user_guide.txt

lfe_guide(7)                                                      lfe_guide(7)



NAME
       lfe_guide - Lisp Flavoured Erlang User Guide

SYNPOSIS
       Note: {{ ...  }} is use 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 #b*10101 #*-1100

       · Octal notation:

           #o377 #o-111

       · Explicitly decimal notation:

           #d1234 #d-123 #d0

       · Hexadecimal notation:

           #xc0ffe 0x-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.  #36rHel‐
       loWorld is valid and the same number  as  #36Rhelloworld  and  #36rHEL‐
       LOWORLD.

       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 the part be‐
       fore 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 viewable
       with regular fonts.  E.g.  "Cat: \\x1f639;" might  be  easier  to  type
       (and view on output devices without a Unicode font) then typing the ac‐
       tual 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 quot‐
       ed (\\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| (simi‐
       larly 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-get m k) (map-set m k v ...) (map-update m k v ...)
              (lambda (arg ...) ...)
              (match-lambda
                ((arg ... ) {{(when e ...)}} ...)           - Matches clauses
                ... )
              (function func-name arity)                    - Function references
              (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 ignore) {{(when e ...)}}
                                              - Must be tuple of length 3!
                    ... )
                   ... )}}
                {{(after ... )}})
              (funcall func arg ... )
              (call mod func arg ... )        - Call to Mod:Func(Arg, ... )

              (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)

   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 ...
                    {{(?= pat expr)}}
                    ... )
              (andalso ... )
              (orelse ... )
              (fun func arity)
              (fun mod func arity)
              (lc (qual ...) ...)
              (list-comp (qual ...) ...)
              (bc (qual ...) ...)
              (binary-comp (qual ...) ...)
              (match-spec ...)

   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 ...)

   Older Scheme inspired macros
              (define (name arg ...) ...)
              (define name lambda|match-lambda)
              (define-syntax name
                (syntax-rules (pat exp) ...)|(macro (pat body) ...))
              (let-syntax ((name ...)
                           ...)
                ...)
              (begin ...)
              (define-record 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 ...)
              (progn gtest ...)           - Sequence of guard tests
              (if gexpr gexpr gexpr)
              (type-test e)
              (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 prefined imported functions, a whopping big (import (from er‐
       lang ...)).  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
              (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 ((f1 2) sune) ((f2 1) kurt) ... ))
                (import (prefix mod mod-prefix))      - NYI
                (attr-1 value-1 value-2)
                ... )

       Can have multiple export and import declarations within module declara‐
       tion.   The (export all) declaration is allowed together with other ex‐
       port declarations and overrides them.  Other attributes which  are  not
       recognised  by the compiler are allowed and are simply passed on to the
       module and can be accessed through module_info/0-1.

Parameterized modules
              (defmodule (name par1 par2 ... )
                ... )

       Define a parameterized module which behaves the same way as in  vanilla
       Erlang.  For now avoid defining functions 'new' and 'instance'.

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 argument 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.

       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.  These are supported with defsyntax and
       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:

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

       N.B.  These are definitely NOT hygienic.

       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
       Cond has been extended with the extra test (?= pat expr) which tests if
       the  result  of  expr matches pat.  If so it binds the variables in pat
       which can be used in the cond.  A optional guard is allowed  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))
                    ... )

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'.

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

       Will  create access functions/macros for creation and accessing fields.
       The  make-,  match-  and  set-  forms  takes  optional  argument  pairs
       field-name value to get non-default values.  E.g.  for

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

       the following will be generated:

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

       · (emp-person age '$1)  - Create an Ets Match Pattern for record person
         where the age field is set to $1 and all other fields are set to '_.

       · (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.

       · (set-person-address john "back street") - Sets the address  field  of
         the person record john to "back street".

       · (set-person john age 35 address "front street")   -   In  the  person
         record john set 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.

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:

              (map key value ... )

       To access maps there are the following forms:

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

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

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

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

       There is also an alternate short form map, mref, mset,  mupd  based  on
       the  Maclisp  array  reference  forms.  They take the same arguments 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
       Apart  from  (emp-record ...)  macros for ETS Match Patterns, which are
       also valid in Mnesia, LFE also supports match specifications and  Query
       List  Comprehensions.  The syntax for a match specification is the same
       as for match-lambdas:

              (match-spec
                ((arg ... ) {{(when e ...)}} ...)             - Matches clauses
                ... )

       For example:

              (ets:select db (match-spec
                               ([(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.  The same match-spec macro can
       also be used with the dbg module.  The same restrictions as to what can
       be done apply 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 arithmentic 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 substituition 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))

Notes
       · NYI - Not Yet Implemented

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

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

AUTHORS
       Robert Virding.



                                   2008-2016                      lfe_guide(7)