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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 generating
     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 generating 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 data:

	    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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