A reference about every use cases of underscores in the Scala programming language

1. Scala dreaded _
François Armand

2. Too many use for _ ?
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3. Too many use for _ ?
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4. 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”
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5. 1. Import all
Java “import *”
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6. 2. var initialization to default value
Highly discourage in idiomatic Scala:
– Prefer val against var
– Try to always initialize a variable
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7. 3. Unused variables
In higher order functions
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8. 3. Unused variables
In pattern matching
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9. 4. Anonymous parameters
In higher order functions
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10. 4. Anonymous parameters
In partial functions
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11. 5. Don't import name in namespace
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12. 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
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13. 6. Syntactic sugar for existential type
And in pattern matching
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14. 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
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15. Q&A
Thanks !
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