如何从pongo模板调用Go函数

Question

I need to create JSON data using few keys of map and need to incorporate into html generated. I am using pongo2 library and want to write custom filter to achieve the same.

<script> {{ CategoryMapping|MycustomFilter }} </script>

and coded custom filter like below.

func init() {

    pongo2.RegisterFilter("superfilter", GetCategoryJsonData)


}

func GetCategoryJsonData(CatAttributeMapping *map[string]interface{}, param *int) (*string, *error) {
.....
}

But I am getting below error.

src/util/TemplateFilters.go:10: cannot use GetCategoryJsonData (type func(*int, *int) (*string, *error)) as type pongo2.FilterFunction in argument to pongo2.RegisterFilter

I am following below documentation - https://godoc.org/github.com/flosch/pongo2#FilterFunction

I am new to go and unable to understand what wrong I am doing here. Please guide me for the same.

Answer 1

The problem is that your filter function does not accept or return the right types to match what pongo2 is requiring. Let's walk through the docs and see what they want.

First, take a look at the godoc for RegisterFilterFunction. It says

func RegisterFilter(name string, fn FilterFunction)

This is in the pongo2 package so you should read this as RegisterFilter is a function that accepts two arguments and returns no values. The first argument name is of the builtin type string and the second argument fn is of the type pongo2.FilterFunction. But what is a pongo2.FilterFunction? Well clicking on it we see further down in the doc

type FilterFunction func(in *Value, param *Value) (out *Value, err *Error)

In Go you can make your own types based on any other types including functions. So what pongo2 has done is to create a named type called FilterFunction that is any func which accepts two arguments (both of type *pongo2.Value) and returns two values (one of type *pongo2.value and one of type *pongo2.Error).

To bring it all together we would do something like this:

package main

import (
    "fmt"
    "log"
    "strings"

    "github.com/flosch/pongo2"
)

func init() {
    pongo2.RegisterFilter("scream", Scream)
}

// Scream is a silly example of a filter function that upper cases strings
func Scream(in *pongo2.Value, param *pongo2.Value) (out *pongo2.Value, err *pongo2.Error) {
    if !in.IsString() {
        return nil, &pongo2.Error{
            ErrorMsg: "only strings should be sent to the scream filter",
        }
    }

    s := in.String()
    s = strings.ToUpper(s)

    return pongo2.AsValue(s), nil
}

func main() {
    tpl, err := pongo2.FromString("Hello {{ name|scream }}!")
    if err != nil {
        log.Fatal(err)
    }
    // Now you can render the template with the given
    // pongo2.Context how often you want to.
    out, err := tpl.Execute(pongo2.Context{"name": "stack overflow"})
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println(out) // Output: Hello STACK OVERFLOW!
}