proposal: fmt: use "%##v" "%++v" to format a value with indented-multi-lines style string

[vipally]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go 1.9

Does this issue reproduce with the latest release?

yes

Current custom go data printing style "%#v" show all data in a single line, which is not easy to read, especiall in a debugging process.
"%#v":
fmt_test.Z{G:fmt_test.Y{D:fmt_test.X{A:123, B:""b" = 1"}, E:[]int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12}, F:[2]string{"aaa", "bbb"}}, H:"zzz", I:[]string{"c:\x\y\z", "d:\a\b\c"}, J:map[string]int{"abc":456}}

I have made a patch to use "%##v" "%++v" to format a value with indented-multi-lines style string to extends "%#v" "%+v".
I expect this feature can be accepted by go team, and it will be my pleasure to contribute this patch to go repository.

Branch
Example

here is the expected example

func ExamplePrintf_flagV() {
	type X struct {
		A int
		B string
	}
	type Y struct {
		D X
		E []int
		F [2]string
	}
	type Z struct {
		G Y
		H string
		I []string
		J map[string]int
	}
	var z = Z{
		G: Y{
			D: X{
				A: 123,
				B: `"b" = 1`,
			},
			E: []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12},
			F: [2]string{
				`aaa`,
				`bbb`,
			},
		},
		H: `zzz`,
		I: []string{
			`c:\x\y\z`,
			`d:\a\b\c`,
		},
		J: map[string]int{
			`abc`: 456,
		},
	}
	fmt.Printf("-------\n\"%%v\":\n%v\n", z)
	fmt.Printf("-------\n\"%%#v\":\n%#v\n", z)
	fmt.Printf("-------\n\"%%##\"v:\n%##v\n", z)
	fmt.Printf("-------\n\"%%+v\":\n%+v\n", z)
	fmt.Printf("-------\n\"%%++v\":\n%++v\n", z)

	// Output:
	// -------
	// "%v":
	// {{{123 "b" = 1} [1 2 3 4 5 6 7 8 9 10 11 12] [aaa bbb]} zzz [c:\x\y\z d:\a\b\c] map[abc:456]}
	// -------
	// "%#v":
	// fmt_test.Z{G:fmt_test.Y{D:fmt_test.X{A:123, B:"\"b\" = 1"}, E:[]int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12}, F:[2]string{"aaa", "bbb"}}, H:"zzz", I:[]string{"c:\\x\\y\\z", "d:\\a\\b\\c"}, J:map[string]int{"abc":456}}
	// -------
	// "%##"v:
	// fmt_test.Z{
	//     G: fmt_test.Y{
	//         D: fmt_test.X{
	//             A: 123,
	//             B: `"b" = 1`,
	//         },
	//         E: []int{
	//             1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
	//             11, 12,
	//         },
	//         F: [2]string{
	//             `aaa`,
	//             `bbb`,
	//         },
	//     },
	//     H: `zzz`,
	//     I: []string{
	//         `c:\x\y\z`,
	//         `d:\a\b\c`,
	//     },
	//     J: map[string]int{
	//         `abc`: 456,
	//     },
	// }
	// -------
	// "%+v":
	// {G:{D:{A:123 B:"b" = 1} E:[1 2 3 4 5 6 7 8 9 10 11 12] F:[aaa bbb]} H:zzz I:[c:\x\y\z d:\a\b\c] J:map[abc:456]}
	// -------
	// "%++v":
	// {
	//     G: {
	//         D: {
	//             A: 123
	//             B: "b" = 1
	//         }
	//         E: [
	//             1 2 3 4 5 6 7 8 9 10
	//             11 12
	//         ]
	//         F: [
	//             aaa
	//             bbb
	//         ]
	//     }
	//     H: zzz
	//     I: [
	//         c:\x\y\z
	//         d:\a\b\c
	//     ]
	//     J: map[
	//         abc: 456
	//     ]
	// }
}

Branch
Example

[ianlancetaylor]

Personally I think this is more suitable for an external packages, such as https://godoc.org/github.com/kr/pretty.

[ianlancetaylor]

Thanks for writing it up, though.

[martisch]

I would favor this functionallity having its own package instead of making the fmt flag logic and functionality more complex too.

I would also recommend to avoid giving different meaning to multiple occurences of format flags but to use a new one instead to signify pretty printing using multiple lines.

[vipally]

Printing go value for debugging becomes gopher's everyday life, I don't think it's a good idea to make it a third-party package.

Put a new flag to to signify pretty printing is a good idea.
eg '@' (maybe looks like glasses, yet)

[robpike]

I too agree it should be outside. The fmt package is complex enough already, and the features you ask for will complicate it a great deal.

I will make this a proposal so it gets a full review.

[rsc]

The correct link for the code appears to be https://github.com/vipally/go/tree/ally_master/src/fmt, which uses %@#v and %@+v, I assume because the fmt.Formatter API cannot express two of a given flag.

Everyone who has commented has said that this can be best done as a separate package, or multiple separate packages if different uses need different rules. That seems right to me too.

Closing after discussion with proposal-review (everyone agreed).