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Differences with Clojure

(This document was built using the one from ClojureScript as a guide).

Rationale

Erlang is a great language for building safe, reliable and scalable systems. It provides immutable, persistent data structures out of the box and its concurrency semantics are unequalled by any other language.

Clojure is a Lisp and as such comes with all the goodies Lisps provide. Apart from these Clojure also introduces powerful abstractions such as protocols, multimethods and seqs, to name a few.

Clojure was built to simplify the development of concurrent programs and some of its concurrency abstractions could be adapted to Erlang. It is fair to say that combining the power of the Erlang VM with the expressiveness of Clojure could provide an interesting, useful result to make the lives of many programmers simpler and make the world a happier place.

State and Identity

The Erlang VM implements the actor model, which means identity is generally bound to a process which maintains an internal state. This doesn't mean that the Clojure identity model can't be implemented, but since the platform favors the actor model, it will always be more efficient using it than the Clojure model.

clojure.spec

Not implemented (yet).

Dynamic Development

There is a REPL available just like Clojure's.

Functional Programming

Since the Erlang VM already includes native immutable data structures, Clojerl includes most of the same immutable data structures present in Clojure. Some of them are implemented based on other immutable data structures (e.g. sorted sets).

Lisp

Same as in Clojure.

Runtime Polymorphism

Protocols and multimethods are available as in Clojure. Everything is implemented in terms of protocols, there are no interfaces. proxy is not supported.

Concurrent Programming

TBD

Hosted on the JVM

Hosted on the Erlang VM.

Getting Started

Getting started.

The Reader

  • Numbers
    • All integer numbers are mapped to Erlang's representation, which are arbitrary precision integers.
    • Numbers with a decimal part are mapped to Erlang's representation of floating point numbers.
    • Ratio and BigDecimal are not supported.
  • Characters are represented as single-character strings.
  • nil is currently mapped to the :undefined keyword. This is because :undefined is generally used in Erlang to specify 'nothing/no-value'.
  • true and false are equivalent to :true and :false respectively.
  • Lists, Vectors, Maps and Sets are the same as in Clojure. There is no support yet for the Map namespace syntax.
  • Macro characters
    • Because there is no character type in Erlang, \ produces a single-character string.

The REPL and main

  • See Getting started for instructions on the Clojerl REPL.
  • The bin/clojerl and bin/clje scripts can be used the same as bin/clojure and bin/clj.

Evaluation

  • Clojerl has the same evaluation rules as Clojure.

Special Forms

The following Clojerl special forms are identical to their Clojure cousins: if, do, let, letfn, quote and loop.

  • fn
    • Compile to an Erlang function and therefore can't have any metadata.
  • recur
    • recur is compiled to an actual recursive call that is only allowed in tail position, since the Erlang VM implements tail call optimization.
  • def
    • When the init expression is a fn, def produces one or more Erlang functions, depending on the arities specified in the fn declaration.
    • When not an fn, the init expression (which is evaluated at compile-time) must return a constant literal, otherwise it's a compile-time error.
  • throw
    • There is no error type in Erlang, any value can be thrown.
  • try..catch..finally
    • An exception in Erlang consists of three things: the class of the exception (throw, error or exit), the exit reason and the stack-trace. There is no specific type for exceptions.
    • The spec for the catch clause is (catch error-type error & body), where error-type is one of the keywords :throw, :error, :exit or _. The last one will catch all the error types.
  • var
    • Vars are not reified as in Clojure, they are more similar to what ClojureScript does, which returns compile time information about the var, and during runtime the information available is static (i.e. can't be modified without recompiling).
    • Vars can't have watches or a validator.
    • It's not possible to change the root binding of a Var.
  • monitor-enter, monitor-exit, and locking are not supported.

Macros

Macros in Clojerl work the same as in Clojure.

Other Useful Functions and Macros

  • Regex support is the one provided by the re Erlang module.

Data Structures

  • nil's type is clojerl.Nil. It is equivalent to :undefined.
  • Numbers
    • All integers are Erlang integers (arbitrary precision).
  • Strings are UTF-8 encoded Erlang binaries.
  • Characters are single-char strings.
  • Collections
    • Clojerl uses the same hash computations as Clojure.
    • When possible a native Erlang data structure is used (i.e. map, list, tuple) in the underlying implementation.
      • List uses an Erlang list.
      • Vector uses clj_vector, which is an Erlang implementation of Clojure's persistent vector.
      • Map uses an Erlang map.
      • Sorted Maps use Robert Virding's implementation of red-black trees.
      • Sets use Erlang maps.
      • StructMap will not be implemented since "most uses of StructMaps would now be better served by records".
      • ArrayMap's closest relative in Clojerl is TupleMap.
  • Literal Erlang collections can be included in code by using the #erl dispatch reader macro:
    • #erl () Erlang lists
    • #erl [] Erlang tuples
    • #erl {} Erlang maps

Datatypes

  • defprotocol and deftype, extend-type, extend-protocol work as in Clojure.
  • Protocols are not reified as in Clojure, there are no runtime protocol objects.
  • Some reflective capabilities (satisfies?) work as in Clojure.
  • extend is not implemented.
  • reify is not implemented.

Sequences

  • Seqs are the same as in Clojure.

Transient Data Structures

Transients are currently not implemeted.

Transducers

Stateful transducers are currently not implemeted.

Multimethods and Hierarchies

  • Multimethods works as in Clojure.
  • Hierarchies are not implemented.

Metadata

Works as in Clojure for all the same data structures, except for fns.

Anonymous functions are compiled into Erlang closures, for which there is no way to bolt arbitrary information.

Namespaces

Namespaces are compiled into Erlang modules. They are available at runtime for inspection, as in Clojure.

Libs

Existing Clojure libs will have to conform to the Clojerl subset in order to work in Clojerl.

Vars and the Global Environment

  • def, binding and set! work as in Clojure
    • def yields the Var itself as in Clojure.
    • Root bindings can't be modified (e.g. with alter-var-root or with-redefs).
    • Vars can't have watches or a validator.
  • Atom and Agent implementations are experimental.
  • Refs are not currently implemented.

Refs and Transactions

Refs and Transactions are not implemented.

Agents

Agent implementation is experimental.

Atoms

Atom implementation is experimental.

Reducers

Reducers are not implemented.

Host Interop

  • Member access
    • All types in Clojerl have an implementation module with the same name as the type. "Member access" is translated to a function call to the specified function in the type's module where the first argument is the value.
  • Type Hinting
    • There are no primitive types in Erlang so there is no use for int, ints, etc.
    • Type hints are used to resolve the target's type at compile-time and avoid having to figure out at run-time.

  • Calling Clojerl from Erlang

    • Since Clojerl namespaces are Erlang modules, calling a function from the clojure.core module is as simple as:

    'clojure.core':inc(1). %%= 2

Ahead-of-time Compilation and Class Generation

  • Each namespace is generally compiled into one Erlang module, except when defrecord, deftype or one of the extend-* functions is used.
  • gen-class, gen-interface, etc. are unnecessary and unimplemented in Clojerl.

Other Included Libraries

  • clojure.erlang.io is an attempt to provide the same polymorphic I/O utility functions for Erlang.
  • clojure.erlang.erldocs (Missing)
  • clojure.erlang.shell (Missing)
  • clojure.repl
  • clojure.set
  • clojure.string
  • clojure.test
  • clojure.walk
  • clojure.xml
  • clojure.zip
  • clojure.core.reducers (Missing)
  • clojure.spec (Missing)
  • clojure.pprint