more erlang

rip joe :(

pe/chapter5.erl

@@ -0,0 +1,23 @@
+%% chapter 5: maps and records
+-module(chapter5).
+-include("records.hrl").
+-export([make_urgent/1, count_characters/1]).
+
+%% Maps are defined with a #{ Key => Value, ... } syntax
+%% =>: update existing key or add new key
+%% :=: update existing key
+
+make_urgent(M) ->
+    M#{status := urgent}.
+
+%% Valid: make_urgent(#{status => reminder, who => kyle}).
+%% Invalid: make_urgent(#{who => kyle}).
+
+count_characters(Str) ->
+    count_characters(Str, #{}).
+count_characters([H|T], #{ H := N }=X) ->
+    count_characters(T, X#{H := N + 1});
+count_characters([H|T], X) ->
+    count_characters(T, X#{H => 1});
+count_characters([], X) -> X.
+

pe/error_test.erl

@@ -0,0 +1,45 @@
+-module(error_test).
+-export([generate_exc/1, demo1/0, demo2/0, demo3/0, catcher/1]).
+
+generate_exc(1) -> a;
+generate_exc(2) -> throw(a);
+generate_exc(3) -> exit(a);
+generate_exc(4) -> {'EXIT', a};
+generate_exc(5) -> error(a).
+
+demo1() ->
+    [catcher(I) || I <- lists:seq(1, 5)].
+
+demo2() ->
+    [{I, (catch generate_exc(I))} || I <- [1,2,3,4,5]].
+
+catcher(N) ->
+    try generate_exc(N) of
+	Val -> {N, normal, Val}
+    catch
+	throw:X -> {N, caught, thrown, X};
+	exit:X  -> {N, caught, exited, X};
+	error:X -> {N, caught, error, X}
+    end.
+
+
+%% NB: catch _ -> ... assumes default tag throw, need to specify
+%%     catch _:_ -> ...
+
+demo3() ->
+    try generate_exc(5)
+    catch
+	error:X ->
+	    {X, erlang:get_stacktrace()}
+    end.
+
+%% In Erlang, when an error is detected internally by the system or is
+%% detected by program logic, the correct approach is to crash
+%% immediately and generate a meaningful error message. We crash
+%% immediately so as not to make matters worse. The error message should
+%% be written to a permanent error log and be sufficiently detailed so
+%% that we can figure out what went wrong later. 
+%%  
+%% Second, fail politely means that only the programmer should see the
+%% detailed error messages produced when a program crashes. A user of the
+%% program should never see these messages.

pe/math_functions.erl

@@ -0,0 +1,32 @@
+-module(math_functions).
+-export([even/1, odd/1, filter/2, split/1, splitacc/1]).
+
+even(N) ->
+    N rem 2 == 0.
+
+odd(N) ->
+    N rem 2 /= 0.
+
+filter(F, [H|T]) ->
+    case F(H) of
+	true  -> [H | filter(F, T)];
+	false -> filter(F, T)
+    end;
+filter(_, []) ->
+    [].
+
+split(L) ->
+    {filter(fun (N) -> even(N) end, L),
+     filter(fun (N) -> odd(N) end, L)}.
+
+splitacc_inner([], Even, Odd) -> {Even, Odd};
+splitacc_inner([H|T], Even, Odd) ->
+    case even(H) of
+	true ->
+	    splitacc_inner(T, [H|Even], Odd);
+	false ->
+	    splitacc_inner(T, Even, [H|Odd])
+    end.
+    
+
+splitacc(L) -> splitacc_inner(L, [], []).

pe/records.hrl

@@ -0,0 +1 @@
+-record(todo, {status=reminder, who=kyle, text}).