Struct value_bag::ValueBag

source ·

pub struct ValueBag<'v> { /* private fields */ }

Expand description

A dynamic structured value.

Capturing values

There are a few ways to capture a value:

Using the ValueBag::capture_* and ValueBag::from_* methods.
Using the standard From trait.
Using the Fill API.

Using the `ValueBag::capture_*` methods

ValueBag offers a few constructor methods that capture values of different kinds. These methods require a T: 'static to support downcasting.

use value_bag::ValueBag;

let value = ValueBag::capture_debug(&42i32);

assert_eq!(Some(42), value.to_i64());

Capturing a value using these methods will retain type information so that the contents of the bag can be serialized using an appropriate type.

For cases where the 'static bound can’t be satisfied, there’s also a few constructors that exclude it.

use value_bag::ValueBag;

let value = ValueBag::from_debug(&42i32);

assert_eq!(None, value.to_i64());

These ValueBag::from_* methods are lossy though and ValueBag::capture_* should be preferred.

Using the standard `From` trait

Primitive types can be converted into a ValueBag using the standard From trait.

use value_bag::ValueBag;

let value = ValueBag::from(42i32);

assert_eq!(Some(42), value.to_i64());

Using the `Fill` API

The fill module provides a way to bridge APIs that may not be directly compatible with other constructor methods.

The Fill trait is automatically implemented for closures, so can usually be used in libraries that can’t implement the trait themselves.

use value_bag::{ValueBag, fill::Slot};

let value = ValueBag::from_fill(&|slot: Slot| {
    #[derive(Debug)]
    struct MyShortLivedValue;

    slot.fill_debug(&MyShortLivedValue)
});

assert_eq!("MyShortLivedValue", format!("{:?}", value));

The trait can also be implemented manually:

use value_bag::{ValueBag, Error, fill::{Slot, Fill}};

struct FillDebug;

impl Fill for FillDebug {
    fn fill(&self, slot: Slot) -> Result<(), Error> {
        slot.fill_debug(&42i32 as &dyn Debug)
    }
}

let value = ValueBag::from_fill(&FillDebug);

assert_eq!(None, value.to_i64());

Inspecting values

Once you have a ValueBag there are also a few ways to inspect it:

Using std::fmt
Using sval
Using serde
Using the ValueBag::visit method.
Using the ValueBag::to_* methods.
Using the ValueBag::downcast_ref method.

Using the `ValueBag::visit` method

The visit module provides a simple visitor API that can be used to inspect the structure of primitives stored in a ValueBag. More complex datatypes can then be handled using std::fmt, sval, or serde.

#[cfg(not(feature = "std"))] fn main() {}
#[cfg(feature = "std")]
use value_bag::{ValueBag, Error, visit::Visit};

// Implement some simple custom serialization
struct MyVisit(Vec<u8>);
impl<'v> Visit<'v> for MyVisit {
    fn visit_any(&mut self, v: ValueBag) -> Result<(), Error> {
        // Fallback to `Debug` if we didn't visit the value specially
        write!(&mut self.0, "{:?}", v).map_err(|_| Error::msg("failed to write value"))
    }

    fn visit_u64(&mut self, v: u64) -> Result<(), Error> {
        self.0.extend_from_slice(itoa_fmt(v).as_slice());
        Ok(())
    }

    fn visit_i64(&mut self, v: i64) -> Result<(), Error> {
        self.0.extend_from_slice(itoa_fmt(v).as_slice());
        Ok(())
    }

    fn visit_f64(&mut self, v: f64) -> Result<(), Error> {
        self.0.extend_from_slice(ryu_fmt(v).as_slice());
        Ok(())
    }

    fn visit_str(&mut self, v: &str) -> Result<(), Error> {
        self.0.push(b'\"');
        self.0.extend_from_slice(escape(v.as_bytes()));
        self.0.push(b'\"');
        Ok(())
    }

    fn visit_bool(&mut self, v: bool) -> Result<(), Error> {
        self.0.extend_from_slice(if v { b"true" } else { b"false" });
        Ok(())
    }
}

let value = ValueBag::from(42i64);

let mut visitor = MyVisit(vec![]);
value.visit(&mut visitor)?;

Using `std::fmt`

Any ValueBag can be formatted using the std::fmt machinery as either Debug or Display.

use value_bag::ValueBag;

let value = ValueBag::from(true);

assert_eq!("true", format!("{:?}", value));

Using `sval`

When the sval2 feature is enabled, any ValueBag can be serialized using sval. This makes it possible to visit any typed structure captured in the ValueBag, including complex datatypes like maps and sequences.

sval doesn’t need to allocate so can be used in no-std environments.

First, enable the sval2 feature in your Cargo.toml:

[dependencies.value-bag]
features = ["sval2"]

Then stream the contents of the ValueBag using sval.

use value_bag::ValueBag;

let value = ValueBag::from(42i64);
let json = sval_json::stream_to_string(value)?;

Using `serde`

When the serde1 feature is enabled, any ValueBag can be serialized using serde. This makes it possible to visit any typed structure captured in the ValueBag, including complex datatypes like maps and sequences.

serde needs a few temporary allocations, so also brings in the std feature.

First, enable the serde1 feature in your Cargo.toml:

[dependencies.value-bag]
features = ["serde1"]

Then stream the contents of the ValueBag using serde.

use value_bag::ValueBag;

let value = ValueBag::from(42i64);
let json = serde_json::to_string(&value)?;

Also see serde.rs for more examples of writing your own serializers.

Using the `ValueBag::to_*` methods

ValueBag provides a set of methods for attempting to pull a concrete value out. These are useful for ad-hoc analysis but aren’t intended for exhaustively serializing the contents of a ValueBag.

use value_bag::ValueBag;

let value = ValueBag::capture_display(&42u64);

assert_eq!(Some(42u64), value.to_u64());

Using the `ValueBag::downcast_ref` method

When a ValueBag is created using one of the capture_* constructors, it can be downcast back to its original value. This can also be useful for ad-hoc analysis where there’s a common possible non-primitive type that could be captured.

use value_bag::ValueBag;

let timestamp = now();
let value = ValueBag::capture_debug(&timestamp);

assert!(value.downcast_ref::<SystemTime>().is_some());

Working with sequences

The seq feature of value-bag enables utilities for working with values that are sequences. First, enable the seq feature in your Cargo.toml:

[dependencies.value-bag]
features = ["seq"]

A sequence captured with either sval or serde can have its elements extracted:

use value_bag::ValueBag;

let value = ValueBag::from_serde1(&[1.0, 2.0, 3.0]);

let seq = value.to_f64_seq::<Vec<Option<f64>>>().ok_or("not a sequence")?;

assert_eq!(vec![Some(1.0), Some(2.0), Some(3.0)], seq);

Struct value_bag::ValueBag

Implementations§

impl<'v> ValueBag<'v>

pub fn try_capture<T>(value: &'v T) -> Option<Self>where T: ?Sized + 'static,

pub fn to_u64(&self) -> Option<u64>

pub fn to_i64(&self) -> Option<i64>

pub fn to_u128(&self) -> Option<u128>

pub fn to_i128(&self) -> Option<i128>

pub fn to_f64(&self) -> Option<f64>

pub fn as_f64(&self) -> f64

pub fn to_bool(&self) -> Option<bool>

pub fn to_char(&self) -> Option<char>

pub fn to_borrowed_str(&self) -> Option<&'v str>

pub fn is_empty(&self) -> bool

pub fn is<T: 'static>(&self) -> bool

pub fn downcast_ref<T: 'static>(&self) -> Option<&T>

impl<'v> ValueBag<'v>

pub const fn from_fill<T>(value: &'v T) -> Selfwhere T: Fill,

impl<'v> ValueBag<'v>

pub fn capture_debug<T>(value: &'v T) -> Selfwhere T: Debug + 'static,

pub fn capture_display<T>(value: &'v T) -> Selfwhere T: Display + 'static,

pub const fn from_debug<T>(value: &'v T) -> Selfwhere T: Debug,

pub const fn from_display<T>(value: &'v T) -> Selfwhere T: Display,

pub const fn from_dyn_debug(value: &'v dyn Debug) -> Self

pub const fn from_dyn_display(value: &'v dyn Display) -> Self

impl<'v> ValueBag<'v>

pub fn visit(&self, visitor: impl Visit<'v>) -> Result<(), Error>

impl<'v> ValueBag<'v>

pub const fn empty() -> ValueBag<'v>

pub fn from_option(v: Option<impl Into<ValueBag<'v>>>) -> ValueBag<'v>

pub const fn from_u8(v: u8) -> ValueBag<'v>

pub const fn from_u16(v: u16) -> ValueBag<'v>

pub const fn from_u32(v: u32) -> ValueBag<'v>

pub const fn from_u64(v: u64) -> ValueBag<'v>

pub const fn from_usize(v: usize) -> ValueBag<'v>

pub const fn from_u128_ref(v: &'v u128) -> ValueBag<'v>

pub const fn from_i8(v: i8) -> ValueBag<'v>

pub const fn from_i16(v: i16) -> ValueBag<'v>

pub const fn from_i32(v: i32) -> ValueBag<'v>

pub const fn from_i64(v: i64) -> ValueBag<'v>

pub const fn from_isize(v: isize) -> ValueBag<'v>

pub const fn from_i128_ref(v: &'v i128) -> ValueBag<'v>

pub const fn from_f32(v: f32) -> ValueBag<'v>

pub const fn from_f64(v: f64) -> ValueBag<'v>

pub const fn from_bool(v: bool) -> ValueBag<'v>

pub const fn from_str(v: &'v str) -> ValueBag<'v>

pub const fn from_char(v: char) -> ValueBag<'v>

pub const fn by_ref<'u>(&'u self) -> ValueBag<'u>

Trait Implementations§

impl<'v> Clone for ValueBag<'v>

fn clone(&self) -> ValueBag<'v>

fn clone_from(&mut self, source: &Self)

impl<'v> Debug for ValueBag<'v>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

impl<'v> Display for ValueBag<'v>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

impl<'v, 'u> From<&'v &'u str> for ValueBag<'v>where 'u: 'v,

fn from(v: &'v &'u str) -> Self

impl<'v> From<&'v ()> for ValueBag<'v>

fn from(_: &'v ()) -> Self

impl<'v> From<&'v bool> for ValueBag<'v>

fn from(v: &'v bool) -> Self

impl<'v> From<&'v char> for ValueBag<'v>

fn from(v: &'v char) -> Self

impl<'v> From<&'v f32> for ValueBag<'v>

fn from(v: &'v f32) -> Self

impl<'v> From<&'v f64> for ValueBag<'v>

fn from(v: &'v f64) -> Self

impl<'v> From<&'v i128> for ValueBag<'v>

fn from(v: &'v i128) -> Self

impl<'v> From<&'v i16> for ValueBag<'v>

fn from(v: &'v i16) -> Self

impl<'v> From<&'v i32> for ValueBag<'v>

fn from(v: &'v i32) -> Self

impl<'v> From<&'v i64> for ValueBag<'v>

fn from(v: &'v i64) -> Self

impl<'v> From<&'v i8> for ValueBag<'v>

fn from(v: &'v i8) -> Self

impl<'v> From<&'v isize> for ValueBag<'v>

fn from(v: &'v isize) -> Self

pub fn try_capture<T>(value: &'v T) -> Option<Self>
where T: ?Sized + 'static,

pub const fn from_fill<T>(value: &'v T) -> Self
where T: Fill,

pub fn capture_debug<T>(value: &'v T) -> Self
where T: Debug + 'static,

pub fn capture_display<T>(value: &'v T) -> Self
where T: Display + 'static,

pub const fn from_debug<T>(value: &'v T) -> Self
where T: Debug,

pub const fn from_display<T>(value: &'v T) -> Self
where T: Display,

impl<'v, 'u> From<&'v &'u str> for ValueBag<'v>
where 'u: 'v,