In Scala, what exactly does 'val a: A = _' (underscore) mean?
What exactly does val a: A = _ initialize a value to? Is this a typed null? Thanks.
val a: A = _ is a compile error. For example:
scala> val a: String = _
<console>:1: error: unbound placeholder parameter
val a: String = _
^
What does work is var a: A = _ (note var instead of val). As Chuck says in his answer, this initialises the variable to a default value. From the Scala Language Specification:
0 if T is Int or one of its subrange types, 0L if T is Long, 0.0f if T is Float, 0.0d if T is Double, false if T is Boolean, () if T is Unit, null for all other types T.
It initializes a to the default value of the type A. For example, the default value of an Int is 0 and the default value of a reference type is null.
NotNull trait? :-)
_ trumps NotNull.
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val?val a: Int = _is probably a compilation error because it would be bad practice if it worked. It would just be an obfuscated way of writingval a: Int = 0. Setting avarto a default value makes sense since avaris expected to change, but avalis fixed so best practice would be to assign a value explicitly.