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    <p>The behavior you are describing has little to do with lazy evaluation: root of the problem is order of execution.</p> <p>In your code (without the <code>log()</code> on <code>block_pos_at_mousedown</code>) <code>mouseDeltaFromClick</code> seems to change before <code>block_pos_at_mousedown</code> (I must say that I don't know how exactly <code>log()</code> changes the order). Let's hold on that.</p> <p>The <code>observable.combine</code> method expects <code>Property</code> as a first argument - <code>EventStream</code> you have passed will be automatically converted. Now, <code>mouseDeltaAndBlockPos</code> changes (and consequently fires all callbacks) whenever <code>mouseDeltaFromClick</code> <strong>or</strong> <code>block_pos_at_mousedown</code> changes.</p> <p>So, when <code>mouseDeltaFromClick</code> fires before <code>block_pos_at_mousedown</code> the callback at the end of code is called with new delta <strong>but with old block position</strong> (because <code>back_pos_at_mousedown</code> was converted to <code>Property</code>). The old value is <code>[0,0]</code> so the block snaps to the top left corner on each click.</p> <p>How to fix it? The safe way is to assume nothing about order of execution of unrelated callbacks and write it again with this in mind. I came up with this: </p> <pre><code>function xyFromEvent(v){ return [v.clientX, v.clientY]; } function getDelta(t){ var a = t[1]; var b = t[0]; return [a[0]-b[0], a[1]-b[1]]; } function add(p1, p2) { return [p1[0] + p2[0], p1[1] + p2[1]]; } $().ready(function () { var block = $("#clickable-block"); var html = $("html"); var blockDragging = block.asEventStream('mousedown').map(true) .merge(html.asEventStream('mouseup').map(false)) .toProperty(false); var deltas = html.asEventStream('mousemove').map(xyFromEvent).slidingWindow(2,2).map(getDelta); // Just like deltas, but [0,0] when user is not dragging the box. var draggingDeltas = Bacon.combineWith(function(delta, dragging) { if(!dragging) { return [0, 0]; } return delta; }, deltas, blockDragging); var blockPosition = draggingDeltas.scan([0,0], add); blockPosition.onValue(function(pos) { block.css({ top : pos[1] + "px", left : pos[0] + "px" }); }); }); </code></pre> <p>And jsFiddle: <a href="http://jsfiddle.net/aknNh/25/" rel="nofollow">http://jsfiddle.net/aknNh/25/</a></p> <p>EDIT: In comments <strong>raimohanska</strong> had suggested another solution using <code>flatMap</code>: <a href="http://jsfiddle.net/TFPge/1/" rel="nofollow">http://jsfiddle.net/TFPge/1/</a></p>
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    1. POBacon.js lazy evaluation, mouse drag example breaks without log() statement
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    1. USŁukasz Dąbek
    1. USŁukasz Dąbek
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    1. POBacon.js lazy evaluation, mouse drag example breaks without log() statement
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    1. COlazy evaluation and order of evaluation go hand in hand...
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      1. USPaul Kaplan
    2. COI agree that that lazy evaluation causes unexpected behavior if your code relies on a *map* function being called at a certain time. Lukasz's answer still looks pretty good to me. Except I'd have used flatMap, as in Wolfflow's drawing example: http://codepen.io/wolfflow/pen/cudiJ There's also a related thread in Google Groups: https://groups.google.com/forum/#!topic/baconjs/OR0C2jPCF9g
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    3. COAnd here's my fiddle with flatMap: http://jsfiddle.net/TFPge/
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