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460 lines
12 KiB
Markdown
---
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language: "Lisp Flavoured Erlang(LFE)"
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filename: lispflavourederlang.lfe
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contributors:
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- ["Pratik Karki", "https://github.com/prertik"]
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---
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Lisp Flavoured Erlang(LFE) is a functional, concurrent, general-purpose programming
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language and Lisp dialect(Lisp-2) built on top of Core Erlang and the Erlang Virtual Machine(BEAM).
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LFE can be obtained from [LFE](https://github.com/rvirding/lfe)
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The classic starting point is [LFE DOCS.](http://docs.lfe.io)
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Another new site is being built to replace it.[LFE DEV.](http://docs.lfe.io/dev)
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```lisp
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;;; 0. Syntax
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;;; General form.
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;; Lisp comprises of two syntax called: the ATOM and the S-expression.
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;; `forms` are known as grouped S-expressions.
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8 ; an atom; it evaluates to itself
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:ERLANG ;Atom; evaluates to the symbol :ERLANG.
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t ; another atom which denotes true.
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(* 2 21) ; an S- expression
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'(8 :foo t) ;another one
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;;; Comments
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;; Single line comments start with a semicolon; use two for normal
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;; comments, three for section comments, and four fo file-level
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;; comments.
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;; Block Comment
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#| comment text |#
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;;; Environment
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;; LFE is the de-facto standard.
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;; Libraries can be used directly from the Erlang ecosystem. Rebar3 is the build tool.
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;; LFE is usually developed with a text editor(preferably Emacs) and a REPL
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;; (Read Evaluate Print Loop) running at the same time. The REPL
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;; allows for interactive exploration of the program as it is "live"
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;; in the system.
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;;; 1. Literals and Special Syntactic Rules
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;;; Integers
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1234 -123 ; Regular decimal notation
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#b0 #b10101 ; Binary notation
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#0 #10101 ; Binary notation (alternative form)
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#o377 #o-111 ; Octal notation
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#d123456789 #d+123 ; Explicitly decimal notation
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#xc0ffe 0x-01 ; Hexadecimal notation
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#2r1010 #8r377 ;Notation with explicit base (up to 36)
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#\a #$ #\ä #\🐭 ;Character notation (the value is the Unicode code point of the character)
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#\x1f42d; ;Character notation with the value in hexadecimal
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;;; Floating point numbers
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1.0 +2.0 -1.5 1.0e10 1.111e-10
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;;; Strings
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"any text between double quotes where \" and other special characters like \n can be escaped".
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; List String
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"Cat: \x1f639;" ; writing unicode in string for regular font ending with semicolon.
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#"This is a binary string \n with some \"escaped\" and quoted (\x1f639;) characters"
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; Binary strings are just strings but function different in the VM.
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; Other ways of writing it are: #B("a"), #"a", and #B(97).
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;;; Character escaping
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\b ; => Backspace
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\t ; => Tab
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\n ; => Newline
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\v ; => Vertical tab
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\f ; => Form Feed
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\r ; => Carriage Return
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\e ; => Escape
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\s ; => Space
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\d ; => Delete
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;;; Binaries
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;; It is used to create binaries with any contents.
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#B((#"a" binary) (#"b" binary)) ; #"ab" (Evaluated form)
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;;; Lists are: () or (foo bar baz)
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;;; Tuples are written in: #(value1 value2 ...). Empty tuple #() is also valid.
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;;; Maps are written as: #M(key1 value1 key2 value2 ...). Empty map #M() is also valid.
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;;; Symbols: Things that cannot be parsed. Eg: foo, Foo, foo-bar, :foo
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| foo | ; explicit construction of symbol by wrapping vertical bars.
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;;; Evaluation
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;; #.(... some expression ...). E.g. '#.(+ 1 1) will evaluate the (+ 1 1) while it ;; reads the expression and then be effectively '2.
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;; List comprehension in LFE REPL
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lfe> (list-comp
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((<- x '(0 1 2 3)))
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(trunc (math:pow 3 x)))
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(1 3 9 27)
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; 2. Core forms
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; These forms are same as those found at Common Lisp and Scheme.
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(quote e)
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(cons head tail)
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(car e)
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(cdr e)
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(list e ... )
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(tuple e ... )
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(binary seg ... )
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(map key val ...), (map-get m k), (map-set m k v ...), (map-update m k v ...)
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(lambda (arg ...) ...)
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(match-lambda
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((arg ... ) {{(when e ...)}} ...) ; Matches clauses
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... )
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(let ((pat {{(when e ...)}} e)
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...)
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... )
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(let-function ((name lambda|match-lambda) ; Only define local
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... ) ; functions
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... )
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(letrec-function ((name lambda|match-lambda) ; Only define local
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... ) ; functions
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... )
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(let-macro ((name lambda-match-lambda) ; Only define local
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...) ; macros
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...)
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(progn ... )
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(if test true-expr {{false-expr}})
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(case e
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(pat {{(when e ...)}} ...)
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... ))
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(receive
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(pat {{(when e ...)}} ... )
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...
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(after timeout ... ))
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(catch ... )
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(try
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e
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{{(case ((pat {{(when e ...)}} ... )
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... ))}}
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{{(catch
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; Next must be tuple of length 3!
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(((tuple type value ignore) {{(when e ...)}}
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... )
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... )}}
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{{(after ... )}})
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(funcall func arg ... )
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(call mod func arg ... ) - Call to Erlang Mod:Func(Arg, ... )
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(define-module name declaration ... )
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(extend-module declaration ... ) - Define/extend module and declarations.
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(define-function name lambda|match-lambda)
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(define-macro name lambda|match-lambda) - Define functions/macros at top-level.
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; 3. Macros
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; Macros are part of the language to allow you to create abstractions
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;; on top of the core language and standard library that move you closer
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;; toward being able to directly express the things you want to express.
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;; Top-level function
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(defun name (arg ...) ...)
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;; Adding comments in functions
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(defun name
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"Toplevel function with pattern-matching arguments"
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((argpat ...) ...)
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...)
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;; Top-level macro
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(defmacro name (arg ...) ...)
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(defmacro name arg ...)
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;; Top-level macro with pattern matching arguments
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(defmacro name
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((argpat ...) ...)
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...)
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;; Top-level macro using Scheme inspired syntax-rules format
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(defsyntax name
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(pat exp)
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...)
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;;; Local macros in macro or syntax-rule format
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(macrolet ((name (arg ... ) ... )
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... )
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... )
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(syntaxlet ((name (pat exp) ...)
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...)
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...)
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;; Like CLISP
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(prog1 ...)
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(prog2 ...)
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;; Erlang LFE module
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(defmodule name ...)
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;; Erlang LFE record
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(defrecord name ...)
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; 4. Patterns and Guards
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; Using patterns in LFE compared to that of Erlang
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;; Erlang ;; LFE
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;; {ok, X} (tuple 'ok x)
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;; error 'error
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;; {yes, [X|Xs]} (tuple 'yes (cons x xs))
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;; <<34,F/float>> (binary 34 (f float))
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;; [P|Ps]=All (= (cons p ps) all)
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_ ; => is don't care while pattern matching
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(= pattern1 pattern2) ; => easier, better version of pattern matching
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;; Guards
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;; Whenever pattern occurs(let, case, receive, lc, etc) it can be followed by an optional
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;; guard which has the form (when test ...).
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(progn gtest ...) ;; => Sequence of guard tests
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(if gexpr gexpr gexpr)
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(type-test e)
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(guard-bif ...) ;; => Guard BIFs, arithmetic, boolean and comparison operators
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;;; REPL
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lfe>(set (tuple len status msg) #(8 ok "Trillian"))
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#(8 ok "Trillian")
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lfe>msg
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"Trillian"
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;;; Program illustrating use of Guards
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(defun right-number?
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((x) (when (orelse (== x 42) (== x 276709)))
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'true)
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((_) 'false))
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; 5. Functions
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; A simple function using if.
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(defun max (x y)
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"The max function."
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(if (>= x y) x y))
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;; Same function using more clause
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(defun max
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"The max function."
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((x y) (when (>= x y)) x)
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((x y) y))
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;; Same function using similar style but using local functions defined by flet or fletrec
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(defun foo (x y)
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"The max function."
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(flet ((m (a b) "Local comment."
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(if (>= a b) a b)))
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(m x y)))
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;; LFE being Lisp-2 has separate namespaces for variables and functions
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;; Both variables and function/macros are lexically scoped.
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;; Variables are bound by lambda, match-lambda and let.
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;; Functions are bound by top-level defun, flet and fletrec.
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;; Macros are bound by top-level defmacro/defsyntax and by macrolet/syntaxlet.
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;; (funcall func arg ...) like CL to call lambdas/match-lambdas
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;; (funs) bound to variables are used.
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;; separate bindings and special for apply.
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apply _F (...),
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apply _F/3 ( a1, a2, a3 )
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;; Cons'ing in function heads
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(defun sum (l) (sum l 0))
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(defun sum
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(('() total) total)
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(((cons h t) total) (sum t (+ h total))))
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;; ``cons`` literal instead of constructor form
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(defun sum (l) (sum l 0))
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(defun sum
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(('() total) total)
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((`(,h . ,t) total) (sum t (+ h total))))
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;; Matching records in function heads
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(defun handle_info
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(('ping (= (match-state remote-pid 'undefined) state))
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(gen_server:cast (self) 'ping)
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`#(noreply ,state))
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(('ping state)
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`#(noreply ,state)))
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;; Receiving Messages
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(defun universal-server ()
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(receive
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((tuple 'become func)
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(funcall func))))
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;; another way for receiving messages
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(defun universal-server ()
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(receive
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(`#(become ,func)
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(funcall func))))
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;; Composing a complete function for specific tasks
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(defun compose (f g)
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(lambda (x)
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(funcall f
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(funcall g x))))
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(defun check ()
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(let* ((sin-asin (compose #'sin/1 #'asin/1))
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(expected (sin (asin 0.5)))
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(compose-result (funcall sin-asin 0.5)))
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(io:format "Expected answer: ~p~n" (list expected))
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(io:format "Answer with compose: ~p~n" (list compose-result))))
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; 6. Concurrency
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; Message passing as done by Erlang's light-weight "processes".
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(defmodule messenger-back
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(export (print-result 0) (send-message 2)))
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(defun print-result ()
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(receive
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((tuple pid msg)
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(io:format "Received message: '~s'~n" (list msg))
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(io:format "Sending message to process ~p ...~n" (list pid))
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(! pid (tuple msg))
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(print-result))))
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(defun send-message (calling-pid msg)
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(let ((spawned-pid (spawn 'messenger-back 'print-result ())))
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(! spawned-pid (tuple calling-pid msg))))
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;; Multiple simultaneous HTTP Requests:
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(defun parse-args (flag)
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"Given one or more command-line arguments, extract the passed values.
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For example, if the following was passed via the command line:
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$ erl -my-flag my-value-1 -my-flag my-value-2
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One could then extract it in an LFE program by calling this function:
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(let ((args (parse-args 'my-flag)))
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...
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)
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In this example, the value assigned to the arg variable would be a list
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containing the values my-value-1 and my-value-2."
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(let ((`#(ok ,data) (init:get_argument flag)))
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(lists:merge data)))
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(defun get-pages ()
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"With no argument, assume 'url parameter was passed via command line."
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(let ((urls (parse-args 'url)))
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(get-pages urls)))
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(defun get-pages (urls)
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"Start inets and make (potentially many) HTTP requests."
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(inets:start)
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(plists:map
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(lambda (x)
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(get-page x)) urls))
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(defun get-page (url)
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"Make a single HTTP request."
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(let* ((method 'get)
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(headers '())
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(request-data `#(,url ,headers))
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(http-options ())
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(request-options '(#(sync false))))
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(httpc:request method request-data http-options request-options)
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(receive
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(`#(http #(,request-id #(error ,reason)))
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(io:format "Error: ~p~n" `(,reason)))
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(`#(http #(,request-id ,result))
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(io:format "Result: ~p~n" `(,result))))))
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;; Check out Erlang's documentation for more concurrency and OTP docs.
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```
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## Further Reading
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* [LFE DOCS](http://docs.lfe.io)
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* [LFE GitBook](https://lfe.gitbooks.io/reference-guide/index.html)
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* [LFE Wiki](https://en.wikipedia.org/wiki/LFE_(programming_language))
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## Extra Info
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* [LFE PDF](http://www.erlang-factory.com/upload/presentations/61/Robertvirding-LispFlavouredErlang.pdf)
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* [LFE mail](https://groups.google.com/d/msg/lisp-flavoured-erlang/XA5HeLbQQDk/TUHabZCHXB0J)
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## Credits
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Lots of thanks to Robert Virding for creating LFE, Duncan McGreggor for documenting it and other LFE contributors who made LFE awesome.
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