Golang for loop with multiple brackets

Question

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?

Answer 1

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.

Answer 2

To clean up the code for better readability and understanding. Whatever you gave was equivalent to:

type TestStruct struct {
  missingField string
  errorMessage string
}

testCase := TestStruct {
  {
    missingField: "Name",
    errorMessage: "missing name for receiver",
  }
  (...)
}


for _, test := range(testCase) {

  fields := removeFromStrSlice(mandatory, test.missingField)
}

configErrorTestRunner(t, config, test.errorMessage) is probably from a parent test func