Struct Value

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pub struct Value<'a> { /* private fields */ }

Expand description

A Nix value.

This represents any value in the Nix language, including primitives, collections, and functions. Values are GC-managed; this struct holds a reference count that is released on drop.

Implementations§

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impl Value<'_>

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pub fn get_attr(&self, key: &str) -> Result<Value<'_>>

Get an attribute by name.

Returns the value associated with the given attribute name.

§Errors

Returns an error if the value is not an attribute set or the key does not exist.

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pub fn attr_keys(&self) -> Result<Vec<String>>

Get all attribute keys.

Returns a vector of all attribute names in this attribute set.

§Errors

Returns an error if the value is not an attribute set.

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pub fn has_attr(&self, key: &str) -> Result<bool>

Check if an attribute exists.

Returns true if the attribute set contains the given key.

§Errors

Returns an error if the value is not an attribute set.

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pub fn attrs(&self) -> Result<AttrIterator<'_>>

Create an iterator over key-value pairs.

§Returns

An iterator over all key-value pairs in the attribute set.

§Errors

Returns an error if the value is not an attribute set.

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impl Value<'_>

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pub fn is_list(&self) -> bool

Check if this value is a list.

§Example

use std::sync::Arc;

use nix_bindings::{Context, EvalStateBuilder, Store};
fn main() -> Result<(), Box<dyn std::error::Error>> {
  let ctx = Arc::new(Context::new()?);
  let store = Arc::new(Store::open(&ctx, None)?);
  let state = EvalStateBuilder::new(&store)?.build()?;
  let list = state.eval_from_string("[1 2 3]", "<eval>")?;
  assert!(list.is_list());
  Ok(())
}

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pub fn list_len(&self) -> Result<usize>

Get the length of this list.

§Errors

Returns an error if this value is not a list.

§Example

let list = state.eval_from_string("[1 2 3]", "<eval>")?;
assert_eq!(list.list_len()?, 3);

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pub fn list_get(&self, idx: usize) -> Result<Value<'_>>

Get an element from this list by index.

§Arguments

idx - The index of the element to retrieve (0-based)

§Errors

Returns an error if this value is not a list or the index is out of bounds.

§Example

let list = state.eval_from_string("[1 2 3]", "<eval>")?;
let first = list.list_get(0)?;
assert_eq!(first.as_int()?, 1);

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pub fn list_iter(&self) -> Result<ListIterator<'_>>

Create an iterator over the elements of this list.

§Errors

Returns an error if this value is not a list.

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impl Value<'_>

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pub fn force(&mut self) -> Result<()>

Force evaluation of this value.

If the value is a thunk, this will evaluate it to its final form.

§Errors

Returns an error if evaluation fails.

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pub fn force_deep(&mut self) -> Result<()>

Force deep evaluation of this value.

Forces evaluation of the value and all its nested components.

§Errors

Returns an error if evaluation fails.

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pub fn value_type(&self) -> ValueType

Get the type of this value.

Does not force; a lazy attribute or list element reports as ValueType::Thunk until forced. Use force or any as_* accessor (which force implicitly) first.

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pub fn type_name(&self) -> String

Get the Nix type name of this value as reported by the C API.

Wraps nix_get_typename. Returns Nix’s source-of-truth string (e.g. "int", "thunk", "lambda"). Falls back to the local [ValueType::to_string] if the C call returns null.

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pub fn as_int(&self) -> Result<i64>

Convert this value to an integer. Forces the value first.

§Errors

Returns an error if forcing fails or the resolved value is not an integer.

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pub fn as_float(&self) -> Result<f64>

Convert this value to a float. Forces the value first.

§Errors

Returns an error if forcing fails or the resolved value is not a float.

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pub fn as_bool(&self) -> Result<bool>

Convert this value to a boolean. Forces the value first.

§Errors

Returns an error if forcing fails or the resolved value is not a boolean.

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pub fn as_string(&self) -> Result<String>

Convert this value to a string.

Realises any string context. Forces the value first.

§Errors

Returns an error if forcing fails or the value is not a string.

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pub fn as_string_with_context(&self) -> Result<(String, Vec<StorePath>)>

Convert this value to a string and return its store-path context.

Extended form of as_string returning both content and any store paths embedded in the string’s context. For ordinary strings the context vector is empty.

§Errors

Returns an error if the value is not a string.

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pub fn as_path(&self) -> Result<PathBuf>

Convert this value to a filesystem path. Forces the value first.

§Errors

Returns an error if forcing fails or the value is not a path.

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pub fn call(&self, arg: &Value<'_>) -> Result<Value<'_>>

Call this value as a function with a single argument.

§Errors

Returns an error if this value is not a function or the call fails.

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pub fn call_multi(&self, args: &[&Value<'_>]) -> Result<Value<'_>>

Call this value as a curried function with multiple arguments.

§Errors

Returns an error if this value is not a function or the call fails.

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pub fn make_thunk<'a>( fn_val: &'a Value<'a>, arg: &'a Value<'a>, ) -> Result<Value<'a>>

Create a lazy thunk that applies a function to an argument.

Unlike call, this does not perform the call immediately; it stores it as a thunk. Useful for lazy attribute sets and lists.

§Errors

Returns an error if the thunk cannot be created.

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pub fn copy(&self) -> Result<Value<'_>>

Copy this value into a new owned value slot.

§Errors

Returns an error if the copy fails.

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pub fn to_nix_string(&self) -> Result<String>

Format this value as Nix syntax.

Forces the value first and recursively renders attribute sets and lists. Functions, thunks, and external values render as opaque placeholders (<lambda>, <thunk>, <external>) the same way nix-instantiate --eval does.