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Implementing custom control structures in Scala

• davidw 17y ago
  Tcl is quite good at that sort of thing too: http://wiki.tcl.tk/685 http://wiki.tcl.tk/685 Because everything is a command. I have a question though.. what's the 'Unit'? That's how they're passing in a block of code, but http://www.scala-lang.org/docu/files/api/scala/Unit.html http://www.scala-lang.org/docu/files/api/scala/Unit.html doesn't seem to have much information.
  • warfangle 17y ago
    Unit is Scala's void. Basically. It's the type returned by functions which don't have a return value.
    • davidw 17y ago
      Ah, I misunderstood then, that's the return type. What type is 'code' and how does it work that you can just put it on a line all its own and it runs? I get that it's figuring out the type via type inference, I'm just curious what the 'magic properties' are.
      • nimbix 17y ago
        'code' is actually a function. But because it doesn't take any parameters it's called without '()' at the end.
        • samstokes 17y ago
          Actually, 'code' isn't a function - that would require different syntax for declaration and usage: def dont(code: () => Unit) = // ... dont { () => println("foo") } It's actually a lazily-evaluated parameter of type Unit - Scala has optional laziness. This means rather than having to pass in a no-arg anonymous function (which requires the "() => expr" syntax as above) you can just pass an expression, and it's evaluated when (and if) needed instead of when it's passed to the "dont" method. Laziness is the reason that this looks like a control structure, and not just a regular old higher-order function.