Multiple values in single-value context - go

Due to error handling in Go, I often end up with multiple values functions. So far, the way I have managed this has been very messy and I am looking for best practices to write cleaner code.
Let's say I have the following function:
type Item struct {
Value int
Name string
}
func Get(value int) (Item, error) {
// some code
return item, nil
}
How can I assign a new variable to item.Value elegantly. Before introducing the error handling, my function just returned item and I could simply do this:
val := Get(1).Value
Now I do this:
item, _ := Get(1)
val := item.Value
Isn't there a way to access directly the first returned variable?

In case of a multi-value return function you can't refer to fields or methods of a specific value of the result when calling the function.
And if one of them is an error, it's there for a reason (which is the function might fail) and you should not bypass it because if you do, your subsequent code might also fail miserably (e.g. resulting in runtime panic).
However there might be situations where you know the code will not fail in any circumstances. In these cases you can provide a helper function (or method) which will discard the error (or raise a runtime panic if it still occurs).
This can be the case if you provide the input values for a function from code, and you know they work.
Great examples of this are the template and regexp packages: if you provide a valid template or regexp at compile time, you can be sure they can always be parsed without errors at runtime. For this reason the template package provides the Must(t *Template, err error) *Template function and the regexp package provides the MustCompile(str string) *Regexp function: they don't return errors because their intended use is where the input is guaranteed to be valid.
Examples:
// "text" is a valid template, parsing it will not fail
var t = template.Must(template.New("name").Parse("text"))
// `^[a-z]+\[[0-9]+\]$` is a valid regexp, always compiles
var validID = regexp.MustCompile(`^[a-z]+\[[0-9]+\]$`)
Back to your case
IF you can be certain Get() will not produce error for certain input values, you can create a helper Must() function which would not return the error but raise a runtime panic if it still occurs:
func Must(i Item, err error) Item {
if err != nil {
panic(err)
}
return i
}
But you should not use this in all cases, just when you're sure it succeeds. Usage:
val := Must(Get(1)).Value
Go 1.18 generics update: Go 1.18 adds generics support, it is now possible to write a generic Must() function:
func Must[T any](v T, err error) T {
if err != nil {
panic(err)
}
return v
}
This is available in github.com/icza/gog, as gog.Must() (disclosure: I'm the author).
Alternative / Simplification
You can even simplify it further if you incorporate the Get() call into your helper function, let's call it MustGet:
func MustGet(value int) Item {
i, err := Get(value)
if err != nil {
panic(err)
}
return i
}
Usage:
val := MustGet(1).Value
See some interesting / related questions:
How to pass multiple return values to a variadic function?
Return map like 'ok' in Golang on normal functions

Yes, there is.
Surprising, huh? You can get a specific value from a multiple return using a simple mute function:
package main
import "fmt"
import "strings"
func µ(a ...interface{}) []interface{} {
return a
}
type A struct {
B string
C func()(string)
}
func main() {
a := A {
B:strings.TrimSpace(µ(E())[1].(string)),
C:µ(G())[0].(func()(string)),
}
fmt.Printf ("%s says %s\n", a.B, a.C())
}
func E() (bool, string) {
return false, "F"
}
func G() (func()(string), bool) {
return func() string { return "Hello" }, true
}
https://play.golang.org/p/IwqmoKwVm-
Notice how you select the value number just like you would from a slice/array and then the type to get the actual value.
You can read more about the science behind that from this article. Credits to the author.

No, but that is a good thing since you should always handle your errors.
There are techniques that you can employ to defer error handling, see Errors are values by Rob Pike.
ew := &errWriter{w: fd}
ew.write(p0[a:b])
ew.write(p1[c:d])
ew.write(p2[e:f])
// and so on
if ew.err != nil {
return ew.err
}
In this example from the blog post he illustrates how you could create an errWriter type that defers error handling till you are done calling write.

No, you cannot directly access the first value.
I suppose a hack for this would be to return an array of values instead of "item" and "err", and then just do
item, _ := Get(1)[0]
but I would not recommend this.

How about this way?
package main
import (
"fmt"
"errors"
)
type Item struct {
Value int
Name string
}
var items []Item = []Item{{Value:0, Name:"zero"},
{Value:1, Name:"one"},
{Value:2, Name:"two"}}
func main() {
var err error
v := Get(3, &err).Value
if err != nil {
fmt.Println(err)
return
}
fmt.Println(v)
}
func Get(value int, err *error) Item {
if value > (len(items) - 1) {
*err = errors.New("error")
return Item{}
} else {
return items[value]
}
}

Here's a generic helper function with assumption checking:
func assumeNoError(value interface{}, err error) interface{} {
if err != nil {
panic("error encountered when none assumed:" + err.Error())
}
return value
}
Since this returns as an interface{}, you'll generally need to cast it back to your function's return type.
For example, the OP's example called Get(1), which returns (Item, error).
item := assumeNoError(Get(1)).(Item)
The trick that makes this possible: Multi-values returned from one function call can be passed in as multi-variable arguments to another function.
As a special case, if the return values of a function or method g are equal in number and individually assignable to the parameters of another function or method f, then the call f(g(parameters_of_g)) will invoke f after binding the return values of g to the parameters of f in order.
This answer borrows heavily from existing answers, but none had provided a simple, generic solution of this form.

Related

Pass a generic variadic function as a parameter in Go

I am working on a ETL type application and a large majority of the error handling I do is just retrying API requests until they succeed (they randomly fail on occasion due to connection etc.).
Therefore I have noticed a lot of code duplication that looks a lot like
for err != nil {
a,b,err = myfunc(c, d, e)
}
return a, b
So basically just keep doing the function until the error goes away (I put a sleep and other error checking as necessary as well to avoid rate limiting).
What I would like to do is simplify this to just one function that takes an arbitrary function, finds the error type in its output (if it has one) and runs it recursively until err!=nil. My problem is that although go seems to let you use any (interface{}) as an input it is not variadic on function definitions e.g.
(type func(a int) (int, int, error)) as the type func(...any) []any
I am wondering if this is impossible to do in go and if so any suggestions to get around it/get similar functionality more idiomatically.
Trying to test with something like this but the compiler does not like it.
func main() {
Deal(SometimesFail, 10)
}
func Deal(f func(...any) []any, inputs ...any) []any {
outputs := f(inputs)
for _, val := range outputs {
err, ok := val.(error)
if ok {
for err != nil {
outputs = Deal(f, inputs...)
}
return outputs
}
continue
}
return outputs
}
func SometimesFail(a int) (int, int, error) {
random := rand.Intn(a)
if random%2 == 0 {
return random, random, nil
} else {
return random, random, errors.New("error")
}
}
I guess what I could do to get around this is create a type for each function out/input scheme and allow the generic function to take any of these. This would keep the code duplication at a minimum while still achieving the goal.
The following: func(any), func(any, any), func(...any) are all different types and you can't assign one type to another. There is no single function type that would include all of them.
One way to work around this is to decouple function invocation (which must know the exact type of the function) from the retrial logic:
type result struct {
vals []any
err error
}
func main() {
vals := repeatUntilSuccess(func(inputs ...any) result {
val, err := failingRandomly(10)
return result{[]any{val}, err}
})
fmt.Println(vals)
}
func repeatUntilSuccess(fn func(...any) result) []any {
res := fn()
for res.err != nil {
res = fn()
}
return res.vals
}
func failingRandomly(i int) (int, error) {
random := rand.Intn(i)
if random%2 == 0 {
return random, nil
} else {
return random, errors.New("bad luck")
}
}

Can I make an optional return value in golang? [duplicate]

In Go, the following works (note one use of the map has one return, the other has two returns)
package main
import "fmt"
var someMap = map[string]string { "some key": "hello" }
func main() {
if value, ok := someMap["some key"]; ok {
fmt.Println(value)
}
value := someMap["some key"]
fmt.Println(value)
}
However, I have no idea how to do this same thing with my own function. Is it possible to have similar behavior with an optional return like map?
For example:
package main
import "fmt"
func Hello() (string, bool) {
return "hello", true
}
func main() {
if value, ok := Hello(); ok {
fmt.Println(value)
}
value := Hello()
fmt.Println(value)
}
Wont compile (due to the error multiple-value Hello() in single-value context) ... is there a way to make this syntax work for the function Hello()?
map is different because it is a built-in type and not a function. The 2 forms of accessing an element of a map is specified by the Go Language Specification: Index Expressions and backed by the compiler.
With functions you can't do this. If a function has 2 return values, you have to "expect" both of them or none at all.
However you are allowed to assign any of the return values to the Blank identifier:
s, b := Hello() // Storing both of the return values
s2, _ := Hello() // Storing only the first
_, b3 := Hello() // Storing only the second
You can also choose not to store any of the return values:
Hello() // Just executing it, but storing none of the return values
Note: you could also assign both of the return values to the blank identifier, although it has no use (other than validating that it has exactly 2 return values):
_, _ = Hello() // Storing none of the return values; note the = instead of :=
You can also try these on the Go Playground.
Helper function
If you use it many times and you don't want to use the blank identifier, create a helper function which discards the 2nd return value:
func Hello2() string {
s, _ := Hello()
return s
}
And now you can do:
value := Hello2()
fmt.Println(value)
Go 1.18 generics update: Go 1.18 adds generics support, it is now possible to write a generic First() function which discards the second (or any further) return values:
func First[T any](first T, _ ...any) T {
return first
}
This is available in github.com/icza/gog, as gog.First() (disclosure: I'm the author).
Using it:
value := First(Hello())
fmt.Println(value)
In addition to the explanation of #icza:
I don't recommend using a helper function there. Especially if the Hello function is your own function.
However, if you can't control it, then it's fine to use a helper.
If it's your own function, it's better to change the signature of your function. Probably, you made a design mistake somewhere.
You can also do this:
package main
import "fmt"
func Hello() (string, bool) {
return "hello", true
}
func main() {
// Just move it one line above: don't use a short-if
value, ok := Hello()
if ok {
fmt.Println(value)
}
}

Error handling of executed slice of functions in Go

I must run an unknown number of functions in a for cycle and I want to create meaningful errors when something goes wrong (when error returns from one of them)
Here some code:
package storage
import (
"github.com/attilasatan/ankara/engine/indexer"
)
type NewHandler func(*indexer.Document) error
var NewHandlers []NewHandler
func AppendNewHandler(handler NewHandler) {
NewHandlers = append(NewHandlers, handler)
}
func New(document *indexer.Document) (err error) {
for i, handler := range NewHandlers {
err = handler(document)
if err != nil {
err = errors.New(`New Handler error at index ` + string(i) + `
original:
` + err.Error())
return
}
}
return
}
This is my solution for error handling but i don't feel comfortable with it because I only return the index of the function that I executed.
My question is. Can I collect more information about the function that returned not nil error.
Also any kind of advises would be appreciated.
Use a struct that contains the func and any metadata instead of just a func. Something like this.
type NewHandler struct {
Handler func(*indexer.Document) error
Data string // or whatever data
}
Also make sure your slice holds pointers because go is pass-by-value.
var NewHandlers []*NewHandler
Then when you for loop, it goes like this.
for i, handler := range NewHandlers {
err = handler.Handler(document)
....
And you can include your Data in the error.

Return map like 'ok' in Golang on normal functions

In Go, the following works (note one use of the map has one return, the other has two returns)
package main
import "fmt"
var someMap = map[string]string { "some key": "hello" }
func main() {
if value, ok := someMap["some key"]; ok {
fmt.Println(value)
}
value := someMap["some key"]
fmt.Println(value)
}
However, I have no idea how to do this same thing with my own function. Is it possible to have similar behavior with an optional return like map?
For example:
package main
import "fmt"
func Hello() (string, bool) {
return "hello", true
}
func main() {
if value, ok := Hello(); ok {
fmt.Println(value)
}
value := Hello()
fmt.Println(value)
}
Wont compile (due to the error multiple-value Hello() in single-value context) ... is there a way to make this syntax work for the function Hello()?
map is different because it is a built-in type and not a function. The 2 forms of accessing an element of a map is specified by the Go Language Specification: Index Expressions and backed by the compiler.
With functions you can't do this. If a function has 2 return values, you have to "expect" both of them or none at all.
However you are allowed to assign any of the return values to the Blank identifier:
s, b := Hello() // Storing both of the return values
s2, _ := Hello() // Storing only the first
_, b3 := Hello() // Storing only the second
You can also choose not to store any of the return values:
Hello() // Just executing it, but storing none of the return values
Note: you could also assign both of the return values to the blank identifier, although it has no use (other than validating that it has exactly 2 return values):
_, _ = Hello() // Storing none of the return values; note the = instead of :=
You can also try these on the Go Playground.
Helper function
If you use it many times and you don't want to use the blank identifier, create a helper function which discards the 2nd return value:
func Hello2() string {
s, _ := Hello()
return s
}
And now you can do:
value := Hello2()
fmt.Println(value)
Go 1.18 generics update: Go 1.18 adds generics support, it is now possible to write a generic First() function which discards the second (or any further) return values:
func First[T any](first T, _ ...any) T {
return first
}
This is available in github.com/icza/gog, as gog.First() (disclosure: I'm the author).
Using it:
value := First(Hello())
fmt.Println(value)
In addition to the explanation of #icza:
I don't recommend using a helper function there. Especially if the Hello function is your own function.
However, if you can't control it, then it's fine to use a helper.
If it's your own function, it's better to change the signature of your function. Probably, you made a design mistake somewhere.
You can also do this:
package main
import "fmt"
func Hello() (string, bool) {
return "hello", true
}
func main() {
// Just move it one line above: don't use a short-if
value, ok := Hello()
if ok {
fmt.Println(value)
}
}

How to get the reflect.Type of an interface?

In order to determine whether a given type implements an interface using the reflect package, you need to pass a reflect.Type to reflect.Type.Implements(). How do you get one of those types?
As an example, trying to get the type of an uninitialized error (interface) type does not work (it panics when you to call Kind() on it)
var err error
fmt.Printf("%#v\n", reflect.TypeOf(err).Kind())
Do it like this:
var err error
t := reflect.TypeOf(&err).Elem()
Or in one line:
t := reflect.TypeOf((*error)(nil)).Elem()
Even Shaws response is correct, but brief. Some more details from the reflect.TypeOf method documentation:
// As interface types are only used for static typing, a common idiom to find
// the reflection Type for an interface type Foo is to use a *Foo value.
writerType := reflect.TypeOf((*io.Writer)(nil)).Elem()
fileType := reflect.TypeOf((*os.File)(nil)).Elem()
fmt.Println(fileType.Implements(writerType))
For googlers out there I just ran into the dreaded scannable dest type interface {} with >1 columns (XX) in result error.
Evan Shaw's answer did not work for me. Here is how I solved it. I am also using the lann/squirrel library, but you could easily take that out.
The solution really isn't that complicated, just knowing the magic combination of reflect calls to make.
The me.GetSqlx() function just returns an instance to *sqlx.DB
func (me *CommonRepo) Get(query sq.SelectBuilder, dest interface{}) error {
sqlst, args, err := query.ToSql()
if err != nil {
return err
}
// Do some reflection magic so that Sqlx doesn't hork on interface{}
v := reflect.ValueOf(dest)
return me.GetSqlx().Get(v.Interface(), sqlst, args...)
}
func (me *CommonRepo) Select(query sq.SelectBuilder, dest interface{}) error {
sqlst, args, err := query.ToSql()
if err != nil {
return err
}
// Do some reflection magic so that Sqlx doesn't hork on interface{}
v := reflect.ValueOf(dest)
return me.GetSqlx().Select(v.Interface(), sqlst, args...)
}
Then to invoke it you can do:
func (me *myCustomerRepo) Get(query sq.SelectBuilder) (rec Customer, err error) {
err = me.CommonRepo.Get(query, &rec)
return
}
func (me *myCustomerRepo) Select(query sq.SelectBuilder) (recs []Customer, err error) {
err = me.CommonRepo.Select(query, &recs)
return
}
This allows you to have strong types all over but have all the common logic in one place (CommonRepo in this example).

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