLboA

What do you not understand in the compiler message? If it is not
clear, it should be improved.

Rémy.

[davy zhang]

Thanks a lot for the correction,

but if the method delared as 

(t *time) func Year(){

...

}

this workaround won't  compile, am I right?

[David DENG]

func (t *time) Year() {

}

the receiver follows keyword func.

David

[David DENG]

I have no idea of what your design is for, but another alternative way to make your program compiled is to use closure:

http://play.golang.org/p/2izY1BtZiD

David

[davy zhang]

method t.Year is not an expression, must be called

the error is the same as the first version, but Deng did help me out about the type definition.

thanks for that

[David DENG]

Some suggestion like `Use Time.Year instead` may help.

David

On Tuesday, March 5, 2013 4:49:43 PM UTC+8, Rémy Oudompheng wrote:

[davy zhang]

Thanks, this is another way to do it, and can be used as common idiom

[Zizon Qiu]

you may wrap it in a clause.

like: fc.update( func() int(return t.Year()},...)

sounds like function is not truly first class citizen here.

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[Rémy Oudompheng]

On 2013/3/5 Zizon Qiu <zzd...@gmail.com> wrote:
> you may wrap it in a clause.
> like: fc.update( func() int(return t.Year()},...)
>
> sounds like function is not truly first class citizen here.

I don't understand what makes you say that. The issue is that t.Year
is not a function.

Rémy.

[davy zhang]

Yes, the issue is "t.Year is not a function", but the question is how can I pass t.Year into another function as parameter, so the follow up post solved this problem include zizon Qi

1. Wrap it as a closure 

2. Use time.Year as parameter not the instance method

[Zizon Qiu]

what if t.Year is an `attribute`.

like in javascript or something equivalent.

it is legal to use t.Year as an expression whatever it is.

[Jan Mercl]

On Tue, Mar 5, 2013 at 10:39 AM, Zizon Qiu <zzd...@gmail.com> wrote:
> what if t.Year is an `attribute`.
> like in javascript or something equivalent.
>
> it is legal to use t.Year as an expression whatever it is.

http://golang.org/ref/spec#Method_expressions

-j

[Nate Finch]

I hit this too, when I was starting, and I think the error message is unclear. The compiler knows the parameter is a function, and it looks like we're trying to pass a method into that function. It should say something like "Methods cannot be passed as functions, specify the type's function explicitly" (or something along those lines). It's a lot less likely this bug is due to you forgetting the parentheses for a method that returns a function... and even if that was the bug, it would be obvious when you look at that line. However, the method/function thing is much less obvious to anyone who either hasn't dealt with functions as objects, or to someone coming from a language where you can pass methods this way.

[Jan Mercl]

On Tue, Mar 5, 2013 at 1:57 PM, Nate Finch <nate....@gmail.com> wrote:
> I hit this too, when I was starting, and I think the error message is
> unclear. The compiler knows the parameter is a function, and it looks like
> we're trying to pass a method into that function. It should say something
> like "Methods cannot be passed as functions, specify the type's function
> explicitly" (or something along those lines).

if U cannot be assigned to T then the correct message is "Cannot
assign U to T". There is an unbound number of such U's and T's so
treating one particular pair specially is opening a can of worms.

-j

[Nate Finch]

On Tuesday, March 5, 2013 8:19:01 AM UTC-5, Jan Mercl wrote:

if U cannot be assigned to T then the correct message is "Cannot 
assign U to T". There is an unbound number of such U's and T's so 
treating one particular pair specially is opening a can of worms. 

 

Yes, that's true, and I'd be happy if it said something like that.  For example, if it said:

"cannot use t.Time (Method) as type func() in function argument" 

But it doesn't. It says:

"method t.Year is not an expression, must be called"

There's two possible reasons for someone writing t.Year: 

1. It's a simple syntax error (forgetting the parentheses) which is relatively unlikely and pretty obvious when looking at the code
2. They're trying to pass the method as a function (a not uncommon newbie error), which is a significantly more complex error to understand.
   

The compiler currently assumes the reason is the first one, and complains about that.... but it is by far the easier error to see and fix... evidenced by the fact that you don't see long threads about how to fix missing parentheses in function calls.

Any error message for case #1 that mentions t.Year at all would be enough to tell but the most neophyte coder how to fix the code, if all they were missing was the parentheses. 

So, if you have to pick one message to show, why not the one that works relatively well for #1, and is not completely useless for #2?

[Nate Finch]

Note, example of when you are simply forgetting the parentheses:

func Foo(i int) {}

Foo(t.Year)

error:  "Cannot use t.Time (method) as type int in function argument"

Still perfectly clear what the problem is. 

[minux]

the original problem (allowing method currying), is https://code.google.com/p/go/issues/detail?id=2280

As Go 1.1 already changed the representation of func, fix that issue is not that hard (certainly not XL).

[Kevin Gillette]

Does this mean that memory pages dynamically allocated to Go will now be marked non-executable on platforms that support it?

[minux]

On Wed, Mar 6, 2013 at 2:30 AM, Kevin Gillette
<extempor...@gmail.com> wrote:
> Does this mean that memory pages dynamically allocated to Go will now be
> marked non-executable on platforms that support it?

Yes. Go 1.1 on all Unix platforms will use non-executable heap and stack.
Windows still needs some callback trampoline that must be allocated.