Re: [go-nuts] How to generalize parameters for a function
On Wed, 2023-03-15 at 12:55 -0400, Frank Juedes wrote: > > > Hi Dan, > Thank you very much for your answer, so that's the data structure > behind maps, very interesting. > I had actually thought about using unsafe pointers and then type- > casting, but that is how i would have done it in the C-dekades, not > with Go. The source for this is here https://cs.opensource.google/go/go/+/refs/tags/go1.20.2:src/runtime/map.go -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/794a286af50a58c9ac1e38fec6f7f10ce5d4f4a9.camel%40kortschak.io.
Re: [go-nuts] How to generalize parameters for a function
On 2023-03-15 06:06, Jan Mercl wrote: On Wed, Mar 15, 2023 at 11:03 AM Frank Jüdes wrote: Well, that didn't go very far:https://go.dev/play/p/UtsrRar9J0Q Fails with the messages ./prog.go:15:11: invalid operation: cannot index p_Map (variable of type MAP constrained by DataMap) ./prog.go:16:26: cannot range over p_Map (variable of type MAP constrained by DataMap) (MAP has no core type) https://go.dev/play/p/6KN36eeomML Hi Jan, that looks great and is exactly what i was looking for, just need to wrap my head around that function declaration… I understand /that/*//*declaration, but do not fully understand this in general. I guess i will have to do some more reading… Thank you very much! Best regards from Charleston (WV), Frank/2 -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/5d13aa53-2ca8-13c4-cf12-38cb61c9e7f3%40gmail.com.
Re: [go-nuts] How to generalize parameters for a function
Which is also the reason why you can't insert into a nil map - you have to make() one. On Wednesday, 15 March 2023 at 10:58:09 UTC Dan Kortschak wrote: > On Tue, 2023-03-14 at 19:56 -0700, Kurtis Rader wrote: > > Maps are a special-case. You can't pass them "by value" in the sense > > you mean because a "map" value is a very tiny structure that contains > > a pointer to the actual map. > > The passed value is a pointer to the header, otherwise changes to maps > would require the altered map to be returned as with slices and append. > > https://go.dev/play/p/avBBkNwVZRY > > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/0ffd9d0d-b80a-4ec5-a772-40dfb102b43an%40googlegroups.com.
Re: [go-nuts] How to generalize parameters for a function
On Tue, 2023-03-14 at 19:56 -0700, Kurtis Rader wrote: > Maps are a special-case. You can't pass them "by value" in the sense > you mean because a "map" value is a very tiny structure that contains > a pointer to the actual map. The passed value is a pointer to the header, otherwise changes to maps would require the altered map to be returned as with slices and append. https://go.dev/play/p/avBBkNwVZRY -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/4af2d37285e3b6d2a7594342c18f5c4160b7b86f.camel%40kortschak.io.
Re: [go-nuts] How to generalize parameters for a function
On Wed, Mar 15, 2023 at 11:03 AM Frank Jüdes wrote: > > Well, that didn't go very far: https://go.dev/play/p/UtsrRar9J0Q > Fails with the messages > ./prog.go:15:11: invalid operation: cannot index p_Map (variable of type MAP > constrained by DataMap) > ./prog.go:16:26: cannot range over p_Map (variable of type MAP constrained by > DataMap) (MAP has no core type) https://go.dev/play/p/6KN36eeomML -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/CAA40n-WAqUuSR_896rz4C%3DSDOH3LGxZhC4DQ8%2BEVqh%2BM0JKUKw%40mail.gmail.com.
Re: [go-nuts] How to generalize parameters for a function
Well, that didn't go very far: https://go.dev/play/p/UtsrRar9J0Q
Fails with the messages
./prog.go:15:11: invalid operation: cannot index p_Map (variable of type
MAP constrained by DataMap)
./prog.go:16:26: cannot range over p_Map (variable of type MAP constrained
by DataMap) (MAP has no core type)
I think i have to to some more research here - or just copy and paste three
different typed functions…
Best regards from Charleston (WV),
Frank/2
Frank Jüdes schrieb am Mittwoch, 15. März 2023 um 05:51:10 UTC-4:
> Hi Kurtis,
> good to know that a map will always be "kind of" passed by reference to
> functions. I have verified that with a short test-program on the playground
> -> https://go.dev/play/p/HxdgpM-QozM
> So in the next code-review all those *[maptype] references will be
> removed. Things are very different in this case in other programming
> languages…
>
> I understand the misconception and that a map[type]interface{} won't
> automagically convert the data-type from the parameter into anything else.
> My intention was to explain the compiler to just accept whatever is fed
> into the function and just pass the data through to a function that uses
> interface{} for a single parameter. That's all this function needs to do:
> Accept a map of "something", get single values of "something" and pass them
> on to another function that accepts a single parameter of "something".
>
> I think i have found a solution in using a "generic" interface, stumbled
> upon this web-site:
> https://www.geeksforgeeks.org/generics-in-golang/?ref=rp
>
> And created a little test-program: https://go.dev/play/p/mOu1pmmfdX8
>
> This should allow me to pass different types of maps - actually i only
> need three types - to a generic function that passes the values on to
> another (generic) function.
>
> Thank you very much for your help.
>
> Best regards from Charleston (WV),
> Frank/2
>
> Kurtis Rader schrieb am Dienstag, 14. März 2023 um 22:57:13 UTC-4:
>
>> On Tue, Mar 14, 2023 at 7:07 PM Frank Jüdes wrote:
>>
>>> Thank you very much Ian!
>>> I am using a pointer to the map because i want to pass the map by
>>> reference, not by value.
>>> Those maps can become pretty large and are being handed down through a
>>> couple of function calls and i don't want them to be copied over and over
>>> again.
>>>
>>
>> Maps are a special-case. You can't pass them "by value" in the sense you
>> mean because a "map" value is a very tiny structure that contains a pointer
>> to the actual map. Possibly the oldest public discussion of the fact maps
>> are reference type is this blog post: https://go.dev/blog/maps. Using a
>> pointer does, in fact, avoid copying that very small data structure but is
>> not necessary to keep from copying the entire map.
>>
>>
>>> I read the document you referenced, but am not able to understand…
>>> So i can use interface{} or any just to generalize simple types?
>>> Why is the compiler accepting then a p_Values map[string]interface{} as
>>> a parameter of a function? - What would be a compatible data-type to pass
>>> as this parameter?
>>>
>>
>> The compiler accepts a type of "map[string]interface{}" because that is
>> well defined and obviously useful, Functions, such as those in the fmt
>> package, would not be useful if a function could not deal with the
>> "interface{}" type directly or indirectly. Your confusion is very common
>> and arises from the misconception that the Go map implementation will
>> magically convert map values from a concrete type (e.g., "int") to the
>> "any" type if that is what is needed in a particular context. One reason it
>> doesn't do so is the same reason it doesn't auto-magically convert any
>> other map value type; e.g., "int" to "float".
>>
>> --
>> Kurtis Rader
>> Caretaker of the exceptional canines Junior and Hank
>>
>
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Re: [go-nuts] How to generalize parameters for a function
Hi Kurtis,
good to know that a map will always be "kind of" passed by reference to
functions. I have verified that with a short test-program on the playground
-> https://go.dev/play/p/HxdgpM-QozM
So in the next code-review all those *[maptype] references will be removed.
Things are very different in this case in other programming languages…
I understand the misconception and that a map[type]interface{} won't
automagically convert the data-type from the parameter into anything else.
My intention was to explain the compiler to just accept whatever is fed
into the function and just pass the data through to a function that uses
interface{} for a single parameter. That's all this function needs to do:
Accept a map of "something", get single values of "something" and pass them
on to another function that accepts a single parameter of "something".
I think i have found a solution in using a "generic" interface, stumbled
upon this web-site: https://www.geeksforgeeks.org/generics-in-golang/?ref=rp
And created a little test-program: https://go.dev/play/p/mOu1pmmfdX8
This should allow me to pass different types of maps - actually i only need
three types - to a generic function that passes the values on to another
(generic) function.
Thank you very much for your help.
Best regards from Charleston (WV),
Frank/2
Kurtis Rader schrieb am Dienstag, 14. März 2023 um 22:57:13 UTC-4:
> On Tue, Mar 14, 2023 at 7:07 PM Frank Jüdes wrote:
>
>> Thank you very much Ian!
>> I am using a pointer to the map because i want to pass the map by
>> reference, not by value.
>> Those maps can become pretty large and are being handed down through a
>> couple of function calls and i don't want them to be copied over and over
>> again.
>>
>
> Maps are a special-case. You can't pass them "by value" in the sense you
> mean because a "map" value is a very tiny structure that contains a pointer
> to the actual map. Possibly the oldest public discussion of the fact maps
> are reference type is this blog post: https://go.dev/blog/maps. Using a
> pointer does, in fact, avoid copying that very small data structure but is
> not necessary to keep from copying the entire map.
>
>
>> I read the document you referenced, but am not able to understand…
>> So i can use interface{} or any just to generalize simple types?
>> Why is the compiler accepting then a p_Values map[string]interface{} as a
>> parameter of a function? - What would be a compatible data-type to pass as
>> this parameter?
>>
>
> The compiler accepts a type of "map[string]interface{}" because that is
> well defined and obviously useful, Functions, such as those in the fmt
> package, would not be useful if a function could not deal with the
> "interface{}" type directly or indirectly. Your confusion is very common
> and arises from the misconception that the Go map implementation will
> magically convert map values from a concrete type (e.g., "int") to the
> "any" type if that is what is needed in a particular context. One reason it
> doesn't do so is the same reason it doesn't auto-magically convert any
> other map value type; e.g., "int" to "float".
>
> --
> Kurtis Rader
> Caretaker of the exceptional canines Junior and Hank
>
--
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Re: [go-nuts] How to generalize parameters for a function
On Tue, Mar 14, 2023 at 7:07 PM Frank Jüdes wrote:
> Thank you very much Ian!
> I am using a pointer to the map because i want to pass the map by
> reference, not by value.
> Those maps can become pretty large and are being handed down through a
> couple of function calls and i don't want them to be copied over and over
> again.
>
Maps are a special-case. You can't pass them "by value" in the sense you
mean because a "map" value is a very tiny structure that contains a pointer
to the actual map. Possibly the oldest public discussion of the fact maps
are reference type is this blog post: https://go.dev/blog/maps. Using a
pointer does, in fact, avoid copying that very small data structure but is
not necessary to keep from copying the entire map.
> I read the document you referenced, but am not able to understand…
> So i can use interface{} or any just to generalize simple types?
> Why is the compiler accepting then a p_Values map[string]interface{} as a
> parameter of a function? - What would be a compatible data-type to pass as
> this parameter?
>
The compiler accepts a type of "map[string]interface{}" because that is
well defined and obviously useful, Functions, such as those in the fmt
package, would not be useful if a function could not deal with the
"interface{}" type directly or indirectly. Your confusion is very common
and arises from the misconception that the Go map implementation will
magically convert map values from a concrete type (e.g., "int") to the
"any" type if that is what is needed in a particular context. One reason it
doesn't do so is the same reason it doesn't auto-magically convert any
other map value type; e.g., "int" to "float".
--
Kurtis Rader
Caretaker of the exceptional canines Junior and Hank
--
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Re: [go-nuts] How to generalize parameters for a function
Thank you very much Ian!
I am using a pointer to the map because i want to pass the map by
reference, not by value.
Those maps can become pretty large and are being handed down through a
couple of function calls and i don't want them to be copied over and over
again.
I read the document you referenced, but am not able to understand…
So i can use interface{} or any just to generalize simple types?
Why is the compiler accepting then a p_Values map[string]interface{} as a
parameter of a function? - What would be a compatible data-type to pass as
this parameter?
Thank you very much in advance for your help.
Best regards from Charleston (WV),
Frank/2
Ian Lance Taylor schrieb am Dienstag, 14. März 2023 um 21:40:27 UTC-4:
> On Tue, Mar 14, 2023 at 6:27 PM Frank Jüdes wrote:
> >
> > i still somewhat new with Go and need a little tutoring: I have to
> create Excel files, using the excelize package and wrote a little function
> to store data from a map[string]int64 into a column within the worksheet:
> >
> > //
> -
> > // Store a slice of int64 data items into the cells of a column
> > //
> -
> > func SetColumnValuesInt64(xf *excelize.File,
> > p_SheetName string,
> > p_StartRow int,
> > p_Column rune,
> > p_CellStyle *excelize.Style,
> > p_Keys *UTL_StringSlice.T_StringSlice,
> > p_Values *map[string]int64) {
> > Row := p_StartRow
> > StyleID,_ := xf.NewStyle(p_CellStyle)
> > for _,Key := range(*p_Keys) {
> > CellAddress := fmt.Sprintf("%c%d",p_Column,Row)
> > xf.SetCellValue(p_SheetName,CellAddress,(*p_Values)[Key])
> > xf.SetCellStyle(p_SheetName,CellAddress,CellAddress,StyleID)
> > Row++
> > } // END for
> > } // END SetColumnValuesInt64
> >
> > Basically the function iterates through a slice of strings, containing
> the key-values for the data-map and then calls the SetCellValue function
> from the excelize package to store the number into the cell.
> > Now i have another set of data, stored into a map[string]float64 …
> > So i duplicated the above function, just replacing the red line with
> > p_Values *map[string]float64) {
> >
> > Basically it is the same code, just the parameter-type is different. I
> looked into the definition of the function SetCellValue and saw that it is
> using the empty interface as parameter for the value, so i tried to define
> my function with
> > p_Values *map[string]interface{}) {
> >
> > The function compiles fine, but if i try to use it with a
> *map[string]int64 as a parameter, the compiler objectst with the message
> »cannot use Amounts (variable of type *map[string]int64) as type
> *map[string]interface{} in argument to ExcelFormats.SetColumnValues«
> >
> > Can somebody please give me hint what i am doing wrong?
>
> First I'll note that it's quite unusual to use a pointer to a map.
> Maps are reference types. If you pass a map to a function, and that
> function adds values to the map, the caller will also see the new map
> entries.
>
> That said, the error message seems clear enough: a function that
> expects a pointer to map[string]any can't take an argument that is a
> pointer to map[string]int64. The types any and int64 are different
> types. You can't interchange them. See
> https://go.dev/doc/faq#covariant_types for a related issue.
>
> Ian
>
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Re: [go-nuts] How to generalize parameters for a function
On Tue, Mar 14, 2023 at 6:27 PM Frank Jüdes wrote:
>
> i still somewhat new with Go and need a little tutoring: I have to create
> Excel files, using the excelize package and wrote a little function to store
> data from a map[string]int64 into a column within the worksheet:
>
> //
> -
> // Store a slice of int64 data items into the cells of a column
> //
> -
> func SetColumnValuesInt64(xf *excelize.File,
> p_SheetName string,
> p_StartRow int,
> p_Column rune,
> p_CellStyle *excelize.Style,
> p_Keys *UTL_StringSlice.T_StringSlice,
> p_Values *map[string]int64) {
>Row := p_StartRow
>StyleID,_ := xf.NewStyle(p_CellStyle)
>for _,Key := range(*p_Keys) {
> CellAddress := fmt.Sprintf("%c%d",p_Column,Row)
> xf.SetCellValue(p_SheetName,CellAddress,(*p_Values)[Key])
> xf.SetCellStyle(p_SheetName,CellAddress,CellAddress,StyleID)
> Row++
>} // END for
> } // END SetColumnValuesInt64
>
> Basically the function iterates through a slice of strings, containing the
> key-values for the data-map and then calls the SetCellValue function from the
> excelize package to store the number into the cell.
> Now i have another set of data, stored into a map[string]float64 …
> So i duplicated the above function, just replacing the red line with
> p_Values *map[string]float64) {
>
> Basically it is the same code, just the parameter-type is different. I looked
> into the definition of the function SetCellValue and saw that it is using
> the empty interface as parameter for the value, so i tried to define my
> function with
> p_Values *map[string]interface{}) {
>
> The function compiles fine, but if i try to use it with a *map[string]int64
> as a parameter, the compiler objectst with the message »cannot use Amounts
> (variable of type *map[string]int64) as type *map[string]interface{} in
> argument to ExcelFormats.SetColumnValues«
>
> Can somebody please give me hint what i am doing wrong?
First I'll note that it's quite unusual to use a pointer to a map.
Maps are reference types. If you pass a map to a function, and that
function adds values to the map, the caller will also see the new map
entries.
That said, the error message seems clear enough: a function that
expects a pointer to map[string]any can't take an argument that is a
pointer to map[string]int64. The types any and int64 are different
types. You can't interchange them. See
https://go.dev/doc/faq#covariant_types for a related issue.
Ian
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