How to report a line number on error when decoding custom struct?

[GreatDanton]

When decoding built in types, I found out that toml.Decode() method returns a ParseError which contains lots of context about the error. When unmarshalling custom structs, however, I don't know how am I supposed to obtain such context (I would like to know at least the name of the field or the line number of the user text).

func decodeConfig() (*xxx, error) {
        config := &MyConfig{}
	md, err := toml.Decode(tomlData, config)
	if err != nil {
		// pretty print errors if possible
		var pErr toml.ParseError
		if errors.As(err, &pErr) {
			return nil, fmt.Errorf("toml: config parse error: %v", pErr.ErrorWithUsage())
		}

		// err could not be cast to a toml.ParseError, which means our own unmarshaller returned an error
		// TODO: how do I get a line number or the field here? md.context is a private member.
		return nil, fmt.Errorf("toml: config parse error:\n  %v", err)
	}

         ... more code

More context about my problem:
In my case I have a custom type which tries to parse user value and return an error on invalid enum:

func (n *NodeReferenceType) UnmarshalText(text []byte) error {
	t := strings.TrimSpace(string(text))
	switch t {
	case "number_and_text":
		*n = NodeReference_NumberAndText
	case "number":
		*n = NodeReference_Number
	case "text":
		*n = NodeReference_Text
	default:
               // TODO: how can I get a line number or the name of a currently decoded key here?
               // I am trying to decode multiple fields of this same type (NodeReferenceType), so I 
               // need some way to distinguish between them.
		return fmt.Errorf("invalid reference type: available options [%s, %s, %s], but got: %q", NodeReference_NumberAndText, NodeReference_Number, NodeReference_Text, t)
	}
	return nil
}

[arp242]

It would be helpful if you can provide a full example (either as a program or test) that I can copy/paste to demonstrate/reproduce the problem. In your example there's lots of missing references and such.

[GreatDanton]

I hope this is better

package main

import (
	"errors"
	"fmt"
	"github.com/BurntSushi/toml"
	"log"
	"strings"
)

type CustomType uint8

func (n *CustomType) UnmarshalText(text []byte) error {
	t := strings.TrimSpace(string(text))
	switch t {
	case "number_and_text":
		*n = 0
	case "number":
		*n = 1
	case "text":
		*n = 2
	default:
		// TODO: how can I get a line number or the name of a currently decoded field here?
		// I am trying to decode multiple fields of this same type (CustomType), so I
		// need some way to distinguish between them.
		return fmt.Errorf("invalid reference type: available options [%d, %d, %d], but got: %q", 0, 1, 2, t)
	}
	return nil
}

type MyConfig struct {
	Field1 CustomType `toml:"field1"`
	Field2 CustomType `toml:"field2"`
	Field3 CustomType `toml:"field3"`
}

func main() {
	CONFIG := `
	field1 = "number_and_text"
	field2 = "number"
	field3 = "invalid enum"
    `

	config := &MyConfig{}
	md, err := toml.Decode(CONFIG, config)
	if err != nil {
		var pErr toml.ParseError
		if errors.As(err, &pErr) {
			log.Fatalf("toml: config parse error: %v", pErr.ErrorWithUsage())
		}

		// err could not be cast to a toml.ParseError, which means our own unmarshaller
		// returned an error. Here I would need some way to report which field has invalid
                // value (field1, field2 or field3?)
		if false {
			// md contains context with our field that reported a decode error, but 
                        // it's a private member
			fmt.Println(md)
		}
		log.Fatalf("toml: config parse error: %v", err)
	}
}

[arp242]

Cheers; that's helpful. I'll have a look tomorrow.

[arp242]

Sorry, I forgot about your issue.

I fixed it; if you use go get github.com/BurntSushi/toml@master you should automatically get a ParseError, and your code should work.