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doc: Compile-time reflection section with *new features* (#5962)

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@ -79,7 +79,7 @@ you can do in V.
* [Debugging generated C code](#debugging-generated-c-code)
* [Conditional compilation](#conditional-compilation)
* [Compile time pseudo variables](#compile-time-pseudo-variables)
* [Reflection via codegen](#reflection-via-codegen)
* [Compile-time reflection](#compile-time-reflection)
* [Limited operator overloading](#limited-operator-overloading)
* [Inline assembly](#inline-assembly)
* [Translating C/C++ to V](#translating-cc-to-v)
@ -2268,26 +2268,38 @@ that does nothing.
`if _unlikely_(bool expression) {` similar to `_likely_(x)`, but it hints that
the boolean expression is highly improbable. In the JS backend, that does nothing.
## Reflection via codegen
<a id='Reflection via codegen'>
## Compile-time reflection
Having built-in JSON support is nice, but V also allows you to create efficient
serializers for any data format:
serializers for any data format. V has compile-time `if` and `for` constructs:
```v
// TODO: not implemented yet
struct User {
name string
age int
}
// Note: T should be passed a struct name only
fn decode<T>(data string) T {
mut result := T{}
for field in T.fields {
if field.typ == 'string' {
result.$field = get_string(data, field.name)
} else if field.typ == 'int' {
result.$field = get_int(data, field.name)
// compile-time `for` loop
// T.fields gives an array of a field metadata type
$for field in T.fields {
$if field.Type is string {
// $(string_expr) produces an identifier
result.$(field.name) = get_string(data, field.name)
} else $if field.Type is int {
result.$(field.name) = get_int(data, field.name)
}
}
return result
}
// generates to:
// `decode<User>` generates:
fn decode_User(data string) User {
mut result := User{}
result.name = get_string(data, 'name')