user_guide.txt

lfe_guide(7)                                                      lfe_guide(7)



NAME
       lfe_guide - Lisp Flavoured Erlang User Guide

SYNOPSIS
       Note: {{ ... }} is used to denote optional syntax.

LITERALS AND SPECIAL SYNTACTIC RULES
   Integers
       Integers can be written in various forms and number bases:

       · Regular decimal notation:

           1234 -123 0

       · Binary notation:

           #b0 #b10101 #b-1100

       · Binary notation (alternative form):

           #*0 #*10101 #*-1100

       · Octal notation:

           #o377 #o-111

       · Explicitly decimal notation:

           #d1234 #d-123 #d0

       · Hexadecimal notation:

           #xc0ffe #x-01

       · Notation with explicit base (up to 36):

           #2r1010 #8r377 #36rhelloworld

       · Character  notation (the value is the Unicode code point of the char‐
         acter):

           #\a #\$ #\ä

       · Character notation with the value in hexadecimal:

           #\x1f42d;

       In all these forms, the case of the indicating letter is  not  signifi‐
       cant, i.e. #b1010 and #B1010 are identical as are #16rf00 and #16Rf00.

       Similarly,  the  case is not significant for digits beyond 9 (i.e. `a',
       `b', `c', ... for number bases larger than 10), e.g. #xabcd is the same
       as #xABCD and can even be mixed in the same number, e.g. #36rHelloWorld
       is valid and the same number as #36Rhelloworld and #36rHELLOWORLD.

       The character notation using hexadecimal code representation (#\x....;)
       is  basically  the same thing as the regular hexadecimal notation #x...
       except that it conveys to the reader that a character is  intended  and
       that  it  does  a  sanity check on the value (e.g. negative numbers and
       value outside the Unicode range are not permitted).

   Floating point numbers
       There is only one type of floating point numbers and the  literals  are
       written  in the usual way, e.g. these are all valid floating point num‐
       bers:

              1.0 +1.0 -1.0 1.0e10 1.111e-10

       The one thing to watch out for is that you cannot omit the part  before
       or  after the decimal point if it is zero.  E.g.  the following are not
       valid forms: 100. or .125.

   Strings
       There are two forms of strings: list strings and binary strings.

   List Strings
       List strings are just lists of integers (where the values  have  to  be
       from a certain set of numbers that are considered valid characters) but
       they have their own syntax for literals (which will also  be  used  for
       integer  lists  as  an output representation if the list contents looks
       like it is meant to be a string): “any text between double quotes where
       " and other special characters like \n can be escaped”.

       As  a  special  case you can also write out the character number in the
       form \xHHH; (where  “HHH”  is  an  integer  in  hexadecimal  notation),
       e.g. "\x61;\x62;\x63;" is a complicated way of writing "abc".  This can
       be convenient when writing Unicode letters not easily typeable or view‐
       able  with  regular  fonts.   E.g.  "Cat: \\x1f639;" might be easier to
       type (and view on output devices without a Unicode  font)  then  typing
       the actual Unicode letter.

   Binary Strings
       Binary strings are just like list strings but they are represented dif‐
       ferently in the virtual machine.  The simple  syntax  is  #"...",  e.g.
       #"This  is  a  binary  string  \n  with  some  \"escaped\"  and  quoted
       (\\x1f639;) characters"

       You can also use the general format for creating binaries (#B(...), de‐
       scribed  below), e.g. #B("a"), #"a", and #B(97) are all the same binary
       string.

   Character Escaping
       Certain control characters can be more readably included by using their
       escaped name:

                | Escaped name | Character       |
                |--------------+-----------------|
                | \b           | Backspace       |
                | \t           | Tab             |
                | \n           | Newline         |
                | \v           | Vertical tab    |
                | \f           | Form Feed       |
                | \r           | Carriage Return |
                | \e           | Escape          |
                | \s           | Space           |
                | \d           | Delete          |

       Alternatively  you  can  also  use  the hexadecimal character encoding,
       e.g. "a\nb" and "a\x0a;b" are the same string.

   Binaries
       We have already seen binary strings, but the #B(...) syntax can be used
       to  create binaries with any contents.  Unless the contents is a simple
       integer you need to annotate it with a type and/or size.

       Example invocations are that show the various annotations:

              > #B(42 (42 (size 16)) (42 (size 32)))
              #B(42 0 42 0 0 0 42)
              > #B(-42 111 (-42 (size 16)) 111 (-42 (size 32)))
              #B(-42 111 (-42 (size 16)) 111 (-42 (size 32)))
              > #B((42 (size 32) big-endian) (42 (size 32) little-endian))
              #B(0 0 0 42 42 0 0 0)
              > #B((1.23 float) (1.23 (size 32) float) (1.23 (size 64) float))
              #B(63 243 174 20 122 225 71 174 63 157 112 164 63 243 174 20
                 122 225 71 174)
              > #B((#"a" binary) (#"b" binary))
              #"ab"

       Learn more about “segments” of binary data e.g. in “Learn You Some  Er‐
       lang  (http://learnyousomeerlang.com/starting-out-for-real#bit-syntax)”
       <http://learnyousomeerlang.com/starting-out-for-real#bit-syntax>.

   Lists
       Lists are formed either as ( ... ) or [ ... ] where the  optional  ele‐
       ments  of the list are separated by some form or whitespace.  For exam‐
       ple:

              ()
              (the empty list)
              (foo bar baz)
              (foo
               bar
               baz)

   Tuples
       Tuples are written as #(value1 value2 ...).  The empty tuple #() is al‐
       so valid.

   Maps
       Maps  are  written  as #M(key1 value1 key2 value2 ...) The empty map is
       also valid and written as #M().

   Symbols
       Things that cannot be parsed as any of the above are usually considered
       as a symbol.

       Simple  examples  are  foo, Foo, foo-bar, :foo.  But also somewhat sur‐
       prisingly 123foo and 1.23e4extra (but note that  illegal  digits  don’t
       make  a  number  a symbol when using the explicit number base notation,
       e.g. #b10foo gives an error).

       Symbol names can contain a surprising breadth or characters,  basically
       all of the latin-1 character set without control character, whitespace,
       the various brackets, double quotes and semicolon.

       Of these, only |, \', ', ,, and # may not be the first character of the
       symbol’s name (but they are allowed as subsequent letters).

       I.e.  these are all legal symbols: foo, foo, µ#, ±1, 451°F.

       Symbols  can be explicitly constructed by wrapping their name in verti‐
       cal bars, e.g. |foo|, |symbol name with spaces|.  In this case the name
       can  contain any character of in the range from 0 to 255 (or even none,
       i.e. || is a valid symbol).  The vertical bar in the symbol name  needs
       to  be  escaped: |symbol with a vertical bar \| in its name| (similarly
       you will obviously have to escape the escape character as well).

   Comments
       Comments come in two forms: line comments and block comments.

       Line comments start with a semicolon (;) and finish with the end of the
       line.

       Block comments are written as #| comment text |# where the comment text
       may span multiple lines but  my  not  contain  another  block  comment,
       i.e. it may not contain the character sequence #|.

   Evaluation While Reading
       #.(... some expression ...).  E.g.  #.(+ 1 1) will evaluate the (+ 1 1)
       while it reads the expression and then be effectively 2.

Supported forms
   Core forms
              (quote e)
              (cons head tail)
              (car e)
              (cdr e)
              (list e ... )
              (tuple e ... )
              (tref tuple index)
              (tset tuple index val)
              (binary seg ... )
              (map key val ...)
              (map-size map) (msiz m)
              (map-get map key) (mref m k)
              (map-set map key val ...) (mset m k v ...)
              (map-update map key val ...) (mupd m k v ...)
              (map-remove map key ...) (mrem m k k ...)
              (lambda (arg ...) ...)
              (match-lambda
                ((arg ... ) {{(when e ...)}} ...)           - Matches clauses
                ... )
              (function func-name arity)                    - Function reference
              (function mod-name func-name arity)
              (let ((pat {{(when e ...)}} e)
                    ...)
                ... )
              (let-function ((name lambda|match-lambda)     - Local functions
                             ... )
                ... )
              (letrec-function ((name lambda|match-lambda)  - Local functions
                                ... )
                ... )
              (let-macro ((name lambda-match-lambda)        - Local macros
                          ...)
                ...)
              (progn ... )
              (if test true-expr {{false-expr}})
              (case e
                (pat {{(when e ...)}} ...)
                ... ))
              (receive
                (pat {{(when e ...)}} ... )
                ...
                (after timeout ... ))
              (catch ... )
              (try
                e
                {{(case ((pat {{(when e ...)}} ... )
                        ... ))}}
                {{(catch
                   ((tuple type value stacktrace)|_ {{(when e ...)}}
                                          - Must be tuple of length 3 or just _!
                    ... )
                   ... )}}
                {{(after ... )}})
              (funcall func arg ... )
              (call mod func arg ... )    - Call to Mod:Func(Arg, ... )

              (define-record name fields)
              (make-record name field val ...)
              (record-index name field)
              (record-field record name field)
              (record-update record name field val ...)

              (define-module name meta-data attributes)
              (extend-module meta-data attributes)

              (define-function name meta-data lambda|match-lambda)
              (define-macro name meta-data lambda|match-lambda)

              (define-type type definition)
              (define-opaque-type type definition)
              (define-function-spec func spec)

   Basic macro forms
              (: mod func arg ... ) =>
                      (call 'mod 'func arg ... )
              (mod:func arg ... ) =>
                      (call 'mod 'func arg ... )
              (? {{timeout {{default}} }})
              (++ ... )
              (-- ... )
              (list* ... )
              (let* (... ) ... )
              (flet ((name (arg ...) {{doc-string}} ...)
                     ...)
                ...)
              (flet* (...) ... )
              (fletrec ((name (arg ...) {{doc-string}} ...)
                        ...)
                ...)
              (cond (test body ...)
                    ...
                    ((?= pat expr) ...)
                    ...
                    (else ...))
              (andalso ... )
              (orelse ... )
              (fun func arity)
              (fun mod func arity)
              (lc (qual ...) ...)
              (list-comp (qual ...) ...)
              (bc (qual ...) ...)
              (binary-comp (qual ...) ...)
              (ets-ms ...)
              (trace-ms ...)

   Common Lisp inspired macros
              (defun name (arg ...) {{doc-string}} ...)
              (defun name
                {{doc-string}}
                ((argpat ...) ...)
                ...)
              (defmacro name (arg ...) {{doc-string}} ...)
              (defmacro name arg {{doc-string}} ...)
              (defmacro name
                {{doc-string}}
                ((argpat ...) ...)
                ...)
              (defsyntax name
                (pat exp)
                ...)
              (macrolet ((name (arg ...) {{doc-string}} ...)
                         ...)
                ...)
              (syntaxlet ((name (pat exp) ...)
                          ...)
                ...)
              (prog1 ...)
              (prog2 ...)
              (defmodule name ...)
              (defrecord name ...)

Patterns
       Written as normal data expressions where symbols are variables and  use
       quote  to match explicit values.  Binaries and tuples have special syn‐
       tax.

              {ok,X}                  -> (tuple 'ok x)
              error                   -> 'error
              {yes,[X|Xs]}            -> (tuple 'yes (cons x xs))
              <<34,U:16,F/float>>     -> (binary 34 (u (size 16)) (f float))
              [P|Ps]=All              -> (= (cons p ps) all)

       Repeated variables are supported in patterns and there is an  automatic
       comparison of values.

       _  as the “don’t care” variable is supported.  This means that the sym‐
       bol _, which is a perfectly valid symbol, can never  be  bound  through
       pattern matching.

       Aliases  are defined with the (= pattern1 pattern2) pattern.  As in Er‐
       lang patterns they can be used anywhere in a pattern.

       CAVEAT The lint pass of the compiler checks for aliases and if they are
       possible  to  match.  If not an error is flagged.  This is not the best
       way.  Instead there should be a warning and the  offending  clause  re‐
       moved, but later passes of the compiler can’t handle this yet.

Guards
       Wherever  a  pattern occurs (in let, case, receive, lc, etc.) it can be
       followed by an optional guard which  has  the  form  (when  test  ...).
       Guard  tests are the same as in vanilla Erlang and can contain the fol‐
       lowing guard expressions:

              (quote e)
              (cons gexpr gexpr)
              (car gexpr)
              (cdr gexpr)
              (list gexpr ...)
              (tuple gexpr ...)
              (tref gexpr gexpr)
              (binary ...)
              (make-record ...)           - Also the macro versions
              (record-field ...)
              (record-index ...)
              (map ...)
              (msiz ...) (map-size ...)
              (mref ...) (map-get ...)
              (mset ...) (map-set ...)
              (mupd ...) (map-update ...)
              (type-test e)               - Type tests
              (guard-bif ...)             - Guard BIFs, arithmetic,
                                            boolean and comparison operators

       An empty guard, (when), always succeeds  as  there  is  no  test  which
       fails.  This simplifies writing macros which handle guards.

Comments in Function Definitions
       Inside  functions  defined  with  defun  LFE  permits  optional comment
       strings in the Common Lisp style after the argument list.   So  we  can
       have:

              (defun max (x y)
                "The max function."
                (if (>= x y) x y))

       Optional  comments  are also allowed in match style functions after the
       function name and before the clauses:

              (defun max
                "The max function."
                ((x y) (when (>= x y)) x)
                ((x y) y))

       This is also possible in a similar style in local functions defined  by
       flet and fletrec:

              (defun foo (x y)
                "The max function."
                (flet ((m (a b)
                         "Local comment."
                         (if (>= a b) a b)))
                  (m x y)))

Variable Binding and Scoping
       Variables  are  lexically  scoped and bound by lambda, match-lambda and
       let forms.  All variables which are bound  within  these  forms  shadow
       variables  bound outside but other variables occurring in the bodies of
       these forms will be imported from the surrounding environments.No vari‐
       ables are exported out of the form.  So for example the following func‐
       tion:

              (defun foo (x y z)
                (let ((x (zip y)))
                  (zap x z))
                (zop x y))

       The variable y in the call (zip y) comes from the  function  arguments.
       However, the x bound in the let will shadow the x from the arguments so
       in the call (zap x z) the x is bound in the let while the z comes  from
       the  function arguments.  In the final (zop x y) both x and y come from
       the function arguments as the let does not export x.

Function Binding and Scoping
       Functions are lexically scoped and bound by the top-level defun and  by
       the  macros  flet  and fletrec.  LFE is a Lisp-2 so functions and vari‐
       ables have separate namespaces and when  searching  for  function  both
       name and arity are used.  This means that when calling a function which
       has been bound to a variable using (funcall func-var arg  ...)  is  re‐
       quired  to  call  lambda/match-lambda  bound to a variable or used as a
       value.

       Unqualified functions shadow as stated above which results in the  fol‐
       lowing order within a module, outermost to innermost:

       · Predefined Erlang BIFs

       · Predefined LFE BIFs

       · Imports

       · Top-level defines

       · Flet/fletrec

       · Core forms, these can never be shadowed

       This  means  that  it  is  perfectly  legal  to shadow BIFs by imports,
       BIFs/imports by top-level functions and BIFs/imports/top-level by  fle‐
       trecs.   In this respect there is nothing special about BIFs, they just
       behave as predefined imported functions, a whopping big  (import  (from
       erlang  ...)).   EXCEPT  that  we  know about guard BIFs and expression
       BIFs.  If you want a private version of spawn  then  define  it,  there
       will be no warnings.

       CAVEAT  This  does not hold for the supported core forms.  These can be
       shadowed by imports or redefined but the compiler will always  use  the
       core meaning and never an alternative.  Silently!

Module definition
       The  basic  forms  for defining a module and extending its metadata and
       attributes are:

              (define-module name meta-data attributes)
              (extend-module meta-data attributes)

       The valid meta data is (type typedef ...), (opaque typedef ...),  (spec
       function-spec ...) and (record record-def ...).  Each can take multiple
       definitions in one meta form.

       Attributes declarations have the syntax (attribute value-1  ...)  where
       the attribute value is a list off the values in the declaration

       To simplify defining modules there is a predefined macro:

              (defmodule name
                "This is the module documentation."
                (export (f 2) (g 1) ... )
                (export all)                          ;Export all functions
                (import (from mod (f1 2) (f2 1) ... )
                        (rename mod ((g1 2) m-g1) ((g2 1) m-g2) ... ))
                (module-alias (really-long-module-name rlmn) ...)
                (attr-1 value-1 value-2)
                {meta meta-data ...)
                ... )

       We  can have multiple export and import attributes within module decla‐
       ration.  The (export all) attribute is allowed together with other  ex‐
       port  attributes  and  overrides  them.  Other attributes which are not
       recognized by the compiler are allowed and are simply passed on to  the
       module and can be accessed with the module_info/0-1 functions.

       In  the import attribute the (from mod (f1 2) ...)  means that the call
       (f1 'everything 42) will be converted by the compiler to  (mod:f1  'ev‐
       erything  42))  while the (rename mod ((g2 2) m-g1) ...) means that the
       call (m-g1 'everything 42) will be  converted  to  (mod:g1  'everything
       42).   The rename form can be used as compact way of indicating the im‐
       ported function’s module.  Note that when importing a module

       · the compiler does no checking on that module at all

       · in the rename above the functions g1/2 and g2/1 aren’t  automatically
         imported, only the “renamed” functions.

       · we  do  not  really see in the code that we are calling a function in
         another module

       In the module-alias attribute the (really-long-module-name rlmn) decla‐
       ration  means that the call (lrmn:foo 'everything 42) will be converted
       by the compiler to (really-long-module-name:foo 'everything 42).   This
       is  often  used  to  write  short module names in the code when calling
       functions in modules with long names.  It is in many ways  better  than
       using  import as it does not hide that we are calling a function in an‐
       other module.

Macros
       Macro calls are expanded in both body and patterns.  This can  be  very
       useful to have both make and match macros, but be careful with names.

       A  macro  is function of two arguments which is a called with a list of
       the arguments to the macro call and the current macro environment.   It
       can be either a lambda or a match-lambda.  The basic forms for defining
       macros are:

              (define-macro name meta-data lambda|match-lambda)
              (let-macro ((name lambda|match-lambda)
                ...)

       Macros are definitely NOT hygienic  in  any  form.   However,  variable
       scoping  and  variable  immutability remove most of the things that can
       cause unhygienic macros.  It can be done but you are not going to do it
       by  mistake.   The only real issue is if you happen to be using a vari‐
       able which has the same name as one which the macro generates, that can
       cause  problems.  The work around for this is to give variables created
       in the macro expansion really weird names like | - foo - | which no one
       in their right mind would use.

       To simplify writing macros there are a number of predefined macros:

              (defmacro name (arg ...) ...)
              (defmacro name arg ...)
              (defmacro name ((argpat ...) body) ...)

       Defmacro can be used for defining simple macros or sequences of matches
       depending on whether the arguments are a simple list of symbols or  can
       be  interpreted  as  a  list of pattern/body pairs.  In the second case
       when the argument is just a symbol it will be bound to the whole  argu‐
       ment list.  For example:

              (defmacro double (a) `(+ ,a ,a))
              (defmacro my-list args `(list ,@args))
              (defmacro andalso
                ((list e) `,e)
                ((cons e es) `(if ,e (andalso ,@es) 'false))
                (() `'true))

       The macro definitions in a macrolet obey the same rules as defmacro.

       The  macro functions created by defmacro and macrolet automatically add
       the second argument with the current macro environment  with  the  name
       $ENV.   This  allows  explicit expansion of macros inside the macro and
       also manipulation of the macro environment.  No changes to the environ‐
       ment are exported outside the macro.

       User  defined  macros shadow the predefined macros so it is possible to
       redefine the built-in macro definitions.  However, see the  caveat  be‐
       low!

       Yes,  we have the backquote.  It is implemented as a macro so it is ex‐
       panded at macro expansion time.

       Local functions that are only available at  compile  time  and  can  be
       called by macros are defined using eval-when-compile:

              (defmacro foo (x)
                ...
                (foo-helper m n)
                ...)

              (eval-when-compile
                (defun foo-helper (a b)
                  ...)

                )

       There can be many eval-when-compile forms.  Functions defined within an
       eval-when-compile are mutually recursive but they can only  call  other
       local  functions defined in an earlier eval-when-compile and macros de‐
       fined earlier in the  file.   Functions  defined  in  eval-when-compile
       which  are called by macros can defined after the macro but must be de‐
       fined before the macro is used.

       Scheme’s syntax rules are an easy way to define macros where  the  body
       is just a simple expansion.  The are implemented the the module scm and
       are supported with scm:define-syntax and scm:let-syntax and the equiva‐
       lent  scm:defsyntax and scm:syntaxlet.  Note that the patterns are only
       the arguments to the macro call and do not contain the macro name.   So
       using them we would get:

              (scm:defsyntax andalso
                (() 'true)
                ((e) e)
                ((e . es) (case e ('true (andalso . es)) ('false 'false))))

       There  is an include file “include/scm.lfe” which defines macros so the
       names don’t have to be prefixed with scm:.

       CAVEAT While it is perfectly legal to define a Core  form  as  a  macro
       these will silently be ignored by the compiler.

Comments in Macro Definitions
       Inside  macros  defined  with  defmacro  LFE  permits  optional comment
       strings in the Common Lisp style after the argument list.   So  we  can
       have:

              (defmacro double (a)
                "Double macro."
                `(+ ,a ,a))

       Optional  comments  are  also  allowed  in match style macros after the
       macro name and before the clauses:

              (defmacro my-list args
                "List of arguments."
                `(list ,@args))

              (defmacro andalso
                "The andalso form."
                ((list e) `,e)
                ((cons e es) `(if ,e (andalso ,@es) 'false))
                (() `'true))

       This is also possible in a similar style in local functions defined  by
       macrolet:

              (defun foo (x y)
                "The max function."
                (macrolet ((m (a b)
                             "Poor macro definition."
                             `(if (>= ,a ,b) ,a ,b)))
                  (m x y)))

Extended cond
       The  tests  in  cond are Erlang tests in that they should return either
       true or false.  If no test succeeds then the cond does not generate  an
       exception  but  just returns false.  There is a simple catch-all “test”
       else which must last and can be used to handle when all tests fail.

       Cond has been extended with the extra test (?= pat expr) which tests if
       the  result  of expr matches the pattern pat.  If so it binds the vari‐
       ables in pat which can be used in the cond.  A optional  guard  is  al‐
       lowed here.  An example:

              (cond ((foo x) ...)
                    ((?= (cons x xs) (when (is_atom x)) (bar y))
                     (fubar xs (baz x)))
                    ((?= (tuple 'ok x) (baz y))
                     (zipit x))
                    ...
                    (else 'yay))

Records
       Records  are  tuples with the record name as first element and the rest
       of the fields in order exactly like “normal” Erlang records.   As  with
       Erlang records the default default value is `undefined'.

       The basic forms for defining a record, creating, accessing and updating
       it are:

              (define-record name ((field) | field
                                   (field default-value)
                                   (field default-value type) ...))
              (make-record name field value field value ...)
              (record-index name field)
              (record-field record name field)
              (record-update record name field value field value ...)

       Note that the list of field/value  pairs  when  making  or  updating  a
       record is a flat list.

       We  will explain these forms with a simple example.  To define a record
       we do:

              (define-record person
                             ((name "")
                              (address "" (string))
                              (age)))

       which defines a record person with the fields name (default value  ""),
       address  (default  value "" and type (string)) and age.  To make an in‐
       stance of a person record we do:

              (make-record person name "Robert" age 54)

       The make-record form is also used to define a pattern.

       We can get the value of the address field in a person  record  and  the
       set it by doing (the variable robert references a person record):

              (record-field robert person address)
              (record-update robert person address "my home" age 55)

       Note  that we must include the name of the record when accessing it and
       there is no need to quote the record and field names as these  are  al‐
       ways literal atoms.

       To simplify defining records there is a predefined macro:

              (defrecord name
                (field) | field
                (field default-value)
                (field default-value type)
                ... )

       This  will  create  access macros for record creation and accessing and
       updating fields.  The make-, match- and update-  forms  takes  optional
       argument pairs field-name value to get non-default values.  E.g.  for

              (defrecord person
                (name "")
                (address "" (string))
                (age))

       the following will be generated:

              (make-person {{field value}} ... )
              (match-person {{field value}} ... )
              (is-person r)
              (fields-person)
              (update-person r {{field value}} ... )
              (person-name r)
              (person-name)
              (update-person-name r name)
              (person-age r)
              (person-age)
              (update-person-age r age)
              (person-address r)
              (person-address)
              (update-person-address r address)

       · (make-person  name "Robert" age 54) - Will create a new person record
         with the name field set to “Robert”, the age field set to 54 and  the
         address field set to the default "".

       · (match-person name name age 55) - Will match a person with age 55 and
         bind the variable name to the name field of the record.  Can use  any
         variable name here.

       · (is-person john) - Test if john is a person record.

       · (person-address john) - Return the address field of the person record
         john.

       · (person-address) - Return the index of the address field of a  person
         record.

       · (update-person-address  john  "back  street")  -  Updates the address
         field of the person record john to “back street”.

       · (update-person john age 35 address "front street") -  In  the  person
         record  john  update  the  age  field  to 35 and the address field to
         “front street”.

       · (fields-person) - Returns a list of fields for the record.   This  is
         useful  for  when  using  LFE  with Mnesia, as the record field names
         don’t have to be provided manually in the create_table call.

       · (size-person) - Returns the size of the record tuple.

       Note that the older now deprecated set- forms are still generated.

Binaries/bitstrings
       A binary is

              (binary seg ... )

       where seg is

                      byte
                      string
                      (val integer|float|binary|bitstring|bytes|bits
                           (size n) (unit n)
                           big-endian|little-endian|native-endian
                           big|little|native
                           signed|unsigned)

       val can also be a string in which case the specifiers will  be  applied
       to  every  character in the string.  As strings are just lists of inte‐
       gers these are also valid here.  In a binary constant all literal forms
       are allowed on input but they will always be written as bytes.

Maps
       A map is created with:

              (map key value ... )

       To access maps there are the following forms:

       · (map-size map) - Return the size of a map.

       · (map-get  map  key) - Return the value associated with the key in the
         map.

       · (map-set map key val ... ) - Set the keys in the map to values.

       · (map-update map key val ... ) - Update the keys in the map to values.
         Note that this form requires all the keys to exist.

       · (map-remove map key ... ) - Remove the keys in the map.

       N.B.   This syntax for processing maps has stabilized but may change in
       the future!

       There are also alternate short forms msiz, mref, mset,  mupd  and  mrem
       based  on  the Maclisp array reference forms.  They take the same argu‐
       ments as their longer alternatives.

List/binary comprehensions
       List/binary comprehensions are supported as  macros.   The  syntax  for
       list comprehensions is:

              (lc (qual  ...) expr ... )
              (list-comp (qual  ...) expr ... )

       where the final expr is used to generate the elements of the list.

       The syntax for binary comprehensions is:

              (bc (qual  ...) expr ... )
              (binary-comp (qual  ...) expr ... )

       where  the final expr is a bitseg expr and is used to generate the ele‐
       ments of the binary.

       The supported qualifiers, in both list/binary comprehensions are:

              (<- pat {{guard}} list-expr)        - Extract elements from list
              (<= bin-pat {{guard}} binary-expr)  - Extract elements from binary
              (?= pat {{guard}} expr)  - Match test and bind variables in pat
              expr                     - Normal boolean test

       Some examples:

              (lc ((<- v (when (> v 5)) l1)
                   (== (rem v 2) 0))
                v)

       returns a list of all the even  elements  of  the  list  l1  which  are
       greater than 5.

              (bc ((<= (f float (size 32)) b1)        ;Only bitseg needed
                   (> f 10.0))
                (: io fwrite "~p\n" (list f))
                (f float (size 64)))                  ;Only bitseg needed

       returns  a  binary of floats of size 64 of floats which are larger than
       10.0 from the binary b1 and of size 32.  The returned numbers are first
       printed.

       N.B.  A word of warning when using guards when extracting elements from
       a binary.  When a match/guard fails for a binary no more attempts  will
       be  made  to  extract  data from the binary.  This means that even if a
       value could be extracted from the binary if the guard fails this  value
       will be lost and extraction will cease.  This is NOT the same as having
       following boolean test which may remove an element but  will  not  stop
       extraction.  Using a guard is probably not what you want!

       Normal vanilla Erlang does the same thing but does not allow guards.

ETS and Mnesia
       LFE  also  supports match specifications and Query List Comprehensions.
       The syntax for a match specification is the same as for match-lambdas:

              (ets-ms
                ((arg ... ) {{(when e ...)}} ...)             - Matches clauses
                ... )

       For example:

              (ets:select db (ets-ms
                               ([(tuple _ a b)] (when (> a 3)) (tuple 'ok b))))

       It is a macro which creates the match specification structure which  is
       used  in ets:select and mnesia:select.  For tracing instead of the ets-
       ms macro there is the trace-ms macro which is also used in  conjunction
       with  the dbg module.  The same restrictions as to what can be done ap‐
       ply as for vanilla match specifications:

       · There is only a limited number of BIFs which are allowed

       · There are some special functions only for use with dbg

       · For ets/mnesia it takes a single parameter which must a  tuple  or  a
         variable

       · For dbg it takes a single parameter which must a list or a variable

       N.B.   the  current  macro  neither knows nor cares whether it is being
       used in ets/mnesia or in dbg.  It is up to the user to get this right.

       Macros, especially record macros,  can  freely  be  used  inside  match
       specs.

       CAVEAT  Some  things which are known not to work in the current version
       are andalso, orelse and record updates.

Query List Comprehensions
       LFE supports QLCs for mnesia through the qlc macro.  It  has  the  same
       structure  as  a list comprehension and generates a Query Handle in the
       same way as with qlc:q([...]).  The handle can be  used  together  with
       all the combination functions in the module qlc.

       For example:

              (qlc (lc ((<- (tuple k v) (: ets table e2)) (== k i)) v)
                   {{Option}})

       Macros,  especially record macros, can freely be used inside query list
       comprehensions.

       CAVEAT Some things which are known not to work in the  current  version
       are nested QLCs and let/case/recieve which shadow variables.

Predefined LFE functions
       The following more or less standard lisp functions are predefined:

              (<arith_op> expr ...)
              (<comp_op> expr ...)

       The standard arithmetic operators, + - * /, and comparison operators, >
       >= < =< == /= =:= =/= , can take multiple arguments the same  as  their
       standard lisp counterparts.  This is still experimental and implemented
       using macros.  They do, however, behave like normal functions and eval‐
       uate ALL their arguments before doing the arithmetic/comparisons opera‐
       tions.

              (acons key value list)
              (pairlis keys values {{list}})
              (assoc key list)
              (assoc-if test list)
              (assoc-if-not test list)
              (rassoc value list)
              (rassoc-if test list)
              (rassoc-if-not test list)

       The standard association list functions.

              (subst new old tree)
              (subst-if new test tree)
              (subst-if-not new test tree)
              (sublis alist tree)

       The standard substitution functions.

              (macroexpand-1 expr {{environment}})

       If Expr is a macro call, does one round of expansion, otherwise returns
       Expr.

              (macroexpand expr {{environment}})

       Returns  the  expansion  returned  by calling macroexpand-1 repeatedly,
       starting with Expr, until the result is no longer a macro call.

              (macroexpand-all expr {{environment}})

       Returns the expansion from the expression where all  macro  calls  have
       been expanded with macroexpand.

       NOTE  that  when no explicit environment is given the macroexpand func‐
       tions then only the default built-in macros will be  expanded.   Inside
       macros and in the shell the variable $ENV is bound to the current macro
       environment.

              (eval expr {{environment}})

       Evaluate the expression expr.  Note  that  only  the  pre-defined  lisp
       functions,  erlang  BIFs and exported functions can be called.  Also no
       local variables can be accessed.  To access local variables the expr to
       be evaluated can be wrapped in a let defining these.

       For example if the data we wish to evaluate is in the variable expr and
       it assumes there is a local variable “foo” which  it  needs  to  access
       then we could evaluate it by calling:

              (eval `(let ((foo ,foo)) ,expr))

   Supplemental Common Lisp Functions
       LFE provides the module cl which contains the following functions which
       closely mirror functions defined in the Common  Lisp  Hyperspec.   Note
       that  the  following functions use zero-based indices, like Common Lisp
       (unlike Erlang, which start at index `1').  A major difference  between
       the  LFE  versions  and  the Common Lisp versions of these functions is
       that the boolean values are the LFE 'true and  'false.   Otherwise  the
       definitions  closely  follow the CL definitions and won’t be documented
       here.

              cl:make-lfe-bool cl-value
              cl:make-cl-bool lfe-bool

              cl:mapcar  function  list
              cl:maplist  function  list
              cl:mapc  function  list
              cl:mapl  function  list

              cl:symbol-plist  symbol
              cl:symbol-name  symbol
              cl:get  symbol  pname
              cl:get  symbol  pname  default
              cl:getl  symbol  pname-list
              cl:putprop  symbol  value  pname
              cl:remprop  symbol  pname

              cl:getf  plist  pname
              cl:getf  plist  pname  default
              cl:putf  plist  value  pname  ; This does not exist in CL
              cl:remf  plist  pname
              cl:get-properties  plist  pname-list

              cl:elt  index  sequence
              cl:length  sequence
              cl:reverse  sequence
              cl:some  predicate  sequence
              cl:every  predicate  sequence
              cl:notany  predicate  sequence
              cl:notevery  predicate  sequence
              cl:reduce  function  sequence
              cl:reduce  function  sequence  'initial-value  x
              cl:reduce  function  sequence  'from-end  'true
              cl:reduce  function  sequence  'initial-value  x  'from-end  'true

              cl:remove  item  sequence
              cl:remove-if  predicate  sequence
              cl:remove-if-not  predicate  sequence
              cl:remove-duplicates  sequence

              cl:find  item  sequence
              cl:find-if  predicate  sequence
              cl:find-if-not  predicate  sequence
              cl:find-duplicates  sequence
              cl:position  item  sequence
              cl:position-if  predicate  sequence
              cl:position-if-not  predicate  sequence
              cl:position-duplicates  sequence
              cl:count  item  sequence
              cl:count-if  predicate  sequence
              cl:count-if-not  predicate  sequence
              cl:count-duplicates  sequence

              cl:car  list
              cl:first  list
              cl:cdr  list
              cl:rest  list
              cl:nth  index  list
              cl:nthcdr  index  list
              cl:last  list
              cl:butlast  list

              cl:subst  new  old  tree
              cl:subst-if  new  test  tree
              cl:subst-if-not  new  test  tree
              cl:sublis  alist  tree

              cl:member  item  list
              cl:member-if  predicate  list
              cl:member-if-not  predicate  list
              cl:adjoin  item  list
              cl:union  list  list
              cl:intersection  list  list
              cl:set-difference  list  list
              cl:set-exclusive-or  list  list
              cl:subsetp  list  list

              cl:acons  key  data  alist
              cl:pairlis  list  list
              cl:pairlis  list  list  alist
              cl:assoc  key  alist
              cl:assoc-if  predicate  alost
              cl:assoc-if-not  predicate  alost
              cl:rassoc  key  alist
              cl:rassoc-if  predicate  alost
              cl:rassoc-if-not  predicate  alost

              cl:type-of  object
              cl:coerce  object  type

       Furthermore, there is an include file which  developers  may  which  to
       utilize  in  their  LFE  programs:  (include-lib "lfe/include/cl.lfe").
       Currently this offers Common Lisp predicates,  but  may  include  other
       useful  macros  and  functions  in  the future.  The provided predicate
       macros wrap the various is_* Erlang functions; since these are expanded
       at compile time, they are usable in guards.  The include the following:

              (alivep x)
              (atomp x)
              (binaryp x)
              (bitstringp x)
              (boolp x) and (booleanp x)
              (builtinp x)
              (consp x)
              (floatp x)
              (funcp x) and (functionp x)
              (intp x) and (integerp x)
              (listp x)
              (mapp x)
              (numberp x)
              (pidp x)
              (process-alive-p x)
              (recordp x tag)
              (recordp x tag size)
              (refp x) and (referencep x)
              (tuplep x)
              (vectorp x)

       Non-predicate macros in lfe/include/cl.lfe include:

              (dolist ...)
              (vector ...)

   Supplemental Clojure Functions
       From  LFE’s earliest days, it’s Lisp-cousin Clojure (created around the
       same time) has inspired LFE developers to create similar, BEAM-versions
       of  those  functions.   These  were collected in a separate library and
       then expanded upon, until eventually becoming part of the LFE  standard
       library.

       Function definition macros:

              (clj:defn ...)
              (clj:defn- ...)
              (clj:fn ...)

       Threading macros:

              (clj:-> ...)
              (clj:->> ...)
              (clj:as-> ...)
              (clj:cond-> ...)
              (clj:cond->> ...)
              (clj:some-> ...)
              (clj:some->> ...)
              (clj:doto ...)

       Conditional macros:

              (clj:if-let ...)
              (clj:iff-let ...)
              (clj:condp ...)
              (clj:if-not ...)
              (clj:iff-not ...)
              (clj:when-not ...)
              (clj:not= ...)

       Predicate macros:

              (clj:atom? x)
              (clj:binary? x)
              (clj:bitstring? x)
              (clj:bool? x)
              (clj:boolean? x)
              (clj:even? x)
              (clj:false? x)
              (clj:falsy? x)
              (clj:float? x)
              (clj:func? x)
              (clj:function? x)
              (clj:identical? x)
              (clj:int? x)
              (clj:integer? x)
              (clj:map? x)
              (clj:neg? x)
              (clj:nil? x)
              (clj:number? x)
              (clj:odd? x)
              (clj:pos? x)
              (clj:record? x)
              (clj:reference? x)
              (clj:true? x)
              (clj:tuple? x)
              (clj:undef? x)
              (clj:undefined? x)
              (clj:zero? x)

       Other:

              (clj:str x)
              (clj:lazy-seq x)
              (clj:conj ...)
              (clj:if ...)

       Most  of  the  above  mentioned macros are available in the clj include
       file, the use of which allows developers to forego the clj:  prefix  in
       calls:

              (include-lib "lfe/include/clj.lfe")

Notes
       · NYI - Not Yet Implemented

       · N.B.  - Nota bene (note well)

SEE ALSO
       lfe(1), lfescript(1), lfe_cl(3)

AUTHORS
       Robert Virding.



                                   2008-2020                      lfe_guide(7)