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lfe_gen(3)							    lfe_gen(3)

NAME
       lfe_gen - Lisp Flavoured	Erlang (LFE) dynamic code generator

SYNOPSIS
       This module provides an experimental interface for dynamically generat-
       ing modules.

DATA TYPES
       sexpr()

       An LFE s-expression, a list structure.

EXPORTS
       new_module(Name)	-> ModDef.

       add_exports([[Name,Arity]], ModDef) -> ModDef.

       add_imports([from,Module|Functions], ModDef) -> ModDef.

       add_imports([rename,Module|Renames], ModDef) -> ModDef.

       add_attribute(Attribute,	ModDef)	-> ModDef.

       add_form(Form, ModDef) -> ModDef.

       build_mod(ModDef) -> Forms.

       compile_mod(Mod)	-> CompRet

       where

	      CompRet =	BinRet | ErrRet
	      BinRet = {ok,ModuleName,Binary,Warnings}
	      ErrRet = {error,Errors,Warnings}

       These  functions	are used to incrementally create a module which	can at
       the end be compiled by compile_mod/1.  The various components have  the
       same  formats  as  they	do when	defining a module in a file.  A	simple
       module which defines one	function a/0 could be defined with:

	      M0 = lfe_gen:new_module(foo),
	      M1 = lfe_gen:add_exports([[a,0]],	M0),
	      M2 = lfe_gen:add_form([defun,a,[],[quote,yes]], M1),
	      lfe_gen:compile_mod(M2)

EXAMPLE
       In this example we build	a module of parameters	where  each  parameter
       has  a  number of features which	each have a value.  We will create one
       function	for each parameter and the feature is the functions  argument.
       The value for each feature is returned.

       We are creating code equivalent to:

	      -module(Name).
	      -export([<param1>/1,...]).

	      <param1>(Feature)	->
		  case Feature of
		      <feature1> -> <value1>;
		      ...
		      _	-> erlang:error({unknown_feature,<param1>,Feature)
		  end.
	      ...

       but  generating it and compiling	it directly in memory without generat-
       ing a text file.	 We assume that	we have	collected the data and have it
       in the form:

	      Params = [{Parameter,[{Feature,Value}]}]

       The equivalent LFE code which we	will be	generating is:

	      (defmodule Name
		(export	(<param1> 1) (<param2> 1) ... ))

	      (defun <param1> (f)
		(case f
		  ('<feature1> '<value1>)
		  ...
		  (f (:	erlang error (tuple 'unknown_feature '<param1> f)))))
	      ...

       The following code builds and compiles a	module from the	parameter  da-
       ta:

	      make_module(Name,	Params)	->
		  Mod0 = lfe_gen:new_module(Name),
		  Exps = lists:map(fun ({Param,_}) -> [Param,1]	end, Params),
		  Mod1 = lfe_gen:add_exports(Exps, Mod0),
		  Mod2 = make_funcs(Params, Mod1),
		  lfe_gen:compile_mod(Mod2).

	      make_funcs([{Param,Fs}|Ps], Mod) ->
		  %% Define catch-all which generates more explicit exit value.
		  CatchAll = [f,[':',erlang,error,
				 [tuple,unknown_feature,[quote,Param],f]]],
		  %% Build case	clauses
		  Fold = fun ({Feature,Value}, Cls) ->
				 [[[quote,Feature],[quote,Value]]|Cls]
			 end
		  Cls =	lists:foldr(Fold, [CatchAll], Params),
		  %% Build function.
		  Func = [defun,Param,[f],['case',f,Cls]],
		  make_funcs(Ps, lfe_gen:add_form(Func,	Mod));
	      make_funcs([], Mod) -> Mod.		      %All done

       This  module  could  be generated and then be loaded into the system by
       doing:

	      {ok,ModuleName,Binary,Warnings} =	make_module(Name, Params),
	      code:load_binary(ModuleName, "nofile", Binary)

       The second argument to code:load_binary/3, here "nofile", is irrelevant
       in this case.

ERROR INFORMATION
       The ErrorInfo mentioned above is	the standard ErrorInfo structure which
       is returned from	all IO modules.	 It has	the following format:

       {ErrorLine,Module,ErrorDescriptor}

       A string	describing the error is	obtained with the following call:

	      apply(Module, format_error, ErrorDescriptor)

SEE ALSO
       lfe_comp(3), lfe_macro(3)

AUTHORS
       Robert Virding.

				   2008-2016			    lfe_gen(3)

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