Unexpected behavior when implicit method arguments also support default values?

[Dean Wampler]

Welcome to Scala version 2.11.1 (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0_45).
...

scala> def m(implicit i:Int = 0) = println(i)

m: (implicit i: Int)Unit

scala> m()
0

scala> m
0

scala> implicit val iv = 10
iv: Int = 10

scala> m
10

scala> m()
0

This sort of breaks the "principle of least surprise".

--

Dean Wampler, Ph.D.

Typesafe
@deanwampler

http://typesafe.com

http://polyglotprogramming.com

[Bob Hiestand]

Speaking only for myself, it doesn't seem surprising.  m is defined as not having an argument list, with an implicit list.  Therefore 'm' means "apply an implicit", but 'm()' means "I'm passing the (lack of) implicit parameter explicitly".

bob

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[Jason Zaugg]

On Wednesday, July 9, 2014, Dean Wampler <deanw...@gmail.com> wrote:

Welcome to Scala version 2.11.1 (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0_45).
...

scala> def m(implicit i:Int = 0) = println(i)

m: (implicit i: Int)Unit

scala> m()
0

scala> m
0

scala> implicit val iv = 10
iv: Int = 10

scala> m
10

scala> m()
0

This sort of breaks the "principle of least surprise".

Arguably it is in poor style to define implicit params with defaults, but the results aren't that surprising IMO. The implicit argument list is only provided by the compiler if you don't provide it explicitly. Your second example does provide it explicitly, albeit short argument, which is where defaults are triggered. 

Jason 

[Simon Ochsenreither]

Would it be reasonable to allow mixing default parameters and implicits?

Not sure if there are real world use cases out there which depend on that ...

[Sonnenschein]

Simon: Sure there are...