proposal: hasField function for use with structs

[mbezhanov]

You have a proposal, explain it!

Originally proposed here: Masterminds/sprig#401

I recently worked on a project that used generics in a similar fashion:

type A[T any] struct {
	Metadata T
}

type B struct {
	Foo string
}
type C struct {
	Bar string
}

Then, similar objects were being passed to a template for rendering:

ab := &A[B]{Metadata: B{Foo: "Lorem"}}
ac := &A[C]{Metadata: C{Bar: "Ipsum"}}

A section in the template had to look slightly different depending on the type of metadata being passed in, which I solved by using a custom hasField method:

{{if hasField $.Metadata "Foo"}}
  We have Foo.
{{else}}
  We have no Foo.
{{end}}

Full code example

package main

import (
  "html/template"
  "log"
  "os"
  "reflect"
)

type A[T any] struct {
  Metadata T
}

type B struct {
  Foo string
}
type C struct {
  Bar string
}

const tmpl = `
  {{if hasField $.Metadata "Foo"}}
    We have Foo.
  {{else}}
    We have no Foo.
  {{end}}`

func main() {
  f := template.FuncMap{"hasField": hasField}
  t := template.Must(
  	template.New("base").Funcs(f).Parse(tmpl),
  )
  ab := &A[B]{Metadata: B{Foo: "Lorem"}}
  ac := &A[C]{Metadata: C{Bar: "Ipsum"}}
  if err := t.Execute(os.Stdout, ab); err != nil {
  	log.Fatal(err)
  }
  if err := t.Execute(os.Stdout, ac); err != nil {
  	log.Fatal(err)
  }
}

func hasField(v interface{}, name string) bool {
  rv := reflect.ValueOf(v)
  if rv.Kind() == reflect.Ptr {
  	rv = rv.Elem()
  }
  if rv.Kind() != reflect.Struct {
  	return false
  }
  return rv.FieldByName(name).IsValid()
}

Describe the solution you'd like

I took the hasField method implementation from the following StackOverflow thread: Field detection in Go HTML template.

It looks like this:

func hasField(v interface{}, name string) bool {
	rv := reflect.ValueOf(v)
	if rv.Kind() == reflect.Ptr {
		rv = rv.Elem()
	}
	if rv.Kind() != reflect.Struct {
		return false
	}
	return rv.FieldByName(name).IsValid()
}

I feel something similar can be added to the reflect registry, so it can be used with Sprout out of the box.

I'd be happy to open a PR!

Additional context

No response

Code of Conduct

• I agree to follow this project's Code of Conduct

[42atomys]

Hello @mbezhanov thanks for your proposal and your PR I will take a look soon