Combine multiple error strings

UPDATE for Go 1.13:

As of Go version 1.13, the language's errors package now supports error wrapping directly.

You can wrap an error by using the %w verb in fmt.Errorf:

err := errors.New("Original error")
err = fmt.Errorf("%w; Second error", err)

Use Unwrap to remove the last error added, and return what remains: previousErrors := errors.Unwrap(err)

Playground Example for errors.Unwrap

Two more functions, errors.Is and errors.As provide ways to check for and retrieve a specific type of error.

Playground Example for errors.As and errors.Is

Dave Cheney's excellent errors package (https://github.com/pkg/errors) include a Wrap function for this purpose:

package main

import "fmt"
import "github.com/pkg/errors"

func main() {
        err := errors.New("error")
        err = errors.Wrap(err, "open failed")
        err = errors.Wrap(err, "read config failed")

        fmt.Println(err) // "read config failed: open failed: error"
}

This also allows additional functionality, such as unpacking the cause of the error:

package main

import "fmt"
import "github.com/pkg/errors"

func main() {
    err := errors.New("original error")
    err = errors.Wrap(err, "now this")

    fmt.Println(errors.Cause(err)) // "original error"
}

As well as the option to output a stack trace when specifying fmt.Printf("%+v\n", err).

You can find additional information about the package on his blog: here and here.

String functions don't work on errors because error is really an interface that implements the function Error() string.

You can use string functions on err1.Error() and err2.Error() but not on the "err1" reference itself.

Some errors are structs, like the ones you get from database drivers.

So there's no natural way to use string functions on errors since they may not actually be strings underneath.

As for combining two errors:

Easy, just use fmt.Errorf again.

fmt.Errorf("Combined error: %v %v", err1, err2)

Alternatively:

errors.New(err1.Error() + err2.Error())