Dr. David J. Pearce

Groovy 2.0 uses Flow Typing!

Groovy 2.0 has just been released, and it contains something rather interesting … optional flow typing!  For those who don’t know much about the language, Groovy is a JVM-based dynamically typed language which is similar to Java, but more compact.  And, being dynamically typed means that there’s no need for any cumbersome type declarations.

Anyhow, I just came across a discussion of what’s in Groovy 2.0 over on InfoQ. The main improvement is the introduction of (optional) static type checking.  A special annotation @TypeChecked can be placed on a class or on a single method.  This annotation indicates that the given code should be statically type checked. More important, from my perspective, is that flow typing is used by the static type checker. Here’s the example from the InfoQ article:

import groovy.transform.TypeChecked

@TypeChecked test() {
    def var = 123             // inferred type is int
    var = "123"               // assign var with a String

    println var.toInteger()   // no problem, no need to cast

    var = 123
    println var.toUpperCase() // error, var is int!

The flow typing algorithm tracks the flow and understands that variable var initially had type int, that this was then updated to type String (before the first println) before finally was updated to int again (before the second println).

The reason for including flow typing is highlighted by the following quote from this early discussion dated 2011 on the idea:

Coming from a dynamic language and going static often feels quite limiting. For me the main point of a static type system is to ensure the code I just wrote is not totally stupid.

Anyhow, I won’t go into any more details since the InfoQ article does a good job and there are some slides here as well. But, needless to say, I’m pretty excited to see this feature being used in a mainstream language…