Assign a new value to an already initiated variable in template

[Accipiter Nisus]

Hi,

What are the reasons why it is not possible to assign a new value to an already initiated variable in the text/template package?

Example where it is needed:

Let's say I have a list of people (json):

[

    {"name": "ANisus", "sex":"M"},

    {"name": "Sofia", "sex":"F"},

    {"name": "Anna", "sex":"F"}

]

Using the template package, I want to have this output:

Females:
Sofia
Anna

But the header "Females:" should only show in case there are actually any person with sex set to F.

If I could reassign a value, I would do the following:

{{$hasFemales := 0}}

{{range .}}{{if eq .sex "F"}}{{$hasFemales = 1}}{{end}}{{end}}

{{if $hasFemale}}Female:{{end}}

Example: http://play.golang.org/p/T-Ekx7n9YQ

/ANisus

[Jesse McNelis]

On 10/10/2013 8:54 PM, "Accipiter Nisus" <acci...@gmail.com> wrote:
>
> Hi,
>
> What are the reasons why it is not possible to assign a new value to an already initiated variable in the text/template package?
>

Templates aren't the place for doing complex logic. Do it before you pass the data to the template.

> {{$hasFemales := 0}}
> {{range .}}{{if eq .sex "F"}}{{$hasFemales = 1}}{{end}}{{end}}
> {{if $hasFemale}}Female:{{end}}
>
> Example: http://play.golang.org/p/T-Ekx7n9YQ

You can easily count the females before you pass the slice to the template and pass that in too.

[Ibrahim M. Ghazal]

Actually, you *can* assign new values to variables: http://play.golang.org/p/GzXhsDFpom

package main

import (

"os"

"text/template"

)

func main() {

const tmpl = `

{{$foo := 0}}

$foo is {{$foo}}

{{$foo := 1}}

Now $foo is {{$foo}}

`

t := template.Must(template.New("tmpl").Parse(tmpl))

err := t.Execute(os.Stdout, nil)

if err != nil {

panic(err)

}

}

Output:

$foo is 0

Now $foo is 1

[Accipiter Nisus]

Well, what do you know.

But, it only works within the same scope. I think it is not so much reassigning a value as replacing the previous variable in the scope with a new one.

In my playground code, it doesn't work. $hasFemale in the outer scope remains unchanged.

[Accipiter Nisus]

Templates aren't the place for doing complex logic. Do it before you pass the data to the template.

Whether or not assigning a new value to a variable is considered complex logic might be up for discussion.

When the template package is used in a generic context (eg. Executing an arbitrary template file with an arbitrary json file), you might not have the luxury of doing precalculations.

/ANisus

[Steven Blenkinsop]

You could do something like this if you had to:

http://play.golang.org/p/SufZdsx-1v

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[sorenhoyer...@gmail.com]

I don't think something like this is "complex logic" and when developing a generic application like a CMS, this functionality would be very handy to have, like we can with Razor.

This is the biggest (and only) problems I have with Go. I simply do not understand why this functionality has not been implemented yet.

[ld...@google.com]

encountered same problem, as it seems still an open issue, cannot change value of an existing variable inside of loop/if in template 

https://github.com/golang/go/issues/10608

...

      {{$deprecated := false}}
      {{range $depre_audit := $depre_audits}}
      {{if eq $audit_name $depre_audit}}
      {{$deprecated := true}}   ------> this line doesn't work beyond end
      <span class="audit_name_chart">{{$audit_name}} - deprecated</span>
      {{end}}
      {{end}}
      {{if not $deprecated}}
      <span class="audit_name_chart">{{$audit_name}}</span>
      {{end}}

[Aliaksandr Valialkin]

FYI, complex logic may be easily implemented with alternative template engines like https://github.com/valyala/quicktemplate .

[whut....@gmail.com]

Thanks for this example!

I realized that to modify a variable I have to add a "var" function to return pointer of an object,  then I can use a "set" function to modify it.

And in the beginning I also provided "get" function to return the value referenced by the pointer, however later I suddenly found that I don't need to do that, go template will automatically return the object itself, rather than the pointer.

So this is my version https://play.golang.org/p/C1MtTKvdmGT
I think it is more concise and elegant than the original example :-)

Thanks so much for inspiring me!