Data commit

Task/Queue-Definition/Jq/queue-definition.jq

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+# 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);
+
+
+def fifo: {queue: []};
+
+# Is the input an object that represents the empty queue?
+def isempty:
+  type == "object"
+  and (.queue | length == 0);  # so .queue == null and .queue == [] are equivalent
+
+def push(e): .queue += [e];
+
+def pop: if isempty then empty else .item = .queue[0] | .queue |= .[1:] end;
+
+def pop_or_error: if isempty then error("pop_or_error") else pop end;
+
+# Examples
+# fifo | pop // "nothing" # produces the string "nothing"
+
+fifo
+| observe(push(42); "length after pushing: \(.queue | length)" )
+| observe(push(43); "length after pushing: \(.queue | length)" )
+| pop      # dequeue
+| .emit, .item