toml: Provide a way to set default values when decoding TOML

[syobocat]

Describe the feature

Currently, this code:

import toml

struct Object {
	text    string
	integer int
}

fn main() {
	null_toml := toml.parse_text('')!
	object := null_toml.decode[Object]()!
	println(object.text)
	println(object.integer)
}

returns

toml.Any(toml.Null{})
0

and I believe that there is no way to change this behavior (You can use .reflect() instead of .decode() to get an empty string instead of toml.Any(toml.Null{}), though).

I think it is very useful if Vlang has a way to provide default values:

option 1) by an attribute, like

struct Object {
	text    string [toml_default: 'hello']
	integer int    [toml_default: 123]
}

or

option 2) by respecting the default value of the field, like

struct Object {
	text    string = 'hello'
	integer int    = 123
}

Use Case

This should be useful, for example, when building an application that uses TOML format for its configuration file.

Proposed Solution

No response

Other Information

json.decode() respects the default value of the field:

import json

struct Object {
	text    string = 'hello'
	integer int    = 123
}

fn main() {
	object := json.decode(Object, '{}')!
	println(object.text)
	println(object.integer)
}

hello
123

Acknowledgements

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

Version used

V 0.4.7 620e064

Environment details (OS name and version, etc.)

V full version: V 0.4.7 e9c9580.620e064
OS: macos, macOS, 14.6.1, 23G93
Processor: 8 cpus, 64bit, little endian, Apple M3

getwd: /Users/syobon
vexe: /Users/syobon/.v/v
vexe mtime: 2024-09-07 12:37:14

vroot: OK, value: /Users/syobon/.v
VMODULES: OK, value: /Users/syobon/.vmodules
VTMP: OK, value: /tmp/v_501

Git version: git version 2.46.0
Git vroot status: weekly.2024.36-18-g620e0641 (2 commit(s) behind V master)
.git/config present: true

CC version: Apple clang version 15.0.0 (clang-1500.3.9.4)
thirdparty/tcc status: thirdparty-macos-arm64 713692d4

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