CODEWITHSUNDEEP
scalafunction
Turin
Turin

Reputation: 2250

Subtle differences between val and parameterless def

Ok, this is is not another question about fundamental differences between vals & defs or functions and methods. I see that while this compiles:

val Extractor = new AnyRef { def unapply(s :String) => Some(s) }
val x = "hello" match { case Extractor(s) => s }

changing val Extractor to def Extractor breaks the code. Why is it so? It's a bit disappointing, as I hoped for complete trasnparency that would let me change the implementation from vals (generating methods anyway) to defs, or vice versa. I wonder what else can be done with one but not another?

Upvotes: 2

Views: 213

Answers (1)

Rex Kerr
Rex Kerr

Reputation: 167901

I'm not sure I can supply an exhaustive list, but I can explain what's going on in this case.

Extractor has to be a stable identifier. See Section 8.1.8 of the Scala Language Specification. (A def is not stable; a val is.)

Stable identifiers have certain nice properties that allow them to be more simply computed and have greater optimization possibilities.

Upvotes: 4

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