Neha Patil (Editor)

LFE (programming language)

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Designed by
  
Robert Virding

First appeared
  
2008

Developer
  
Robert Virding

Typing discipline
  
dynamic, strong

LFE (programming language)

Paradigm
  
multi-paradigm: concurrent, functional

Stable release
  
v1.1.1 / 19 June 2016 (2016-06-19)

LFE (Lisp Flavored Erlang) is a functional, concurrent, general-purpose programming language and Lisp dialect built on top of Core Erlang and the Erlang Virtual Machine (BEAM). LFE builds on top of Erlang in order to provide a Lisp syntax for writing distributed, fault-tolerant, soft real-time, non-stop applications. LFE also extends Erlang to support meta-programming with Lisp macros and an improved developer experience with a feature-rich REPL. LFE is actively supported on all recent releases of Erlang; the oldest version of Erlang supported is R14.

Contents

Initial release

Initial work on LFE began in 2007, when Robert Virding started creating a prototype of Lisp running on Erlang. This work was focused primarily on parsing and exploring what an implementation might look like; no version control system was being used at the time, so tracking exact initial dates is somewhat problematic.

Robert Virding announced the first release of LFE on the "Erlang Questions" mail list in March 2008. This release of LFE was very limited: it did not handle recursive letrecs, binarys, receive, or try; it also did not support a Lisp shell.

Initial development of LFE was done with version R12B-0 of Erlang on a Dell XPS laptop.

Motivation

Robert Virding has stated that there were a number of reasons why he started the LFE programming language:.

  • He had previous experience programming in Lisp.
  • Given his previous experience, he was interested in implementing his own Lisp.
  • In particular, he wanted to implement a Lisp in Erlang: not only was he curious to see how it would run on and integrate with Erlang, he wanted to see what it would look like.
  • Since helping to create the Erlang programming language, he had had the goal of making a Lisp which was specifically designed for running on the BEAM and able to fully interact with Erlang/OTP.
  • He wanted to experiment with compiling another language on top of Erlang. As such, he saw LFE as a means of exploring this by generating Core Erlang and plugging it into the backend of the Erlang compiler.
  • He was not working with programming or Erlang at the time, so was looking for some interesting programming projects that were not too large to do in his spare time.
  • He likes implementing languages.
  • He also thought it would be a fun problem to solve, as a solution would have many parts and the problem space was quite open-ended.

    • Features

  • A language targeting Erlang Virtual Machine (BEAM)
  • Seamless Erlang integration: zero-penalty Erlang function calls (and vice versa)
  • Meta programming via macros and the homoiconicity of a Lisp
  • Common Lisp-style documentation via both code comments and docstrings
  • Shared-nothing concurrent programming via message passing (Actor model)
  • Emphasis on recursion and higher-order functions instead of side-effect-based looping
  • A full REPL for interactive development and testing (unlike Erlang's shell, the LFE REPL supports function and macro definitions)
  • Pattern matching
  • Hot loading of code
  • A Lisp-2 separation of namespaces for variables and functions
  • Java inter-operation via JInterface and Erjang
  • Scripting capabilities with both lfe and lfescript

    • Symbolic expressions (S-expressions)

    Like Lisp, LFE is an expression-oriented language. Unlike non-homoiconic programming languages, Lisps make no or little syntactic distinction between "expressions" and "statements": all code and data are written as expressions. LFE brought homoiconicity to the Erlang VM.

    Lists

    In LFE, the list data type is written with its elements separated by whitespace, and surrounded by parentheses. For example, (list 1 2 'foo) is a list whose elements are the integers 1 and 2, and the atom foo. These values are implicitly typed: they are respectively two integers and a Lisp-specific data type called a "symbolic atom", and do not have to be declared as such.

    As seen in the example above, LFE expressions are written as lists, using prefix notation. The first element in the list is the name of a form, i.e., a function, operator, macro, or operator. The remainder of the list are the arguments.

    Operators

    The LFE/Erlang operators are used in the same way. The expression

    evaluates to 42. Unlike functions in Erlang and LFE, arithmetic operators in Lisp are variadic (or n-ary), able to take any number of arguments.

    Lambda expressions and function definition

    LFE has lambda, just like Common Lisp. It also, however, has lambda-match to account for Erlang's pattern-matching capabilities in anonymous function calls.

    Erlang idioms in LFE

    This section does not represent a complete comparison between Erlang and LFE, but should give a taste.

    Pattern matching

    Erlang:

    LFE:

    List comprehensions

    Erlang:

    LFE:

    Or idiomatic functional style:

    Guards

    Erlang:

    LFE:

    cons'ing in function heads

    Erlang:

    LFE:

    or using a ``cons`` literal instead of the constructor form:

    Matching records in function heads

    Erlang:

    LFE:

    Receiving messages

    Erlang:

    LFE:

    or:

    Erlang interoperability

    Calls to Erlang functions take the form (<module>:<function> <arg1> ... <argn>):

    Functional paradigm

    Using recursion to define the Ackermann function:

    Composing functions:

    Concurrency

    Message-passing with Erlang's light-weight "processes":

    Multiple simultaneous HTTP requests:

    References

    LFE (programming language) Wikipedia
    (Text) CC BY-SA


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