Start programming erlang.

.gitignore

@@ -4,6 +4,7 @@
 *.o
 *.pdf
 .d
+*.beam
 
 # ignore build systems
 .ninja_deps

pe/afile_client.erl

@@ -0,0 +1,25 @@
+-module(afile_client).
+-compile([debug_info]).
+-export([ls/1, get_file/2, put_file/3]).
+
+ls(Server) ->
+    Server ! {self(), list_dir},
+    receive
+	{Server, FileList} ->
+	    FileList
+    end.
+
+get_file(Server, File) ->
+    Server ! {self(), {get_file, File}},
+    receive
+	{Server, Content} ->
+	    Content
+    end.
+
+put_file(Server, File, Contents) ->
+    Server ! {self(), {put_file, File, Contents}},
+    receive
+	{Server, Result} ->
+	    Result
+    end.
+

pe/afile_server.erl

@@ -0,0 +1,37 @@
+-module(afile_server).
+-compile([debug_info]).
+-export([start/1, loop/1]).
+
+start(Dir) ->
+    spawn(afile_server, loop, [Dir]).
+
+%% Three significant points to observer about this code:
+%%
+%% + Who to reply to: all the received messages contain
+%%   the variable Client - the PID of the sender.
+%% + Use of self(): the PID of the server.
+%% + Pattern matching to select the message.
+loop(Dir) ->
+    receive
+	{Client, list_dir} ->
+	    Client ! {self(), file:list_dir(Dir)};
+	{Client, {get_file, File}} ->
+	    Full = filename:join(Dir, File),
+	    Client ! {self(), file:read_file(Full)};
+	{Client, {put_file, File, Contents}} ->
+	    Full = filename:join(Dir, File),
+	    Client ! {self(), file:write_file(Full, Contents)},
+	{Client, ping} ->
+	    Client ! {self(), pong};
+	{Client, _} ->
+	    Client ! {self(), unrecognised_message};
+	{_} ->
+	    io:format("unhandled message")
+    end,
+    loop(Dir).
+
+%% Notes:
+%% + function names are always lowercased
+%% + universal receiver: receive X -> X end.
+%% + erlang: use processes to structure the solutions to our problems.
+%% + *ATOMS*ARE*LOWER*CASED*

pe/geometry.erl

@@ -0,0 +1,11 @@
+-module(geometry).
+-export([area/1]).
+
+area({rectangle, Width, Height}) ->
+    Width * Height;
+area({circle, Radius}) ->
+    3.14159 * Radius * Radius;
+area({square, Side}) ->
+    Side * Side.
+
+

pe/heads.erl

@@ -0,0 +1,14 @@
+-module(heads).
+-export([len/1]).
+
+alen([], Count) ->
+    Count;
+
+alen([_|T], Count) ->
+    alen(T, Count+1).
+
+len([_|T]) ->
+    alen(T, 1);
+
+len([]) ->
+    0.

pe/hello.erl

@@ -0,0 +1,5 @@
+-module(hello).
+-export([start/0]).
+
+start() ->
+    io:format("Goodbye, joe.~n").

pe/shop.erl

@@ -0,0 +1,4 @@
+-module(shop).
+-export([cost/1]).
+
+