From 93f1c95894e61d638c6cae082defb20b4fd50fd2 Mon Sep 17 00:00:00 2001
From: Alexander Verbitsky <habibutsu@gmail.com>
Date: Tue, 17 Nov 2015 12:07:06 +0300
Subject: [PATCH] Function for printing stacktrace in human readable form

---
 README.md                   | 13 +++++++++++
 src/lager.erl               | 45 ++++++++++++++++++++++++++++++++++++-
 test/pr_stacktrace_test.erl | 26 +++++++++++++++++++++
 3 files changed, 83 insertions(+), 1 deletion(-)
 create mode 100644 test/pr_stacktrace_test.erl

diff --git a/README.md b/README.md
index 645d3aa..1eb442b 100644
--- a/README.md
+++ b/README.md
@@ -392,6 +392,19 @@ Lager 2.0 changed the backend API, there are various 3rd party backends for
 lager available, but they may not have been updated to the new API. As they
 are updated, links to them can be re-added here.
 
+Exception Pretty Printing
+----------------------
+
+```erlang
+try
+    foo()
+catch
+    Class:Error ->
+        Stacktrace = lists:reverse(erlang:get_stacktrace()),
+        lager:error("~nStacktrace:~s~n~s:~p", [lager:pr_stacktrace(Stacktrace), Class, Error])
+end.
+```
+
 Record Pretty Printing
 ----------------------
 Lager's parse transform will keep track of any record definitions it encounters
diff --git a/src/lager.erl b/src/lager.erl
index 90386c3..f1c07d2 100644
--- a/src/lager.erl
+++ b/src/lager.erl
@@ -33,7 +33,7 @@
         get_loglevel/1, get_loglevel/2, set_loglevel/2, set_loglevel/3, set_loglevel/4, get_loglevels/1,
         update_loglevel_config/1, posix_error/1, set_loghwm/2, set_loghwm/3, set_loghwm/4,
         safe_format/3, safe_format_chop/3, unsafe_format/2, dispatch_log/5, dispatch_log/7, dispatch_log/9,
-        do_log/9, do_log/10, do_log_unsafe/10, pr/2, pr/3]).
+        do_log/9, do_log/10, do_log_unsafe/10, pr/2, pr/3, pr_stacktrace/1]).
 
 -type log_level() :: debug | info | notice | warning | error | critical | alert | emergency.
 -type log_level_number() :: 0..7.
@@ -580,3 +580,46 @@ is_record_known(Record, Module) ->
             end
     end.
 
+%% @doc Print stacktrace in human readable form
+pr_stacktrace([]) ->
+    <<"">>;
+pr_stacktrace([Entry|Stacktrace]) ->
+    Indent = <<"    ">>,
+    {Mod, Func, ArityOrArgs, Location} = Entry,
+
+    BinMod = atom_to_binary(Mod, utf8),
+    BinFunc = atom_to_binary(Func, utf8),
+
+    {Arity, Args} = case ArityOrArgs of
+        A when is_integer(A) -> {A, []};
+        _ ->
+            {length(ArityOrArgs), ArityOrArgs}
+    end,
+    BinArity = integer_to_binary(Arity),
+    FunSpec = <<"fun ", BinMod/binary, ":", BinFunc/binary, "/", BinArity/binary>>,
+
+    BinArgs = case Args of
+        [] -> <<"">>;
+        _ ->
+            BArgs = binary:replace(
+                list_to_binary(io_lib:format("~p", [Args])),
+                <<"\n">>, <<"">>, [global]),
+            <<"\n", Indent/binary, Indent/binary, "args ", BArgs/binary>>
+    end,
+
+    BinLocation = case Location of
+        [{file, File}, {line, Line}] ->
+            BLine = integer_to_binary(Line),
+            BFile = list_to_binary(File),
+            <<
+                "\n", Indent/binary, Indent/binary, "file \"", BFile/binary, "\"",
+                "\n", Indent/binary, Indent/binary, "line ", BLine/binary>>;
+        [] -> <<"">>
+    end,
+
+    Output = <<
+        "\n", Indent/binary, FunSpec/binary,
+            BinArgs/binary,
+            BinLocation/binary>>,
+    Rest = pr_stacktrace(Stacktrace),
+    <<Output/binary, Rest/binary>>.
diff --git a/test/pr_stacktrace_test.erl b/test/pr_stacktrace_test.erl
new file mode 100644
index 0000000..dffa8bb
--- /dev/null
+++ b/test/pr_stacktrace_test.erl
@@ -0,0 +1,26 @@
+-module(pr_stacktrace_test).
+
+-compile([{parse_transform, lager_transform}]).
+
+-include_lib("eunit/include/eunit.hrl").
+
+foo() ->
+    throw(test).
+
+pr_stacktrace_test() ->
+    Result = try
+        foo()
+    catch
+        _Class:_Error ->
+            Stacktrace = lists:reverse(erlang:get_stacktrace()),
+            lager:pr_stacktrace(Stacktrace)
+    end,
+ExpectedPart = <<"
+    fun pr_stacktrace_test:pr_stacktrace_test/0
+        file \"test/pr_stacktrace_test.erl\"
+        line 12
+    fun pr_stacktrace_test:foo/0
+        file \"test/pr_stacktrace_test.erl\"
+        line 8">>,
+
+    ?assertNotEqual(nomatch, binary:match(Result, ExpectedPart, [])).