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Scala Map.mapValues returns a (lazy) view. Hilarity ensues.
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| // | |
| // TIL Scala Map.mapValues returns a lazy view | |
| // | |
| // This has the horrific consequence that supplying a non-pure function | |
| // can yield a result map with unexpected / undesired behavior (see below) | |
| // | |
| // Further discussion here: | |
| // http://stackoverflow.com/questions/14882642/scala-why-mapvalues-produces-a-view-and-is-there-any-stable-alternatives | |
| // https://issues.scala-lang.org/browse/SI-4776 | |
| // | |
| object TrickyMapValues extends App { | |
| // Here comes trouble | |
| var sideFX = 0.0 | |
| def nonPure(x: Double) = { | |
| sideFX += 1.0 | |
| x + sideFX | |
| } | |
| val myMap = Map("a" -> 1.0, "b" -> 2.0) | |
| val mapValues = myMap.mapValues(nonPure) | |
| // If you think these should produce identical values, | |
| // prepare to be delightfully surprised | |
| println("sum1 = %.2f".format(mapValues.values.sum)) | |
| println("sum2 = %.2f".format(mapValues.values.sum)) | |
| println("sum3 = %.2f".format(mapValues.values.sum)) | |
| } |
mapValues is now deprected an replaced by view.mapValues (which is explicitely lazy).
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Ran across your blog and tried this out. So tricky! I have never used mapValues but probably won't for sure now.