proposal: Go 2: exclamation assignment error handling

[emcfarlane]

Proposes an exclamation identifier ! similar to the blank identifier for assignment. A non zero value assigned to ! causes a run-time error. For example:

f, ! := os.Open(filename) // panics on error

Related to the try proposal this proposal aims to simplify error boilerplate. Unlike the try proposal it can be used for any type. The value v assigned to ! effectively calls:

if !reflect.ValueOf(v).IsZero() {
    panic(v)
}

I think this behaviour is similar to how type assertions panic on invalid non handled cases.
Maps can get similar behaviour with value, ! := myMap[key] which would panic on missing key.

As a larger example Go by Example: Reading Files could be written as:

package main

import (
    "bufio"
    "fmt"
    "io"
    "io/ioutil"
    "os"
)

func main() {
    dat, ! := ioutil.ReadFile("/tmp/dat")
    fmt.Print(string(dat))

    f, ! := os.Open("/tmp/dat")

    b1 := make([]byte, 5)
    n1, ! := f.Read(b1)
    fmt.Printf("%d bytes: %s\n", n1, string(b1[:n1]))

    o2, ! := f.Seek(6, 0)
    b2 := make([]byte, 2)
    n2, ! := f.Read(b2)
    fmt.Printf("%d bytes @ %d: ", n2, o2)
    fmt.Printf("%v\n", string(b2[:n2]))

    o3, ! := f.Seek(6, 0)
    b3 := make([]byte, 2)
    n3, ! := io.ReadAtLeast(f, b3, 2)
    fmt.Printf("%d bytes @ %d: %s\n", n3, o3, string(b3))

    _, ! = f.Seek(0, 0)

    r4 := bufio.NewReader(f)
    b4, ! := r4.Peek(5)
    fmt.Printf("5 bytes: %s\n", string(b4))

    f.Close()
}

This removes the check function and calls to check. The panic stack trace would be simplified as the call to check is removed. I could see this being used for example code and simple scripts where panics are already used for error handling.

[ianlancetaylor]

Looks very similar to #22122.

[emcfarlane]

Thanks for linking.

[networkimprov]

FYI, allowing a special character in place of an error variable is one of the most common suggestions to simplify error handling. I think I was the first to suggest it, but it's appeared in many flavors since. Some are on https://github.com/golang/go/wiki/Go2ErrorHandlingFeedback#recurring-themes