tasks a-s

Task/Queue-Definition/Haskell/queue-definition-1.hs

@@ -0,0 +1,16 @@
+data Fifo a = F [a] [a]
+
+emptyFifo :: Fifo a
+emptyFifo = F [] []
+
+push :: Fifo a -> a -> Fifo a
+push (F input output) item = F (item:input) output
+
+pop :: Fifo a -> (Maybe a, Fifo a)
+pop (F input (item:output)) = (Just item, F input output)
+pop (F []    []           ) = (Nothing, F [] [])
+pop (F input []           ) = pop (F [] (reverse input))
+
+isEmpty :: Fifo a -> Bool
+isEmpty (F [] []) = True
+isEmpty _         = False

Task/Queue-Definition/Haskell/queue-definition-2.hs

@@ -0,0 +1,34 @@
+# Use a record to hold a Queue, using a list as the concrete implementation
+record Queue(items)
+
+procedure make_queue ()
+  return Queue ([])
+end
+
+procedure queue_push (queue, item)
+  put (queue.items, item)
+end
+
+# if the queue is empty, this will 'fail' and return nothing
+procedure queue_pop (queue)
+  return pop (queue.items)
+end
+
+procedure queue_empty (queue)
+  return *queue.items = 0
+end
+
+# procedure to test class
+procedure main ()
+  queue := make_queue()
+
+  # add the numbers 1 to 5
+  every (item := 1 to 5) do
+    queue_push (queue, item)
+
+  # pop them in the added order, and show a message when queue is empty
+  every (1 to 6) do {
+    write ("Popped value: " || queue_pop (queue))
+    if (queue_empty (queue)) then write ("empty queue")
+  }
+end

Task/Queue-Definition/Haskell/queue-definition-3.hs

@@ -0,0 +1,30 @@
+# Use a class to hold a Queue, with a list as the concrete implementation
+class Queue (items)
+  method push (item)
+    put (items, item)
+  end
+
+  # if the queue is empty, this will 'fail' and return nothing
+  method take ()
+    return pop (items)
+  end
+
+  method is_empty ()
+    return *items = 0
+  end
+
+  initially () # initialises the field on creating an instance
+    items := []
+end
+
+procedure main ()
+  queue := Queue ()
+
+  every (item := 1 to 5) do
+    queue.push (item)
+
+  every (1 to 6) do {
+    write ("Popped value: " || queue.take ())
+    if queue.is_empty () then write ("empty queue")
+  }
+end