Go method chaining and error handling

Question

I want to create a method chaining API in Go. In all examples I can find the chained operations seem always to succeed which I can't guarantee. I therefore try to extend these to add the error return value.

If I do it like this

package main

import "fmt"

type Chain struct {
}

func (v *Chain)funA() (*Chain, error ) {
    fmt.Println("A")
    return v, nil
}
func (v *Chain)funB() (*Chain, error) {
    fmt.Println("B")
    return v, nil
}
func (v *Chain)funC() (*Chain, error) {
    fmt.Println("C")
    return v, nil
}

func main() {
    fmt.Println("Hello, playground")
    c := Chain{}
    d, err := c.funA().funB().funC() // line 24
}

The compiler tells me chain-err-test.go:24: multiple-value c.funA() in single-value context and won't compile. Is there a good way so funcA, funcB and funcC can report an error and stop that chain?

Answer 1

Is there a good way so funcA, funcB and funcC can report an error and stop that chain?

Unfortunately, no, there is no good solution to your problem. Workarounds are sufficiently complex (adding in error channels, etc) that the cost exceeds the gain.

Method chaining isn't an idiom in Go (at least not for methods that can possibly error). This isn't because there is anything particularly wrong with method chains, but a consequence of the idiom of returning errors instead of panicking. The other answers are workarounds, but none are idiomatic.

Can I ask, is it not idiomatic to chain methods in Go because of the consequence of returning error as we do in Go, or is it more generally a consequence of having multiple method returns?

Good question, but it's not because Go supports multiple returns. Python supports multiple returns, and Java can too via a Tuple<T1, T2> class; method chains are common in both languages. The reason these languages can get away with it is because they idiomatically communicate errors via exceptions. Exceptions stop the method chain immediately and jump to the relevant exception handler. This is the behavior the Go developers were specifically trying to avoid by choosing to return errors instead.

Answer 2

You can try like that: https://play.golang.org/p/dVn_DGWt1p_H

package main

import (
    "errors"
    "fmt"
)

type Chain struct {
    err error
}

func (v *Chain) funA() *Chain {
    if v.err != nil {
        return v
    }
    fmt.Println("A")
    return v
}
func (v *Chain) funB() *Chain {
    if v.err != nil {
        return v
    }
    v.err = errors.New("error at funB")
    fmt.Println("B")
    return v
}
func (v *Chain) funC() *Chain {
    if v.err != nil {
        return v
    }
    fmt.Println("C")
    return v
}

func main() {
    c := Chain{}
    d := c.funA().funB().funC() 
    fmt.Println(d.err)
}

Answer 3

If you have control over the code and the function signature is identical you can write something like:

func ChainCall(fns ...func() (*Chain, error)) (err error) {
    for _, fn := range fns {
        if _, err = fn(); err != nil {
            break
        }
    }
    return
}

playground

Answer 4

You can make your chain lazy by collecting a slice of funtions

package main

import (
    "fmt"
)

type (
    chainFunc func() error
    funcsChain struct {
        funcs []chainFunc
    }
)

func Chain() funcsChain {
    return funcsChain{}
}

func (chain funcsChain) Say(s string) funcsChain {
    f := func() error {
        fmt.Println(s)

        return nil
    }

    return funcsChain{append(chain.funcs, f)}
}


func (chain funcsChain) TryToSay(s string) funcsChain {
    f := func() error {
        return fmt.Errorf("don't speek golish")
    }

    return funcsChain{append(chain.funcs, f)}
}

func (chain funcsChain) Execute() (i int, err error) {
    for i, f := range chain.funcs {
        if err := f(); err != nil {
            return i, err
        }
    }

    return -1, nil
}

func main() {
    i, err := Chain().
        Say("Hello, playground").
        TryToSay("go cannot into chains").
        Execute()

    fmt.Printf("i: %d, err: %s", i, err)
}

Answer 5

You don't actually need channels and/or contexts to get something like this to work. I think this implementation meets all your requirements but needless to say, this leaves a sour taste. Go is not a functional language and it's best not to treat it as such.

package main

import (
    "errors"
    "fmt"
    "strconv"
)

type Res[T any] struct {
    Val  T
    Halt bool
    Err  error
}

// executes arguments until a halting signal is detected
func (r *Res[T]) Chain(args ...func() *Res[T]) *Res[T] {
    temp := r
    for _, f := range args {
        if temp = f(); temp.Halt {
            break
        }
    }

    return temp
}

// example function, converts any type -> string -> int -> string
func (r *Res[T]) funA() *Res[string] {
    s := fmt.Sprint(r.Val)
    i, err := strconv.Atoi(s)
    if err != nil {
        r.Err = fmt.Errorf("wrapping error: %w", err)
    }
    fmt.Println("the function down the pipe is forced to work with Res[string]")

    return &Res[string]{Val: strconv.Itoa(i), Err: r.Err}
}

func (r *Res[T]) funB() *Res[T] {
    prev := errors.Unwrap(r.Err)
    fmt.Printf("unwrapped error: %v\n", prev)

    // signal a halt if something is wrong
    if prev != nil {
        r.Halt = true
    }
    return r
}

func (r *Res[T]) funC() *Res[T] {
    fmt.Println("this one never gets executed...")
    return r
}

func (r *Res[T]) funD() *Res[T] {
    fmt.Println("...but this one does")
    return r
}

func funE() *Res[string] {
    fmt.Println("Chain can even take non-methods, but beware of nil returns")
    return nil
}

func main() {
    r := Res[string]{}
    r.Chain(r.funA, r.funB, r.funC).funD().Chain(funE).funC() // ... and so on
}

Answer 6

How about this approach: Create a struct that delegates Chain and error, and return it instead of two values. e.g.:

package main

import "fmt"

type Chain struct {
}

type ChainAndError struct {
    *Chain
    error
}

func (v *Chain)funA() ChainAndError {
    fmt.Println("A")
    return ChainAndError{v, nil}
}

func (v *Chain)funB() ChainAndError {
    fmt.Println("B")
    return ChainAndError{v, nil}
}

func (v *Chain)funC() ChainAndError {
    fmt.Println("C")
    return ChainAndError{v, nil}
}

func main() {
    fmt.Println("Hello, playground")
    c := Chain{}
    result := c.funA().funB().funC() // line 24
    fmt.Println(result.error)
}