Initial data commit

Task/Stack/Jq/stack-1.jq

@@ -0,0 +1,20 @@
+# create a Stack
+def Stack: {stack: []};
+
+# check an object is a Stack
+def isStack:
+  type == "object" and has("stack") and (.stack|type) == "array";
+
+def pop:
+  if .stack|length == 0 then "pop: stack is empty" | error
+  else {stack: .stack[1:], item: .stack[0]]
+  end;
+
+def push($x):
+  .stack = [$x] + .stack | .item = null;
+
+def size:
+  .stack | length;
+
+def isEmpty:
+  size == 0;

Task/Stack/Jq/stack-2.jq

@@ -0,0 +1 @@
+Stack | push(3) | pop | .item

Task/Stack/Jq/stack-3.jq

@@ -0,0 +1,4 @@
+        Stack
+| size, (push(3)
+| size, (pop
+| size, .item ))

Task/Stack/Jq/stack-4.jq

@@ -0,0 +1,5 @@
+null
+| observe(Stack;   size)
+| observe(push(3); size)
+| observe(pop;     size)
+| .emit, item

Task/Stack/Jq/stack-5.jq

@@ -0,0 +1,12 @@
+# Input: an object
+# Output: the updated object with .emit filled in from `update|emit`.
+# `emit` may produce a stream of values, which need not be strings.
+def observe(update; emit):
+  def s(stream): reduce stream as $_ (null;
+    if $_ == null then .
+    elif . == null then "\($_)"
+    else . + "\n\($_)"
+    end);
+  .emit as $x
+  | update
+  | .emit = s($x // null, emit);