Idiomatic way to implement generic functions in Go

Question

Let's say I want to write a function to check whether a predicate is matched for an element in a slice:

func IsIn(array []T, pred func(elt T) bool) bool {
    for _, obj := range array {
        if pred(obj) { return true;}
    }
    return false;
}

Obviously, the previous code won't compile, since T does not exist. I can replace it with some interface{} like this:

func IsIn(array[]interface{}, pred func(elt interface{}) bool) bool {
    ...
}

As I am happy to let the predicate perform the casting:

IsIn([]interface{}{1,2,3,4}, func(o interface{}) {return o.(int) == 3; });

But then, the function won't accept any array which is not of type []interface{}:

IsIn([]int{1,2,3,4}, func(o interface{}) { return o.(int) == 3; }) // DO NOT COMPILE

And similarly:

func IsIn(arr interface, pred func(o interface{}) bool) bool {
    for _, o := range arr.([]interface{}) { ... }
}
IsIn([]int{1,2,3,4}, func(o interface{}) { return o.(int) == 3; }) // PANICS AT RUNTIME (cannot cast []int to []interface)

The other alternative is to have typed functions for each array type:

IsInInt(arr []int, pred func(i int) bool) { ... }
IsInStr(arr []string, pred func(s string) bool) { ... }
...

But it seems like a LOT of code duplication.

Has anyone come up with an nice way to deal with such situations ?

EDIT

Thanks to jnml's fantastic tips on Go reflection, I think I have found a nice way to express these patterns, by converting every 'iterable' to a channel:

func iter(obj interface{}) chan interface{} {
    c := make(chan interface{})
    v := reflect.ValueOf(obj)
    if (v.Kind() == reflect.Array || v.Kind() == reflect.Slice) {
        go func() {
            for i := 0; i < v.Len(); i++ {
                c<-v.Index(i).Interface()
            }
            close(c)
        }()
    } else if v.Kind() == reflect.Chan {
        go func() {
            x, ok := v.Recv()
            for ok {
                c<-x.Interface()
                x,ok = v.Recv()
            }
            close(c)
        }()
    } else if (... whatever iteration protocol you have ...) {
    } else {
        panic("Cannot iterate !")
    }
    return c;
}

With my initial example rewritten using it on the playground.

Thanks a lot to jnml and ANisus for helping out !

Answer 1

For example:

package main

import (
        "fmt"
        "reflect"
)

func checkSlice(slice interface{}, predicate func(reflect.Value) bool) bool {
        v := reflect.ValueOf(slice)
        if v.Kind() != reflect.Slice {
                panic("not a slice")
        }

        for i := 0; i < v.Len(); i++ {
                if predicate(v.Index(i)) {
                        return true
                }
        }

        return false
}

func main() {
        a := []int{1, 2, 3, 4, 42, 278, 314}
        fmt.Println(checkSlice(a, func(v reflect.Value) bool { return v.Int() == 42 }))

        b := []float64{1.2, 3.4, -2.5}
        fmt.Println(checkSlice(b, func(v reflect.Value) bool { return v.Float() > 4 }))
}

Playground

---

Output:

true
false

Answer 2

I can't say if it is the most idiomatic. But one idiomatic solution would be to do like in the sort package; to define an interface for the array:

type Interface interface {
    Len() int
    Equal(i int, v interface{}) bool
}

func IsIn(array Interface, value interface{}) bool {
    for i := 0; i < array.Len(); i++ {
    if array.Equal(i, value) {
            return true
        }
    }
    return false;
}

As long as your array implements this interface, you can use IsIn().

Working example can he found here