[go] Type converting slices of interfaces

I'm curious why Go does't implicitly convert []T to []interface{} when it will implicitly convert T to interface{}. Is there something non-trivial about this conversion that I'm missing?

Example:

func foo([]interface{}) { /* do something */ }

func main() {
    var a []string = []string{"hello", "world"}
    foo(a)
}

go build complains

cannot use a (type []string) as type []interface {} in function argument

And if I try to do it explicitly, same thing: b := []interface{}(a) complains

cannot convert a (type []string) to type []interface {}

So every time I need to do this conversion (which seems to come up a lot), I've been doing something like this:

b = make([]interface{}, len(a), len(a))
for i := range a {
    b[i] = a[i]
}

Is there a better way to do this, or standard library functions to help with these conversions? It seems kind of silly to write 4 extra lines of code every time I want to call a function that can take a list of e.g. ints or strings.

Convert interface{} into any type.

Syntax:

result := interface.(datatype)

Example:

var employee interface{} = []string{"Jhon", "Arya"}
result := employee.([]string)   //result type is []string.

The thing you are missing is that T and interface{} which holds a value of T have different representations in memory so can't be trivially converted.

A variable of type T is just its value in memory. There is no associated type information (in Go every variable has a single type known at compile time not at run time). It is represented in memory like this:

• value

An interface{} holding a variable of type T is represented in memory like this

• pointer to type T
• value

So coming back to your original question: why go does't implicitly convert []T to []interface{}?

Converting []T to []interface{} would involve creating a new slice of interface {} values which is a non-trivial operation since the in-memory layout is completely different.

In case you need more shorting your code, you can creating new type for helper

type Strings []string

func (ss Strings) ToInterfaceSlice() []interface{} {
    iface := make([]interface{}, len(ss))
    for i := range ss {
        iface[i] = ss[i]
    }
    return iface
}

then

a := []strings{"a", "b", "c", "d"}
sliceIFace := Strings(a).ToInterfaceSlice()

Here is the official explanation: https://github.com/golang/go/wiki/InterfaceSlice

var dataSlice []int = foo()
var interfaceSlice []interface{} = make([]interface{}, len(dataSlice))
for i, d := range dataSlice {
    interfaceSlice[i] = d
}

Try interface{} instead. To cast back as slice, try

func foo(bar interface{}) {
    s := bar.([]string)
    // ...
}