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authorBjörn Gustavsson <[email protected]>2018-05-17 14:10:58 +0200
committerGitHub <[email protected]>2018-05-17 14:10:58 +0200
commit52fe21127a1e2d82a0239e8e46467ad9b778677f (patch)
tree63e426f664f996774ac40a9273235b20674063b4
parentb65538a9598981764b018e1cda409a36957a6ee7 (diff)
parent46237d41e7455ae5801d78e85a82216b73697f44 (diff)
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Merge pull request #1810 from tomas-abrahamsson/bjorn/preprocessor-extensions
Add ?OTP_RELEASE, -if and -elif to the preprocessor OTP-15087
-rw-r--r--lib/stdlib/src/epp.erl127
-rw-r--r--lib/stdlib/test/epp_SUITE.erl171
-rw-r--r--lib/syntax_tools/src/epp_dodger.erl36
-rw-r--r--lib/syntax_tools/src/erl_prettypr.erl7
-rw-r--r--lib/syntax_tools/src/erl_syntax_lib.erl2
-rw-r--r--system/doc/reference_manual/macros.xml35
6 files changed, 342 insertions, 36 deletions
diff --git a/lib/stdlib/src/epp.erl b/lib/stdlib/src/epp.erl
index 77cc88eb08..cc34d4bdd3 100644
--- a/lib/stdlib/src/epp.erl
+++ b/lib/stdlib/src/epp.erl
@@ -38,7 +38,7 @@
-type epp_handle() :: pid().
-type source_encoding() :: latin1 | utf8.
--type ifdef() :: 'ifdef' | 'ifndef' | 'else'.
+-type ifdef() :: 'ifdef' | 'ifndef' | 'if' | 'else'.
-type name() :: atom().
-type argspec() :: 'none' %No arguments
@@ -221,6 +221,8 @@ format_error({illegal_function,Macro}) ->
io_lib:format("?~s can only be used within a function", [Macro]);
format_error({illegal_function_usage,Macro}) ->
io_lib:format("?~s must not begin a form", [Macro]);
+format_error(elif_after_else) ->
+ "'elif' following 'else'";
format_error({'NYI',What}) ->
io_lib:format("not yet implemented '~s'", [What]);
format_error({error,Term}) ->
@@ -571,6 +573,7 @@ init_server(Pid, Name, Options, St0) ->
predef_macros(File) ->
Machine = list_to_atom(erlang:system_info(machine)),
Anno = line1(),
+ OtpVersion = list_to_integer(erlang:system_info(otp_release)),
Defs = [{'FILE', {none,[{string,Anno,File}]}},
{'FUNCTION_NAME', undefined},
{'FUNCTION_ARITY', undefined},
@@ -580,7 +583,8 @@ predef_macros(File) ->
{'BASE_MODULE', undefined},
{'BASE_MODULE_STRING', undefined},
{'MACHINE', {none,[{atom,Anno,Machine}]}},
- {Machine, {none,[{atom,Anno,true}]}}
+ {Machine, {none,[{atom,Anno,true}]}},
+ {'OTP_RELEASE', {none,[{integer,Anno,OtpVersion}]}}
],
maps:from_list(Defs).
@@ -1085,21 +1089,118 @@ scan_else(_Toks, Else, From, St) ->
epp_reply(From, {error,{loc(Else),epp,{bad,'else'}}}),
wait_req_scan(St).
-%% scan_if(Tokens, EndifToken, From, EppState)
+%% scan_if(Tokens, IfToken, From, EppState)
%% Handle the conditional parsing of a file.
-%% Report a badly formed if test and then treat as false macro.
+scan_if([{'(',_}|_]=Toks, If, From, St) ->
+ try eval_if(Toks, St) of
+ true ->
+ scan_toks(From, St#epp{istk=['if'|St#epp.istk]});
+ _ ->
+ skip_toks(From, St, ['if'])
+ catch
+ throw:Error0 ->
+ Error = case Error0 of
+ {_,erl_parse,_} ->
+ {error,Error0};
+ _ ->
+ {error,{loc(If),epp,Error0}}
+ end,
+ epp_reply(From, Error),
+ wait_req_skip(St, ['if'])
+ end;
scan_if(_Toks, If, From, St) ->
- epp_reply(From, {error,{loc(If),epp,{'NYI','if'}}}),
+ epp_reply(From, {error,{loc(If),epp,{bad,'if'}}}),
wait_req_skip(St, ['if']).
+eval_if(Toks0, St) ->
+ Toks = expand_macros(Toks0, St),
+ Es1 = case erl_parse:parse_exprs(Toks) of
+ {ok,Es0} -> Es0;
+ {error,E} -> throw(E)
+ end,
+ Es = rewrite_expr(Es1, St),
+ assert_guard_expr(Es),
+ Bs = erl_eval:new_bindings(),
+ LocalFun = fun(_Name, _Args) ->
+ error(badarg)
+ end,
+ try erl_eval:exprs(Es, Bs, {value,LocalFun}) of
+ {value,Res,_} ->
+ Res
+ catch
+ _:_ ->
+ false
+ end.
+
+assert_guard_expr([E0]) ->
+ E = rewrite_expr(E0, none),
+ case erl_lint:is_guard_expr(E) of
+ false ->
+ throw({bad,'if'});
+ true ->
+ ok
+ end;
+assert_guard_expr(_) ->
+ throw({bad,'if'}).
+
+%% Dual-purpose rewriting function. When the second argument is
+%% an #epp{} record, calls to defined(Symbol) will be evaluated.
+%% When the second argument is 'none', legal calls to our built-in
+%% functions are eliminated in order to turn the expression into
+%% a legal guard expression.
+
+rewrite_expr({call,_,{atom,_,defined},[N0]}, #epp{macs=Macs}) ->
+ %% Evaluate defined(Symbol).
+ N = case N0 of
+ {var,_,N1} -> N1;
+ {atom,_,N1} -> N1;
+ _ -> throw({bad,'if'})
+ end,
+ {atom,0,maps:is_key(N, Macs)};
+rewrite_expr({call,_,{atom,_,Name},As0}, none) ->
+ As = rewrite_expr(As0, none),
+ Arity = length(As),
+ case erl_internal:bif(Name, Arity) andalso
+ not erl_internal:guard_bif(Name, Arity) of
+ false ->
+ %% A guard BIF, an -if built-in, or an unknown function.
+ %% Eliminate the call so that erl_lint will not complain.
+ %% The call might fail later at evaluation time.
+ to_conses(As);
+ true ->
+ %% An auto-imported BIF (not guard BIF). Not allowed.
+ throw({bad,'if'})
+ end;
+rewrite_expr([H|T], St) ->
+ [rewrite_expr(H, St)|rewrite_expr(T, St)];
+rewrite_expr(Tuple, St) when is_tuple(Tuple) ->
+ list_to_tuple(rewrite_expr(tuple_to_list(Tuple), St));
+rewrite_expr(Other, _) ->
+ Other.
+
+to_conses([H|T]) ->
+ {cons,0,H,to_conses(T)};
+to_conses([]) ->
+ {nil,0}.
+
%% scan_elif(Tokens, EndifToken, From, EppState)
%% Handle the conditional parsing of a file.
%% Report a badly formed if test and then treat as false macro.
scan_elif(_Toks, Elif, From, St) ->
- epp_reply(From, {error,{loc(Elif),epp,{'NYI','elif'}}}),
- wait_req_scan(St).
+ case St#epp.istk of
+ ['else'|Cis] ->
+ epp_reply(From, {error,{loc(Elif),
+ epp,{illegal,"unbalanced",'elif'}}}),
+ wait_req_skip(St#epp{istk=Cis}, ['else']);
+ [_I|Cis] ->
+ skip_toks(From, St#epp{istk=Cis}, ['elif']);
+ [] ->
+ epp_reply(From, {error,{loc(Elif),epp,
+ {illegal,"unbalanced",elif}}}),
+ wait_req_scan(St)
+ end.
%% scan_endif(Tokens, EndifToken, From, EppState)
%% If we are in an if body then exit it, else report an error.
@@ -1158,6 +1259,8 @@ skip_toks(From, St, [I|Sis]) ->
skip_toks(From, St#epp{location=Cl}, ['if',I|Sis]);
{ok,[{'-',_Lh},{atom,_Le,'else'}=Else|_Toks],Cl}->
skip_else(Else, From, St#epp{location=Cl}, [I|Sis]);
+ {ok,[{'-',_Lh},{atom,_Le,'elif'}=Elif|Toks],Cl}->
+ skip_elif(Toks, Elif, From, St#epp{location=Cl}, [I|Sis]);
{ok,[{'-',_Lh},{atom,_Le,endif}|_Toks],Cl} ->
skip_toks(From, St#epp{location=Cl}, Sis);
{ok,_Toks,Cl} ->
@@ -1188,11 +1291,21 @@ skip_toks(From, St, []) ->
skip_else(Else, From, St, ['else'|Sis]) ->
epp_reply(From, {error,{loc(Else),epp,{illegal,"repeated",'else'}}}),
wait_req_skip(St, ['else'|Sis]);
+skip_else(_Else, From, St, ['elif'|Sis]) ->
+ skip_toks(From, St, ['else'|Sis]);
skip_else(_Else, From, St, [_I]) ->
scan_toks(From, St#epp{istk=['else'|St#epp.istk]});
skip_else(_Else, From, St, Sis) ->
skip_toks(From, St, Sis).
+skip_elif(_Toks, Elif, From, St, ['else'|_]=Sis) ->
+ epp_reply(From, {error,{loc(Elif),epp,elif_after_else}}),
+ wait_req_skip(St, Sis);
+skip_elif(Toks, Elif, From, St, [_I]) ->
+ scan_if(Toks, Elif, From, St);
+skip_elif(_Toks, _Elif, From, St, Sis) ->
+ skip_toks(From, St, Sis).
+
%% macro_pars(Tokens, ArgStack)
%% macro_expansion(Tokens, Anno)
%% Extract the macro parameters and the expansion from a macro definition.
diff --git a/lib/stdlib/test/epp_SUITE.erl b/lib/stdlib/test/epp_SUITE.erl
index 9123bf2f28..a3e294ffea 100644
--- a/lib/stdlib/test/epp_SUITE.erl
+++ b/lib/stdlib/test/epp_SUITE.erl
@@ -28,7 +28,8 @@
otp_8130/1, overload_mac/1, otp_8388/1, otp_8470/1,
otp_8562/1, otp_8665/1, otp_8911/1, otp_10302/1, otp_10820/1,
otp_11728/1, encoding/1, extends/1, function_macro/1,
- test_error/1, test_warning/1, otp_14285/1]).
+ test_error/1, test_warning/1, otp_14285/1,
+ test_if/1]).
-export([epp_parse_erl_form/2]).
@@ -69,7 +70,7 @@ all() ->
overload_mac, otp_8388, otp_8470, otp_8562,
otp_8665, otp_8911, otp_10302, otp_10820, otp_11728,
encoding, extends, function_macro, test_error, test_warning,
- otp_14285].
+ otp_14285, test_if].
groups() ->
[{upcase_mac, [], [upcase_mac_1, upcase_mac_2]},
@@ -799,7 +800,8 @@ otp_8130(Config) when is_list(Config) ->
PreDefMacs = macs(Epp),
['BASE_MODULE','BASE_MODULE_STRING','BEAM','FILE',
'FUNCTION_ARITY','FUNCTION_NAME',
- 'LINE','MACHINE','MODULE','MODULE_STRING'] = PreDefMacs,
+ 'LINE','MACHINE','MODULE','MODULE_STRING',
+ 'OTP_RELEASE'] = PreDefMacs,
{ok,[{'-',_},{atom,_,file}|_]} = epp:scan_erl_form(Epp),
{ok,[{'-',_},{atom,_,module}|_]} = epp:scan_erl_form(Epp),
{ok,[{atom,_,t}|_]} = epp:scan_erl_form(Epp),
@@ -952,27 +954,7 @@ ifdef(Config) ->
{define_c5,
<<"-\ndefine a.\n">>,
- {errors,[{{2,1},epp,{bad,define}}],[]}},
-
- {define_c6,
- <<"\n-if.\n"
- "-endif.\n">>,
- {errors,[{{2,2},epp,{'NYI','if'}}],[]}},
-
- {define_c7,
- <<"-ifndef(a).\n"
- "-elif.\n"
- "-endif.\n">>,
- {errors,[{{2,2},epp,{'NYI',elif}}],[]}},
-
- {define_c7,
- <<"-ifndef(a).\n"
- "-if.\n"
- "-elif.\n"
- "-endif.\n"
- "-endif.\n"
- "t() -> a.\n">>,
- {errors,[{{2,2},epp,{'NYI','if'}}],[]}}
+ {errors,[{{2,1},epp,{bad,define}}],[]}}
],
[] = compile(Config, Cs),
@@ -1117,6 +1099,147 @@ test_warning(Config) ->
[] = compile(Config, Cs),
ok.
+%% OTP-12847: Test the -if and -elif directives and the built-in
+%% function defined(Symbol).
+test_if(Config) ->
+ Cs = [{if_1c,
+ <<"-if.\n"
+ "-endif.\n"
+ "-if no_parentheses.\n"
+ "-endif.\n"
+ "-if(syntax error.\n"
+ "-endif.\n"
+ "-if(true).\n"
+ "-if(a+3).\n"
+ "syntax error not triggered here.\n"
+ "-endif.\n">>,
+ {errors,[{1,epp,{bad,'if'}},
+ {3,epp,{bad,'if'}},
+ {5,erl_parse,["syntax error before: ","error"]},
+ {11,epp,{illegal,"unterminated",'if'}}],
+ []}},
+
+ {if_2c, %Bad guard expressions.
+ <<"-if(is_list(integer_to_list(42))).\n" %Not guard BIF.
+ "-endif.\n"
+ "-if(begin true end).\n"
+ "-endif.\n">>,
+ {errors,[{1,epp,{bad,'if'}},
+ {3,epp,{bad,'if'}}],
+ []}},
+
+ {if_3c, %Invalid use of defined/1.
+ <<"-if defined(42).\n"
+ "-endif.\n">>,
+ {errors,[{1,epp,{bad,'if'}}],[]}},
+
+ {if_4c,
+ <<"-elif OTP_RELEASE > 18.\n">>,
+ {errors,[{1,epp,{illegal,"unbalanced",'elif'}}],[]}},
+
+ {if_5c,
+ <<"-ifdef(not_defined_today).\n"
+ "-else.\n"
+ "-elif OTP_RELEASE > 18.\n"
+ "-endif.\n">>,
+ {errors,[{3,epp,{illegal,"unbalanced",'elif'}}],[]}},
+
+ {if_6c,
+ <<"-if(defined(OTP_RELEASE)).\n"
+ "-else.\n"
+ "-elif(true).\n"
+ "-endif.\n">>,
+ {errors,[{3,epp,elif_after_else}],[]}},
+
+ {if_7c,
+ <<"-if(begin true end).\n" %Not a guard expression.
+ "-endif.\n">>,
+ {errors,[{1,epp,{bad,'if'}}],[]}}
+
+ ],
+ [] = compile(Config, Cs),
+
+ Ts = [{if_1,
+ <<"-if(?OTP_RELEASE > 18).\n"
+ "t() -> ok.\n"
+ "-else.\n"
+ "a bug.\n"
+ "-endif.\n">>,
+ ok},
+
+ {if_2,
+ <<"-if(false).\n"
+ "a bug.\n"
+ "-elif(?OTP_RELEASE > 18).\n"
+ "t() -> ok.\n"
+ "-else.\n"
+ "a bug.\n"
+ "-endif.\n">>,
+ ok},
+
+ {if_3,
+ <<"-if(true).\n"
+ "t() -> ok.\n"
+ "-elif(?OTP_RELEASE > 18).\n"
+ "a bug.\n"
+ "-else.\n"
+ "a bug.\n"
+ "-endif.\n">>,
+ ok},
+
+ {if_4,
+ <<"-define(a, 1).\n"
+ "-if(defined(a) andalso defined(OTP_RELEASE)).\n"
+ "t() -> ok.\n"
+ "-else.\n"
+ "a bug.\n"
+ "-endif.\n">>,
+ ok},
+
+ {if_5,
+ <<"-if(defined(a)).\n"
+ "a bug.\n"
+ "-else.\n"
+ "t() -> ok.\n"
+ "-endif.\n">>,
+ ok},
+
+ {if_6,
+ <<"-if(defined(not_defined_today)).\n"
+ " -if(true).\n"
+ " bug1.\n"
+ " -elif(true).\n"
+ " bug2.\n"
+ " -elif(true).\n"
+ " bug3.\n"
+ " -else.\n"
+ " bug4.\n"
+ " -endif.\n"
+ "-else.\n"
+ "t() -> ok.\n"
+ "-endif.\n">>,
+ ok},
+
+ {if_7,
+ <<"-if(not_builtin()).\n"
+ "a bug.\n"
+ "-else.\n"
+ "t() -> ok.\n"
+ "-endif.\n">>,
+ ok},
+
+ {if_8,
+ <<"-if(42).\n" %Not boolean.
+ "a bug.\n"
+ "-else.\n"
+ "t() -> ok.\n"
+ "-endif.\n">>,
+ ok}
+ ],
+ [] = run(Config, Ts),
+
+ ok.
+
%% Advanced test on overloading macros.
overload_mac(Config) when is_list(Config) ->
Cs = [
diff --git a/lib/syntax_tools/src/epp_dodger.erl b/lib/syntax_tools/src/epp_dodger.erl
index 0a12e8fd8b..7e741cc649 100644
--- a/lib/syntax_tools/src/epp_dodger.erl
+++ b/lib/syntax_tools/src/epp_dodger.erl
@@ -502,6 +502,10 @@ quickscan_form([{'-', _L}, {atom, La, ifdef} | _Ts]) ->
kill_form(La);
quickscan_form([{'-', _L}, {atom, La, ifndef} | _Ts]) ->
kill_form(La);
+quickscan_form([{'-', _L}, {'if', La} | _Ts]) ->
+ kill_form(La);
+quickscan_form([{'-', _L}, {atom, La, elif} | _Ts]) ->
+ kill_form(La);
quickscan_form([{'-', _L}, {atom, La, else} | _Ts]) ->
kill_form(La);
quickscan_form([{'-', _L}, {atom, La, endif} | _Ts]) ->
@@ -615,8 +619,13 @@ filter_form(T) ->
%% ---------------------------------------------------------------------
%% Normal parsing - try to preserve all information
-normal_parser(Ts, Opt) ->
- rewrite_form(parse_tokens(scan_form(Ts, Opt))).
+normal_parser(Ts0, Opt) ->
+ case scan_form(Ts0, Opt) of
+ Ts when is_list(Ts) ->
+ rewrite_form(parse_tokens(Ts));
+ Node ->
+ Node
+ end.
scan_form([{'-', _L}, {atom, La, define} | Ts], Opt) ->
[{atom, La, ?pp_form}, {'(', La}, {')', La}, {'->', La},
@@ -636,12 +645,26 @@ scan_form([{'-', _L}, {atom, La, ifdef} | Ts], Opt) ->
scan_form([{'-', _L}, {atom, La, ifndef} | Ts], Opt) ->
[{atom, La, ?pp_form}, {'(', La}, {')', La}, {'->', La},
{atom, La, ifndef} | scan_macros(Ts, Opt)];
+scan_form([{'-', _L}, {'if', La} | Ts], Opt) ->
+ [{atom, La, ?pp_form}, {'(', La}, {')', La}, {'->', La},
+ {atom, La, 'if'} | scan_macros(Ts, Opt)];
+scan_form([{'-', _L}, {atom, La, elif} | Ts], Opt) ->
+ [{atom, La, ?pp_form}, {'(', La}, {')', La}, {'->', La},
+ {atom, La, 'elif'} | scan_macros(Ts, Opt)];
scan_form([{'-', _L}, {atom, La, else} | Ts], Opt) ->
[{atom, La, ?pp_form}, {'(', La}, {')', La}, {'->', La},
{atom, La, else} | scan_macros(Ts, Opt)];
scan_form([{'-', _L}, {atom, La, endif} | Ts], Opt) ->
[{atom, La, ?pp_form}, {'(', La}, {')', La}, {'->', La},
{atom, La, endif} | scan_macros(Ts, Opt)];
+scan_form([{'-', _L}, {atom, La, error} | Ts], _Opt) ->
+ Desc = build_info_string("-error", Ts),
+ ErrorInfo = {La, ?MODULE, {error, Desc}},
+ erl_syntax:error_marker(ErrorInfo);
+scan_form([{'-', _L}, {atom, La, warning} | Ts], _Opt) ->
+ Desc = build_info_string("-warning", Ts),
+ ErrorInfo = {La, ?MODULE, {warning, Desc}},
+ erl_syntax:error_marker(ErrorInfo);
scan_form([{'-', L}, {'?', L1}, {Type, _, _}=N | [{'(', _} | _]=Ts], Opt)
when Type =:= atom; Type =:= var ->
%% minus, macro and open parenthesis at start of form - assume that
@@ -657,6 +680,11 @@ scan_form([{'?', L}, {Type, _, _}=N | [{'(', _} | _]=Ts], Opt)
scan_form(Ts, Opt) ->
scan_macros(Ts, Opt).
+build_info_string(Prefix, Ts0) ->
+ Ts = lists:droplast(Ts0),
+ String = lists:droplast(tokens_to_string(Ts)),
+ Prefix ++ " " ++ String ++ ".".
+
scan_macros(Ts, Opt) ->
scan_macros(Ts, [], Opt).
@@ -865,6 +893,10 @@ tokens_to_string([]) ->
format_error(macro_args) ->
errormsg("macro call missing end parenthesis");
+format_error({error, Error}) ->
+ Error;
+format_error({warning, Error}) ->
+ Error;
format_error({unknown, Reason}) ->
errormsg(io_lib:format("unknown error: ~tP", [Reason, 15])).
diff --git a/lib/syntax_tools/src/erl_prettypr.erl b/lib/syntax_tools/src/erl_prettypr.erl
index 60a15c8e3f..6906ef1553 100644
--- a/lib/syntax_tools/src/erl_prettypr.erl
+++ b/lib/syntax_tools/src/erl_prettypr.erl
@@ -675,7 +675,12 @@ lay_2(Node, Ctxt) ->
%% attribute name, without following parentheses.
Ctxt1 = reset_prec(Ctxt),
Args = erl_syntax:attribute_arguments(Node),
- N = erl_syntax:attribute_name(Node),
+ N = case erl_syntax:attribute_name(Node) of
+ {atom, _, 'if'} ->
+ erl_syntax:variable('if');
+ N0 ->
+ N0
+ end,
D = case attribute_type(Node) of
spec ->
[SpecTuple] = Args,
diff --git a/lib/syntax_tools/src/erl_syntax_lib.erl b/lib/syntax_tools/src/erl_syntax_lib.erl
index c7f477c4d2..ced0dba3e2 100644
--- a/lib/syntax_tools/src/erl_syntax_lib.erl
+++ b/lib/syntax_tools/src/erl_syntax_lib.erl
@@ -1317,6 +1317,8 @@ analyze_attribute(Node) ->
include_lib -> preprocessor;
ifdef -> preprocessor;
ifndef -> preprocessor;
+ 'if' -> preprocessor;
+ elif -> preprocessor;
else -> preprocessor;
endif -> preprocessor;
A ->
diff --git a/system/doc/reference_manual/macros.xml b/system/doc/reference_manual/macros.xml
index a341307ab7..760599308c 100644
--- a/system/doc/reference_manual/macros.xml
+++ b/system/doc/reference_manual/macros.xml
@@ -150,6 +150,11 @@ bar(X) ->
<item>The name of the current function.</item>
<tag><c>?FUNCTION_ARITY</c></tag>
<item>The arity (number of arguments) for the current function.</item>
+ <tag><c>?OTP_RELEASE</c></tag>
+ <item>The OTP release that the currently executing ERTS
+ application is part of, as an integer. For details, see
+ <seealso marker="erts:erlang#system_info/1"><c>erlang:system_info(otp_release)</c></seealso>.
+ This macro was introduced in OTP release 21.</item>
</taglist>
</section>
@@ -202,8 +207,16 @@ f() ->
directive. If that condition is false, the lines following
<c>else</c> are evaluated instead.</item>
<tag><c>-endif.</c></tag>
- <item>Specifies the end of an <c>ifdef</c> or <c>ifndef</c>
- directive.</item>
+ <item>Specifies the end of an <c>ifdef</c>, an <c>ifndef</c>
+ directive, or the end of an <c>if</c> or <c>elif</c> directive.</item>
+ <tag><c>-if(Condition).</c></tag>
+ <item>Evaluates the following lines only if <c>Condition</c>
+ evaluates to true.</item>
+ <tag><c>-elif(Condition).</c></tag>
+ <item>Only allowed after an <c>if</c> or another <c>elif</c> directive.
+ If the preceding <c>if</c> or <c>elif</c> directives do not
+ evaluate to true, and the <c>Condition</c> evaluates to true,
+ the lines following the <c>elif</c> are evaluated instead.</item>
</taglist>
<note>
<p>The macro directives cannot be used inside functions.</p>
@@ -231,6 +244,24 @@ or
{ok,m}</pre>
<p><c>?LOG(Arg)</c> is then expanded to a call to <c>io:format/2</c>
and provide the user with some simple trace output.</p>
+
+ <p><em>Example:</em></p>
+ <code type="none">
+-module(m)
+...
+-ifdef(OTP_RELEASE).
+ %% OTP 21 or higher
+ -if(?OTP_RELEASE >= 22).
+ %% Code that will work in OTP 22 or higher
+ -elif(?OTP_RELEASE >= 21).
+ %% Code that will work in OTP 21 or higher
+ -endif.
+-else.
+ %% OTP 20 or lower.
+-endif.
+...</code>
+ <p>The code uses the <c>OTP_RELEASE</c> macro to conditionally
+ select code depending on release.</p>
</section>
<section>