为什么在Golang中使用fmt.Println（slice）打印切片不同

Question

code A:

package main

import "fmt"

func main() {
    slice := IntSlice{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
    fmt.Println(slice)
}

type IntSlice []int

output A:

[0 1 2 3 4 5 6 7 8 9]

code B:

package main

import "fmt"

func main() {
    slice := IntSlice{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
    fmt.Println(slice)
}

type IntSlice []int

func (slice IntSlice) Error() string { return "this is called." }

output B:

this is called.

why the behavior of fmt.Println(slice) is different for these two codes( A and B)?
or why fmt.Println(slice) calls slice.Error() automatically?

Answer 1

This is documented as part of the behaviour of fmt (emphasis mine):

Except when printed using the verbs %T and %p, special formatting considerations apply for operands that implement certain interfaces. In order of application:

1. If the operand is a reflect.Value, the operand is replaced by the concrete value that it holds, and printing continues with the next rule.

2. If an operand implements the Formatter interface, it will be invoked. Formatter provides fine control of formatting.

3. If the %v verb is used with the # flag (%#v) and the operand implements the GoStringer interface, that will be invoked.

   If the format (which is implicitly %v for Println etc.) is valid for a string (%s %q %v %x %X), the following two rules apply:

4. If an operand implements the error interface, the Error method will be invoked to convert the object to a string, which will then be formatted as required by the verb (if any).

5. If an operand implements method String() string, that method will be invoked to convert the object to a string, which will then be formatted as required by the verb (if any).

For compound operands such as slices and structs, the format applies to the elements of each operand, recursively, not to the operand as a whole. Thus %q will quote each element of a slice of strings, and %6.2f will control formatting for each element of a floating-point array.

The fmt package sees that slice implements error and prints value.Error(), rather than iterating over the slice and applying formatting to each element.