Add indent() method to string type

[kbkpbot]

Describe the feature

Add a indent() method to the V string type that formats strings containing curly braces {, } with proper indentation. The method should automatically:

• Insert line breaks after opening braces { and before closing braces }
• Apply configurable indentation levels
• Maintain proper nesting for multiple brace levels
• Preserve content outside braces

May related to PR #25857

Use Case

Currently, V's string type lacks built-in support for brace-based formatting. This method would be particularly useful for:

• Generating properly formatted code
• Creating readable JSON/XML-like outputs
• Formatting configuration files
• Pretty-printing nested data structures
• Use for auto_str(), then we will not need .indent_str() anymore

Proposed Solution

// As a method on the string type
fn (s string) indent(indent_spaces int) string

or

fn (s string) indent(indent int, options ...FormatOption) string

Other Information

a dump can be called as:

dump(output_string.indent(4))

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change

Version used

V 0.4.12 ff6413a.dd066bb

Environment details (OS name and version, etc.)

V full version	V 0.4.12 ff6413a.dd066bb
OS	linux, Ubuntu 24.04.3 LTS
Processor	8 cpus, 64bit, little endian, Intel(R) Core(TM) i7-9700 CPU @ 3.00GHz
Memory	12.37GB/15.51GB
V executable	/media/HD/github/kbkpbot/v/v
V last modified time	2025-12-07 00:07:42
V home dir	OK, value: /media/HD/github/kbkpbot/v
VMODULES	OK, value: /home/mars/.vmodules
VTMP	OK, value: /tmp/v_1000
Current working dir	OK, value: /media/HD/github/kbkpbot/v
Git version	git version 2.43.0
V git status	weekly.2025.46-151-gdd066bbd-dirty
.git/config present	true
cc version	cc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
gcc version	gcc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
clang version	Ubuntu clang version 18.1.3 (1ubuntu1)
tcc version	tcc version 0.9.28rc 2025-02-13 HEAD@f8bd136d (x86_64 Linux)
tcc git status	thirdparty-linux-amd64 696c1d84
emcc version	N/A
glibc version	ldd (Ubuntu GLIBC 2.39-0ubuntu8.6) 2.39

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[kbkpbot]

an example usage:

'fn main() {println("Hello") for i in 0..5 {println(i) if i == 3 {break}}}'.indent(4)

will output:

fn main() {
    println("Hello")
    for i in 0..5 {
        println(i)
        if i == 3 {
            break
        }
    }
}

[JalonSolov]

If you want to print "properly" formatted, you'll need an option to use tabs instead of spaces. Could be a boolean in your FormatOptions struct.

However, if you do allow tabs vs spaces, the name of the first parameter should be just indent or indent_num.

[kbkpbot]

@[params]
pub struct IndentParam {
pub mut:
    open_char  string = '{'   // Character that triggers increased indentation
    close_char string = '}'   // Character that triggers decreased indentation
    indent_char string = ' '  // Character used for indentation (space or tab)
    indent_count int = 4     // Number of indent_char per indentation level
}

// Method on string type
fn (s string) indent(params IndentParam) string

[jorgeluismireles]

Looks helpful. Do you plan to reuse v fmt code ? Since is very similar to:

$ echo "fn main(){ a:=1 }" | v fmt
fn main() {
	a := 1
}

JSON call it prettify it also sounds good.

[kbkpbot]

I am developing a PR attempting to move the previous backend auto_str() logic (such as in cgen) to the frontend, PR #25857

One issue I've encountered is alignment of fields within structs, especially for nested struct displays that require proper indentation.

[a.v:17] x: MyS{
    a: 0
    b: 0
    son: Son{
        k: ''
        m: ''
    }
}

This requires correct indentation within nested structs. My idea is that the internal str() function shouldn't need to handle indentation itself. Instead, an external indent function should handle it, simplifying the design of the internal str() function.

For example, the internal str() would generate a string like this:

MyS{
    a: 0
    b: 0
    son: Son{
    k: ''
    m: ''
}
}

[kbkpbot]

Currently, the approach in cgen is to split the struct's string formatting into two methods: .str() and .indent_str(indent_count int). In .str(), it simply calls indent_str(0), while nested structs require calling indent_str(x).

This approach necessitates implementing two methods for every struct: .str() and .indent_str(), which presents several issues.

[kbkpbot]

A sample implementation :

module main

struct Son {
        a int
        b string
}

struct MyS {
        c int
        d string
        e Son
}

fn (s Son) str() string {
        return 'Son {\n    a: ${s.a}\n    b: \'${s.b}\'\n}'
}

fn main() {
        s := Son{}
        dump(s)
        x := MyS{}
        dump(x)
}

Note that Son has a .str() method, but without a .indent_str() method.

run this code, generated following result:

$ v run struct.v
[struct.v:20] s: Son {
    a: 0
    b: ''
}
[struct.v:22] x: MyS{
    c: 0
    d: ''
    e: Son {
    a: 0
    b: ''
}
}

Direct output Son generated a correct result, but for nested struct MyS, generated wrong indent result.