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loopsgostructcomposite-literals

Golang for loop with multiple brackets

I've stumbled upon this repository,

https://github.com/prometheus-community/jiralert/blob/a0f0e80e575e71cbf7db565d3296a3a984282dff/pkg/config/config_test.go#L148

The for loop has multiple brackets:

for _, test := range []struct {
        missingField string
        errorMessage string
    }{
        {"Name", "missing name for receiver"},
    (...)
    } {

        fields := removeFromStrSlice(mandatory, test.missingField)

    (...)
        }
        configErrorTestRunner(t, config, test.errorMessage)
    }

I haven't been able to find anything about this in the go documentation, what is this construct?

Solution

  • The first bracket pair is part of the struct type definition which has the syntax:

    struct{
    field1 type1
    field2 type2
    ...
}

    The second pair is part of the composite literal value, creating a value for the slice of this struct. It has the syntax of:

    []elementType{value1, value2, ...}

    The inner, embedded brackets are part of the composite literals creating the struct values of the slice. The type is omitted (it's known from the slice type), so each {field1Value, fieldValue2...} is a struct value.

    The third pair defines the block of the for statement. The for statement iterates over the elements of the slice defined by the mentioned composite literal.