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sbt - build tool
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Breeze - numerical computing library
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Spire - numeric types library
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Smile - basic stats and ML
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Rainier - Bayesian modelling and probabilistic programming
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EvilPlot - plotting library
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Mdoc - typechecked Markdown for Scala
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ScalaTest - popular unit testing library
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ScalaCheck - property-based testing
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Apache Spark - big data framework
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Simulacrum - type class support
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Darren's Scala links - a much less selective and less well curated set of Scala links