Added an explanation for haskell language's foldr and foldl.

src/main/Geiserbot.java

@@ -197,56 +197,76 @@ public class Geiserbot {
             handleInputPrivate((PrivateMessage) m);
     }
 
+    private boolean isCommand(String m, String command) {
+        return (m.startsWith(command) &&
+                (!m.substring(command.length()).startsWith("@") ||
+                m.substring(command.length()).startsWith("@" + USERNAME)));
+    }
+
+    private String removeCommandHead(String m, String command) {
+        String m2 = m.substring(command.length()).trim();
+        String uname = "@" + USERNAME;
+        if(m2.startsWith(uname)) m2 = m2.substring(uname.length());
+
+        return m2;
+    }
+
     private void handleInputGroup(GroupMessage m) throws UnirestException {
         try {
             if (m.isEvent()) {
                 System.out.println("\nDetected group event.");
                 handleGroupEvent(m);
-            }
+            } else if (isCommand(m.getText(), "/start"))
-
-            else if (m.getText().startsWith("/start") &&
-                    (!m.getText().substring("/start".length()).startsWith("@") ||
-                            m.getText().substring("/start".length()).startsWith("@" + USERNAME)))
                 handleStart(m.getCid());
 
-            else if (m.getText().startsWith("/help") &&
+            else if (isCommand(m.getText(), "/help"))
-                    (!m.getText().substring("/help".length()).startsWith("@") ||
-                            m.getText().substring("/help".length()).startsWith("@" + USERNAME)))
                 handleHelp(m.getCid());
 
-            else if (m.getText().startsWith("/whoami") &&
+            else if (isCommand(m.getText(), "/whoami"))
-                    (!m.getText().substring("/whoami".length()).startsWith("@") ||
-                            m.getText().substring("/whoami".length()).startsWith("@" + USERNAME)))
                 handleWhoami(m);
 
-            else if (m.getText().startsWith("/list") &&
+            else if (isCommand(m.getText(), "/list"))
-                    (!m.getText().substring("/list".length()).startsWith("@") ||
-                            m.getText().substring("/list".length()).startsWith("@" + USERNAME)))
                 handleList(m);
+
+            else if (isCommand(m.getText(), "/foldl")) {
+                if(removeCommandHead(m.getText(), "/foldl").startsWith("-v"))
+                    explainFoldHaskellVerbose(m.getCid());
+                else explainFoldHaskell(m.getCid());
+            }
+
+            else if (isCommand(m.getText(), "/foldr")) {
+                if(removeCommandHead(m.getText(), "/foldr").startsWith("-v"))
+                    explainFoldHaskellVerbose(m.getCid());
+                else explainFoldHaskell(m.getCid());
+            }
         } catch (NoTextMessageException ignored) {}
     }
 
     private void handleInputPrivate(PrivateMessage m) throws UnirestException {
         try {
-            if (m.getText().startsWith("/start") &&
+            if (isCommand(m.getText(), "/start"))
-                    (!m.getText().substring("/start".length()).startsWith("@") ||
-                            m.getText().substring("/start".length()).startsWith("@" + USERNAME)))
                 handleStart(m.getCid());
 
-            else if (m.getText().startsWith("/help") &&
+            else if (isCommand(m.getText(), "/help"))
-                    (!m.getText().substring("/help".length()).startsWith("@") ||
-                            m.getText().substring("/help".length()).startsWith("@" + USERNAME)))
                 handleHelp(m.getCid());
 
-            else if (m.getText().startsWith("/whoami") &&
+            else if (isCommand(m.getText(), "/whoami"))
-                    (!m.getText().substring("/whoami".length()).startsWith("@") ||
-                            m.getText().substring("/whoami".length()).startsWith("@" + USERNAME)))
                 handleWhoami(m);
 
-            else if (m.getText().startsWith("/list") &&
+            else if (isCommand(m.getText(), "/list"))
-                    (!m.getText().substring("/list".length()).startsWith("@") ||
-                            m.getText().substring("/list".length()).startsWith("@" + USERNAME)))
                 handleList(m);
+
+            else if (isCommand(m.getText(), "/foldl")) {
+                if(removeCommandHead(m.getText(), "/foldl").startsWith("-v"))
+                    explainFoldHaskellVerbose(m.getCid());
+                else explainFoldHaskell(m.getCid());
+            }
+
+            else if (isCommand(m.getText(), "/foldr")) {
+                if(removeCommandHead(m.getText(), "/foldr").startsWith("-v"))
+                    explainFoldHaskellVerbose(m.getCid());
+                else explainFoldHaskell(m.getCid());
+            }
         } catch (NoTextMessageException ignored) {}
     }
 
@@ -273,7 +293,7 @@ public class Geiserbot {
     private void handleWhoami(Message m) throws UnirestException {
         String s = "Estamos en";
 
-        if(m.isPrivate()) s+= " un chat privado";
+        if(m.isPrivate()) s += " un chat privado";
         else s += " el chat " + m.getChatName();
 
         s += " de ID " + m.getCid() + ".\n";
@@ -384,6 +404,39 @@ public class Geiserbot {
         sendMessage(m.getCid(), getWarning(WARNING_NOT_IMPLEMENTED));
     }
 
+    private void explainFoldHaskell(long chat_id) throws UnirestException {
+
+
+        String fold = "The recursion for `foldr f x ys` where `ys = [y1,y2,...,yk]` looks like\n" +
+                "\n" +
+                "`f y1 (f y2 (... (f yk x) ...))`\n" +
+                "\n" +
+                "whereas the recursion for `foldl f x ys` looks like\n" +
+                "\n" +
+                "`f (... (f (f x y1) y2) ...) yk`";
+        sendMessage(chat_id, fold, PARSE_MARKDOWN);
+    }
+
+    private void explainFoldHaskellVerbose(long chat_id) throws UnirestException {
+        String fold = "The recursion for `foldr f x ys` where `ys = [y1,y2,...,yk]` looks like\n" +
+                "\n" +
+                "`f y1 (f y2 (... (f yk x) ...))`\n" +
+                "\n" +
+                "whereas the recursion for `foldl f x ys` looks like\n" +
+                "\n" +
+                "`f (... (f (f x y1) y2) ...) yk`\n" +
+                "\n" +
+                "An important difference here is that if the result of `f x y` can be computed using only the value of `x`, then `foldr` doesn't need to examine the entire list. For example:\n" +
+                "\n" +
+                "`foldr (&&) False (repeat False)`\n" +
+                "returns `False` whereas\n" +
+                "`foldl (&&) False (repeat False)`\n" +
+                "never terminates. (Note: `repeat False` creates an infinite list where every element is `False`.)\n" +
+                "\n" +
+                "On the other hand, `foldl'` is tail recursive and strict. If you know that you'll have to traverse the whole list no matter what (e.g., summing the numbers in a list), then `foldl'` is more space- (and probably time-) efficient than `foldr`.";
+        sendMessage(chat_id, fold, PARSE_MARKDOWN);
+    }
+
     private boolean exists(long chat_id) {
         return  chats.containsKey(chat_id);
     }