Assigning to map in golang

Question

In the following go snippet, what am I doing wrong?

type Element interface{}

func buncode(in *os.File) (e Element) {
    <snip>
    e = make(map[string]interface{})
    for {
        var k string = buncode(in).(string)
        v := buncode(in)
        e[k] = v
    }
    <snip>
}

Compiling gives me this error:

gopirate.go:38: invalid operation: e[k] (index of type Element)

Double ewe T eff?

peterSO · Accepted Answer · 2011-08-12 14:37:48Z

3

In the buncode function you declare e Element, where type e Element interface{}. The variable e is a scalar value, which you are trying to index.

Types

The static type (or just type) of a variable is the type defined by its declaration. Variables of interface type also have a distinct dynamic type, which is the actual type of the value stored in the variable at run-time. The dynamic type may vary during execution but is always assignable to the static type of the interface variable. For non-interface types, the dynamic type is always the static type.

The static type of e is Element, a scalar. The dynamic type of e is map[string]interface{}.

Here's a revised, compilable version of your code.

type Element interface{}

func buncode(in *os.File) (e Element) {
    m := make(map[string]interface{})
    for {
        var k string = buncode(in).(string)
        v := buncode(in)
        m[k] = v
    }
    return m
}

Why are you making the recursive calls to buncode?

edited Aug 12, 2011 at 14:37

answered Aug 12, 2011 at 14:01

peterSO

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4 Comments

Matt Joiner Over a year ago

But I set it's type with the assignment from the make? I gathered from the spec on type assertions that the type would now be that of the "larger" type I'd assigned to it.

Matt Joiner Over a year ago

The recursive calls fetch the nested values for m. If you're interested, and have some idiomatic go-isms to propose, take a look here: code.google.com/p/erutor/source/browse/bencoding.py

SteveMcQwark Over a year ago

@Matt (1) - The compiler doesn't track the dynamic type of an interface{} value, even if it can be statically known. You must use a separate variable with the correct type.

milosgajdos Over a year ago

Are you sure this is going to compile ? You specify Element as a return value and you are returning map. I'm just curious if I'm missing something here.

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