add chapter 3

chapter_3/def.erl

@@ -0,0 +1,38 @@
+-module(def).
+-export([convert/1, convert_def/1]).
+
+convert(Day) ->
+  case Day of
+    monday -> 1;
+    tuesday -> 2;
+    wednesday -> 3;
+    thursday -> 4;
+    friday -> 5;
+    staturday -> 6;
+    sunday -> 7
+end.
+
+% 8> def:convert(amonday).
+% ** exception error: no case clause matching amonday
+%    in function  def:convert/1
+
+convert_def(Day) ->
+  case Day of
+    monday -> 1;
+    tuesday -> 2;
+    wednesday -> 3;
+    thursday -> 4;
+    friday -> 5;
+    staturday -> 6;
+    sunday -> 7;
+    Other -> {error, unknown_day}
+end.
+
+% 2> c(def).
+% ./def.erl:13: Warning: variable 'Other' is unused
+% {ok,def}
+% 3> def:convert(sunday). 
+% 7
+% 4> def:convert(other). 
+% {error,unknown_day}
+