Struct EvalState 

pub struct EvalState { /* private fields */ }

Nix evaluation state for evaluating expressions.

This provides the main interface for evaluating Nix expressions and creating values.

Implementations

impl EvalState

pub fn eval_from_string(&self, expr: &str, path: &str) -> Result<Value<'_>>

Evaluate a Nix expression from a string.

Arguments

• expr - The Nix expression to evaluate
• path - The path to use for error reporting (e.g., "<eval>")

Errors

Returns an error if evaluation fails.

pub fn eval_from_file(&self, path: impl AsRef<Path>) -> Result<Value<'_>>

Evaluate a Nix expression from a file.

Reads the file at path as UTF-8, then evaluates its contents using the parent directory as the base path for relative imports. The base path is passed to Nix as a UTF-8 string; non-UTF-8 components are replaced lossily for the error-reporting label only.

Errors

Returns an error if the file cannot be read as UTF-8 or if evaluation fails.

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

Allocate a new uninitialized value.

Errors

Returns an error if value allocation fails.

pub fn make_int(&self, i: i64) -> Result<Value<'_>>

Create a Nix integer value.

Errors

Returns an error if value allocation or initialization fails.

pub fn make_float(&self, f: f64) -> Result<Value<'_>>

Create a Nix float value.

Errors

Returns an error if value allocation or initialization fails.

pub fn make_bool(&self, b: bool) -> Result<Value<'_>>

Create a Nix boolean value.

Errors

Returns an error if value allocation or initialization fails.

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

Create a Nix null value.

Errors

Returns an error if value allocation or initialization fails.

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

Create a Nix string value.

Errors

Returns an error if value allocation, string conversion, or initialization fails.

pub fn make_path(&self, path: impl AsRef<Path>) -> Result<Value<'_>>

Create a Nix path value.

Pure Evaluation

In pure-eval mode (--pure-eval) the Nix evaluator wraps the filesystem in an AllowListSourceAccessor that rejects any unregistered absolute path. When the shim feature is enabled this method automatically registers absolute paths via the shim’s nix_eval_state_allow_path before constructing the value, mirroring what Nix’s own fetch builtins do. Without shim you must arrange for allowPath yourself, or is_pure_eval returns true.

Errors

Returns an error if value allocation, path conversion, or initialization fails.

pub fn allow_store_path(&self, path: &str) -> Result<()>

Allow a canonical Nix store path in this evaluation state.

This restores access to a previously resolved store path in a fresh evaluator running with restrict-eval. It does not permit arbitrary filesystem paths: Nix parses and validates the supplied store path.

Errors

Returns an error if path is not a valid store path or Nix cannot add it to this evaluation state’s allowlist.

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

Create a Nix list value from a slice of values.

Errors

Returns an error if value allocation or list construction fails.

pub fn make_attrs<'s>( &'s self, pairs: &[(&str, &Value<'_>)], ) -> Result<Value<'s>>

Create a Nix attribute set from key-value pairs.

Errors

Returns an error if value allocation or attribute set construction fails.

pub fn get_derivation(&self, value: &Value<'_>) -> Result<Option<StorePath>>

Determine whether a value is a derivation and return its store path.

Forces value and, if it is a derivation, returns a newly allocated StorePath for its .drvPath. Returns Ok(None) when the value is not a derivation. An exception raised during forcing propagates as Err(...).

Errors

Returns an error if forcing the value raises a Nix exception.

pub fn auto_call_function<'s>( &'s self, auto_args: Option<&Value<'_>>, fn_val: &Value<'_>, ) -> Result<Value<'s>>

Call a function using an attribute set as its argument source.

Forces fn_val and writes the result into a newly allocated value:

• If fn_val is a function with named formals, each formal is looked up in auto_args; formals with defaults that are absent from auto_args use their defaults.
• If auto_args is None, empty bindings are supplied (every formal must then have a default).
• If fn_val is not a function, the value is copied to the result unchanged.

Errors

Returns an error if the call fails.

impl EvalState

pub fn make_external<T: NixExternal>( &self, data: T, ) -> Result<ExternalValueHandle<'_>>

Wrap a NixExternal value and return an ExternalValueHandle.

The handle carries the Nix Value and the raw ExternalValue* needed for downcasting via ExternalValueHandle::as_external. The value’s lifetime is managed by the Nix GC.

Errors

Returns an error if value allocation or external value creation fails.

Trait Implementations

impl Drop for EvalState

fn drop(&mut self)

Executes the destructor for this type.

impl Send for EvalState