Looping over slice of aliased(user defined) types gives type before aliasing

Question

I'm trying to loop over slice of user defined types (in example below these are aliased int), but range produces values of type int, instead of MyInt as I would expect. Casting inside 'if' helps for sure, but I would like to understand why range does not produces values of type MyInt.

package main

import (
    "fmt"
)

type MyInt int

const (
    MYINT00 MyInt = iota
    MYINT01
)

func main() {
    var myInt02 MyInt = 2
    myInts := []MyInt{MYINT00, MYINT01}
    for i := range myInts {
        if i == myInt02 {
            fmt.Println("same")
        }
    }
}

Playground: https://play.golang.org/p/nb77pvTMdkW

Error:

prog.go:18:8: invalid operation: i == myInt02 (mismatched types int and MyInt)

Then I thought that problem can be related to consts and iota, so I used variables declared in function - didn't change anything.

https://play.golang.org/p/0fVRhBtvlOL https://play.golang.org/p/pioDSU4oJdP

I haven't found any info in Effective Go/other questions. If anybody has some input on that, please share!

Here is your cure for a broader understanding of the use-case: https://stackoverflow.com/questions/20185511/golang-range-references-instead-values — vahdet, Jun 15 '18 at 14:52

peterSO · Accepted Answer · 2018-06-15T17:12:20.783

The Go Programming Language Specification

For statements with range clause
Range expression                          1st value          2nd value

array or slice  a  [n]E, *[n]E, or []E    index    i  int    a[i]       E

i is the range index, an int, not the range value.

For example, fixing your code to use the range value,

package main

import (
    "fmt"
)

type MyInt int

const (
    MYINT00 MyInt = iota
    MYINT01
)

func main() {
    var myInt02 MyInt = 2
    myInts := []MyInt{MYINT00, MYINT01}
    for _, v := range myInts {
        if v == myInt02 {
            fmt.Println("same")
        }
    }
}

Looping over slice of aliased(user defined) types gives type before aliasing

1 Answers1