proposal: define a type for (type, error)

[fcracker79]

Proposal Details

It would be nice to have a specific type for the pair (type, error), so as to be treated as a normal parameter.
It would allow, for instance, to simplify error chaining, as follows:

package main

import (
	"errors"
	"fmt"
)

func f(foo int) (int, error) {
	if foo > 100 {
		return -1, errors.New("error")
	}
	return 5, nil
}

// I created this type that would be the nice one to have to represent an error OR a value
type TypeOrError[T any] struct {
	Error error
	Value T
}

func mapError[T any, D any](t TypeOrError[T], gFunc func(T) (*D, error)) (*D, error) {
	if t.Error != nil {
		return nil, t.Error
	}
	
	stringResult, err := gFunc(t.Value)
	if err != nil {
		return nil, err
	}

	return stringResult, nil
}

func g(foo int) (*string, error) {
	if foo > 10 {
		return nil, errors.New("error")
	}
	s := fmt.Sprintf("%d", foo)
	return &s, nil
}

func main() {
	mystring, err := mapError[int, string](f(3), g)
	// Here `err` may come both for f and g
	if err != nil {
		fmt.Println("Error:", err)
	} else {
		fmt.Println(mystring)
	}
}

That would allow to convert "result or error" into a functor and would greatly simplify the error management.

[seankhliao]

Duplicate of #51931

[fcracker79]

@seankhliao in the end it has not to be necessarily an object. It would be just enough to allow functions to pass a result and an error. All in all, it would be nice if I could do mapError[int, string](f(3), g), for instance by accepting something like mapError[T any, D any](T t, err errror, gFunc func(T) (*D, error)) (*D, error)

[seankhliao]

also declined #40387