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Go Language Overview

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Multiple Return Values

5:26 with Jay McGavren

It's possible to return multiple values from a Go function. This is usually used to communicate errors.

  • Can return multiple values from a function. Usually used to communicate errors.
package main

import (
    "fmt"
    "log"
    "math"
)

func main() {
    squareRoot, err := squareRoot(9)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println(squareRoot)
}

func squareRoot(x float64) (float64, error) {
    if x < 0 {
        return 0, fmt.Errorf("can't take square root of negative number")
    }
    return math.Sqrt(x), nil
}
  • Can ignore a return value by assigning to blank identifier, _.
    • This code gives a compile error:
package main

import (
    "fmt"
    "os"
)

func main() {
    fileInfo := os.Stat("existent.txt")
    fmt.Println(fileInfo.Size())
    fileInfo = os.Stat("nonexistent.txt")
    fmt.Println(fileInfo.Size())
}
  • We can get it to compile by adding blank identifiers.
    • But this isn't always a good idea. In this case, checking the size of "existent.txt" works, because the file exists. But our users get a panic, a runtime error, when we try to get the size of "nonexistent.txt", because the file doesn't exist.
  • We can fix this by checking to see if the error value is anything other than nil, and handling the error if it is.
package main

import (
    "fmt"
    "os"
)

func main() {
    fileInfo, error := os.Stat("existent.txt")
    if error != nil {
        fmt.Println(error)
    } else {
        fmt.Println(fileInfo.Size())
    }
    fileInfo, error = os.Stat("nonexistent.txt")
    if error != nil {
        fmt.Println(error)
    } else {
        fmt.Println(fileInfo.Size())
    }
}

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    You set this up by declaring multiple return values within parentheses, 0:00
    following the parentheses that accept parameters for the function. 0:01
    So here, we specify that this squareRoot function is going to return one value 0:05
    that's a float64 type, and then a second value that's an error type. 0:10
    Now, as you might be aware, 0:15
    it's not possible to take a square root of a negative number. 0:17
    So, we've set our square root function up to check for this. 0:21
    It looks at the parameter that got passed in, x, and 0:23
    if it's less than 0 then we return the value 0, which we're going to ignore. 0:27

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