How Do You Assign A Struct Field Variable In Multiple Assignment Statement

Question

How do you assign a struct field variable in a multiple assignment statement? Please refer to code below.

type TestMultipleReturns struct {
    value string
}

func (t *TestMultipleReturns) TestSomething() {
    someMap := make(map[string]string)
    someMap["Test"] = "world"
    t.value, exists := someMap["doesnotexist"] // fails

    // works, but do I really need a 2nd line?
    tmp, exists := someMap["doesnotexist"] 
    t.value = tmp

    if exists == false {
        fmt.Println("t.value is not set.")
    } else {
        fmt.Println(t.value)
    }
}

score 6 · Accepted Answer · 2016-11-29 17:55:37Z

6

Short variable declaration do not support assigning struct receiver properties; they are omitted from the spec definition:

Unlike regular variable declarations, a short variable declaration may redeclare variables provided they were originally declared earlier in the same block (or the parameter lists if the block is the function body) with the same type, and at least one of the non-blank variables is new.

The fix is to define exists before the assignment and not use short variable declarations:

type TestMultipleReturns struct {
    value string
}

func (t *TestMultipleReturns) TestSomething() {
    someMap := make(map[string]string)
    someMap["Test"] = "world"
    var exists bool
    t.value, exists = someMap["doesnotexist"]

    if exists == false {
        fmt.Println("t.value is not set.")
    } else {
        fmt.Println(t.value)
    }
}

edited Nov 29, 2016 at 17:55

answered Nov 29, 2016 at 17:52

user142162

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3 Comments

PressingOnAlways Over a year ago

Thanks, though it is still 2 lines. This seems inconsistent with the behavior where you can have 1 variable already defined and 1 variable not defined in undefined, defined := MultipleReturnCall(). Quirks of Go!

user142162 Over a year ago

@PressingOnAlways: If you think this was just an oversight by the spec authors, I would encourage you to submit a proposal for the change.

Paul Buis Over a year ago

I'm just happy it doesn't require 3 lines!

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