Struct ExpansionCache 

pub struct ExpansionCache(/* private fields */);

Per-call-site slot holding the last macro expansion of a Form.

The tree-walking evaluator otherwise re-expands every macro call on every evaluation (converting the forms to values, running the macro, converting the result back), which dominated the cost of e.g. a when inside a hot loop. Clojure itself expands each call site once, at compile time, so reusing the expansion matches its semantics.

Clones of a list form share the slot (see Form::clone), so a function body copied into many closures expands each call site once in total. The shared cell is allocated lazily, on first store or first clone, so a form that is parsed and evaluated once pays nothing beyond an empty word.

A form whose kind is mutated in place after cloning must not be evaluated as a macro call afterwards, since it would share its source’s expansion; no evaluator path does this (rewrites build fresh forms).

Implementations

impl ExpansionCache

pub fn get(&self, macro_id: u64, ns: &str) -> Option<Arc<Form>>

The cached expansion, if one was made by macro_id in ns.

pub fn store(&self, macro_id: u64, ns: Arc<str>, form: Form) -> Arc<Form>

Record form as the expansion made by macro_id in ns, replacing any previous entry. Returns the stored Arc so the caller evaluates the very form the cache holds.

pub fn clear(&self)

Drop any cached expansion (for every form sharing this slot).

Trait Implementations

impl Debug for ExpansionCache

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

Formats the value using the given formatter.

impl Default for ExpansionCache

fn default() -> Self

Returns the “default value” for a type.