LFE (programming language)


Lisp Flavored Erlang is a functional, concurrent, garbage collected, general-purpose programming language and Lisp dialect built on Core Erlang and the Erlang virtual machine. LFE builds on Erlang to provide a Lisp syntax for writing distributed, fault-tolerant, soft real-time, non-stop applications. LFE also extends Erlang to support metaprogramming with Lisp macros and an improved developer experience with a feature-rich read–eval–print loop. LFE is actively supported on all recent releases of Erlang; the oldest version of Erlang supported is R14.

History

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.
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.

Motives

Robert Virding has stated that there were several reasons why he started the LFE programming language:

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, is a list whose elements are the integers and, 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 need not 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, or macro. 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, 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 abilities 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:

1> =.

2> Msg.
"Trillian"

LFE:

> #)
#
> msg
"Trillian"

List comprehensions

Erlang:

1>

LFE:

>
)


Or idiomatic functional style:

>
')

Guards

Erlang:

right_number when X 42; X 276709 ->
true;
right_number ->
false.

LFE:

))
'true)
)

cons'ing in function heads

Erlang:

sum -> sum.
sum -> Total;
sum -> sum.

LFE:

)
total)
))

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

)
total)
)

Matching records in function heads

Erlang:

handle_info ->
gen_server:cast,
;
handle_info ->
;

LFE:


`#)

`#))

Receiving messages

Erlang:

universal_server ->
receive
->
Func
end.

LFE:



)))

or:



)))

Examples

Erlang interoperability

Calls to Erlang functions take the form :

Functional paradigm

Using recursion to define the Ackermann function:

)

))

Composing functions:


))

)

))

Concurrency

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

))




)))
)))
))

Multiple simultaneous HTTP requests:

"Given one or more command-line arguments, extract the passed values.
For example, if the following was passed via the command line:
$ erl -my-flag my-value-1 -my-flag my-value-2
One could then extract it in an LFE program by calling this function:

...
)
In this example, the value assigned to the arg variable would be a list
containing the values my-value-1 and my-value-2."
))
))
"With no argument, assume 'url parameter was passed via command line."

))
"Start inets and make HTTP requests."


) urls))
"Make a single HTTP request."

)

)
)


)
)))