Scala dreaded underscore

Scala dreaded _ François Armand

Too many use for _ ? © Normation 2010 - 2

Too many use for _ ? © Normation 2010 - 3

It's always a kind of joker 1/ “Everything” 2/ “Give me a variable name but – I don't care of what it is – and/or – don't want to pollute my namespace with it” © Normation 2010 - 4

1. Import all Java “import *” © Normation 2010 - 5

2. var initialization to default value Highly discourage in idiomatic Scala: – Prefer val against var – Try to always initialize a variable © Normation 2010 - 6

3. Unused variables In higher order functions © Normation 2010 - 7

3. Unused variables In pattern matching © Normation 2010 - 8

4. Anonymous parameters In higher order functions © Normation 2010 - 9

4. Anonymous parameters In partial functions © Normation 2010 - 10

6. Syntactic sugar for existential type Existential type ? http://stackoverflow.com/questions/1031042/scalas-existential-types Java: – Type erasure and raw types – “?” generic wildcard Scala © Normation 2010 - 12

And some more _ with an other char 1) Accessors in tuples • ("foo", "bar")._2 // getter for “bar” 2) _* In repeated parameter (Java 1.5 varargs) • String.format("one: %s two: %s", "1" , "2") • String.format("one: %s two: %s", Seq("1","2"):_*) In XML pattern matching • See Scala XML book http://www.scalalang.org/node/131 In pattern matching, as sequence wildcard • Search for UnapplySeq documentation 3) In identifier name ending with an operator • ex: “query?” not valide name, use “query_?” 4) In setter name • _= http://stackoverflow.com/questions/1392862/scala-overriding-a-non-abstract-def-with-a-var/1393389#1393389 © Normation 2010 - 14

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Scala dreaded underscore

by Normation on Jul 01, 2010

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