Multiple values in single-value context

Question

Due to error handling in Go, I often end up with multiple values functions. So far, the way I have managed this has been very messy and I am looking for best practices to write cleaner code.

Let's say I have the following function:

type Item struct {
   Value int
   Name string
}

func Get(value int) (Item, error) {
  // some code

  return item, nil
}

How can I assign a new variable to item.Value elegantly. Before introducing the error handling, my function just returned item and I could simply do this:

val := Get(1).Value

Now I do this:

item, _ := Get(1)
val := item.Value

Isn't there a way to access directly the first returned variable?

Answer 1

In case of a multi-value return function you can't refer to fields or methods of a specific value of the result when calling the function.

And if one of them is an error, it's there for a reason (which is the function might fail) and you should not bypass it because if you do, your subsequent code might also fail miserably (e.g. resulting in runtime panic).

However there might be situations where you know the code will not fail in any circumstances. In these cases you can provide a helper function (or method) which will discard the error (or raise a runtime panic if it still occurs).
This can be the case if you provide the input values for a function from code, and you know they work.
Great examples of this are the template and regexp packages: if you provide a valid template or regexp at compile time, you can be sure they can always be parsed without errors at runtime. For this reason the template package provides the Must(t *Template, err error) *Template function and the regexp package provides the MustCompile(str string) *Regexp function: they don't return errors because their intended use is where the input is guaranteed to be valid.

Examples:

// "text" is a valid template, parsing it will not fail
var t = template.Must(template.New("name").Parse("text"))

// `^[a-z]+\[[0-9]+\]$` is a valid regexp, always compiles
var validID = regexp.MustCompile(`^[a-z]+\[[0-9]+\]$`)

Back to your case

IF you can be certain Get() will not produce error for certain input values, you can create a helper Must() function which would not return the error but raise a runtime panic if it still occurs:

func Must(i Item, err error) Item {
    if err != nil {
        panic(err)
    }
    return i
}

But you should not use this in all cases, just when you're sure it succeeds. Usage:

val := Must(Get(1)).Value

Go 1.18 generics update: Go 1.18 adds generics support, it is now possible to write a generic Must() function:

func Must[T any](v T, err error) T {
    if err != nil {
        panic(err)
    }
    return v
}

This is available in github.com/icza/gog, as gog.Must() (disclosure: I'm the author).

Alternative / Simplification

You can even simplify it further if you incorporate the Get() call into your helper function, let's call it MustGet:

func MustGet(value int) Item {
    i, err := Get(value)
    if err != nil {
        panic(err)
    }
    return i
}

Usage:

val := MustGet(1).Value

See some interesting / related questions:

How to pass multiple return values to a variadic function?

Return map like 'ok' in Golang on normal functions

Answer 2

Yes, there is.

Surprising, huh? You can get a specific value from a multiple return using a simple mute function:

package main

import "fmt"
import "strings"

func µ(a ...interface{}) []interface{} {
    return a
}

type A struct {
    B string
    C func()(string)
}

func main() {
    a := A {
        B:strings.TrimSpace(µ(E())[1].(string)),
        C:µ(G())[0].(func()(string)),
    }

    fmt.Printf ("%s says %s\n", a.B, a.C())
}

func E() (bool, string) {
    return false, "F"
}

func G() (func()(string), bool) {
    return func() string { return "Hello" }, true
}

https://play.golang.org/p/IwqmoKwVm-

Notice how you select the value number just like you would from a slice/array and then the type to get the actual value.

You can read more about the science behind that from this article. Credits to the author.

Answer 3

No, but that is a good thing since you should always handle your errors.

There are techniques that you can employ to defer error handling, see Errors are values by Rob Pike.

ew := &errWriter{w: fd}
ew.write(p0[a:b])
ew.write(p1[c:d])
ew.write(p2[e:f])
// and so on
if ew.err != nil {
    return ew.err
}

In this example from the blog post he illustrates how you could create an errWriter type that defers error handling till you are done calling write.

Answer 4

No, you cannot directly access the first value.

I suppose a hack for this would be to return an array of values instead of "item" and "err", and then just do item, _ := Get(1)[0] but I would not recommend this.

Answer 5

How about this way?

package main

import (
    "fmt"
    "errors"
)

type Item struct {
    Value int
    Name string
}

var items []Item = []Item{{Value:0, Name:"zero"}, 
                        {Value:1, Name:"one"}, 
                        {Value:2, Name:"two"}}

func main() {
    var err error
    v := Get(3, &err).Value
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println(v)

}

func Get(value int, err *error) Item {
    if value > (len(items) - 1) {
        *err = errors.New("error")
        return Item{}
    } else {
        return items[value]
    }
}

Answer 6

Here's a generic helper function with assumption checking:

func assumeNoError(value interface{}, err error) interface{} {
    if err != nil {
        panic("error encountered when none assumed:" + err.Error())
    }
    return value
}

Since this returns as an interface{}, you'll generally need to cast it back to your function's return type.

For example, the OP's example called Get(1), which returns (Item, error).

item := assumeNoError(Get(1)).(Item)

---

The trick that makes this possible: Multi-values returned from one function call can be passed in as multi-variable arguments to another function.

As a special case, if the return values of a function or method g are equal in number and individually assignable to the parameters of another function or method f, then the call f(g(parameters_of_g)) will invoke f after binding the return values of g to the parameters of f in order.

---

This answer borrows heavily from existing answers, but none had provided a simple, generic solution of this form.