Differences from Clojure

This page documents intentional differences, missing features, and behavioural variations between clojurust and Clojure (JVM). The goal is to be a useful reference for porting .cljc code and for understanding what to expect when running Clojure code under clojurust.

No JVM, no Java interop

clojurust runs on Rust, not the JVM. There is no Java class hierarchy, no java.lang.*, and no Java method calls. The . (dot) special form and new have limited implementations:

• (new Exception "msg") and (Exception. "msg") produce clojurust exception values, not Java objects.
• (.method obj args...) is not yet implemented. Use protocols or the built-in equivalents instead.

Code that uses (System/nanoTime), (Thread/sleep n), or any other Java static method must use reader conditionals to supply a :cljrs alternative, or use the clojurust built-ins nanotime and sleep respectively.

Platform key

The reader-conditional platform key is :cljrs, not :clj. The older key :rust is still accepted as an alias. See Reader conditionals.

Missing concurrency features

atom, agent, future, promise, delay, and volatile! are fully implemented.

deftype

deftype is implemented: a positional ->Name constructor, inline protocol/interface method bodies with the fields in scope, .-field access, and mutable fields (^:unsynchronized-mutable / ^:volatile-mutable) written with set! — both (set! field v) inside a method and (set! (.-field inst) v) from outside.

Instances are single-threaded here, so ^:volatile-mutable and ^:unsynchronized-mutable behave identically. Unlike defrecord, deftype generates no map->Name constructor and no map behaviour on the instance — that matches Clojure. Host-class method calls ((.someMethod inst)) on an instance are not supported; protocol methods are called as (proto-fn inst).

Metadata on collections

Metadata is supported on vars and some values, but not yet propagated through collection operations such as assoc. The with-meta and meta functions work on any value that carries metadata.

Sorted collections

sorted-map and sorted-set are implemented. sorted-map-by and sorted-set-by (custom comparators) are not yet implemented.

Hierarchies

make-hierarchy, ancestors, descendants, and parents have stub implementations. derive, underive, and a full isa? hierarchy are not yet implemented. defmulti / defmethod dispatch works but does not consult a custom hierarchy.

amap and areduce

These macros are registered as stubs but not fully implemented.

clojure.pprint

Not implemented.

clojure.zip

Stub — the namespace exists but most functions are not yet implemented.

Numeric tower

The numeric tower (Long → BigInt → Ratio → BigDecimal → Double) follows Clojure conventions. A few differences to be aware of:

• Integer overflow in +, -, * automatically promotes to BigInt (same as Clojure’s checked arithmetic). The promoting variants +', -', *' are also available.
• (/ 1 3) returns a Ratio (1/3), not a Double, same as Clojure.
• BigDecimal precision is controlled by with-precision (the Clojure macro) or the lower-level push-precision! / pop-precision! built-ins.

*clojure-version* / *cljrs-version*

These vars are not yet defined. clojure-version as a function returns a map describing the current runtime.

with-open and close

clojurust provides RAII-style resource management via with-open (a macro) and close (a built-in function). These follow the same protocol as Clojure’s with-open; any value that implements the Resource protocol can be used.

Source file namespace mapping

The namespace→file mapping converts . to / and - to _, the same as Clojure:

myapp.core    →  myapp/core.cljrs  (or .cljc)
my-app.utils  →  my_app/utils.cljrs

Standard library

The following clojure.* namespaces are available: clojure.string, clojure.set, clojure.test, clojure.walk, clojure.edn, clojure.data, clojure.spec.alpha, clojure.spec.test.alpha, and clojure.spec.gen.alpha. clojure.core.async is also available (as the cljrs-async crate — see the Async & I/O chapter).

clojure.zip and clojure.pprint exist as stubs. clojure.spec.alpha has no generators: clojure.spec.gen.alpha and every generative-testing entry point (s/gen, s/exercise, s/exercise-fn, stest/check) throw a clear ex-info instead of generating values. clojure.core.match is not available.