13 - Staying anonymous

Covered in this module:

• anonymous functions
• closures
• currying
• recursion

anonymous functions

Go supports anonymous functions that can be passed around like any other value.

func main() {
    add := func(a, b int) int {
        return a + b
    }

    fmt.Println(add(3, 400))
    fmt.Println(add(10, 59))
    fmt.Println(add(-5, 252))
    fmt.Println(add(80, -311))
    fmt.Println(add(-123, 42))
    fmt.Println(add(0, -65))
}

prints

403
69
247
-231
-81
-65

currying

Functions can return a function to allow for currying. The function returned by sequence() is considered a closure since it encapsulates the sequence value. The returned function essentially takes ownership of the seq variable.

func sequence(startingVal int) func() int {
    seq := startingVal
    return func() int {
        seq++
        return seq
    }
}

func main() {
    nextVal := sequence(25)

    fmt.Println(nextVal())
    fmt.Println(nextVal())
    fmt.Println(nextVal())
    fmt.Println(nextVal())

    moreVal := sequence(0)

    fmt.Println(moreVal())
    fmt.Println(moreVal())
}

prints

26
27
28
29
1
2

recursion

Functions can also invoke themselves, allowing for recursive behavior.

package main

import (
    "errors"
    "fmt"
    "math/rand"
    "time"
)

func mockCall() (int, error) {
    num := rand.Intn(10)
    if num <= 5 {
        return num, errors.New("call failed")
    }
    return num, nil
}


func attemptCall(call func() (int, error), maxRetries, try int) int {
    result, err := call()
    if err != nil {
        if try >= maxRetries {
            fmt.Printf("call failed after %v attempts. last error is: %v\n", maxRetries, err.Error())
            return 0
        }
        return attemptCall(call, maxRetries, try + 1)
    }
    return result
}

func main() {
    rand.Seed(time.Now().UTC().UnixNano())

    result := attemptCall(mockCall, 3, 1)
    fmt.Println("Result is", result)
}

occasionally prints

call failed after 3 attempts. last error is call failed
Result is 0

Hands on!

1. In the repo, open the file ./advanced/13currying.go
2. Complete the TODOs
3. Run make 13 from project root (alternatively, type go run ./13currying.go)
4. Example implementation available on solutions branch