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Friday, May 27, 2011

Groovy Goodness: Command Chain Expressions for Fluid DSLs

Groovy 1.8 introduces command chain expression to further the support for DSLs. We already could leave out parenthesis when invoking top-level methods. But now we can also leave out punctuation when we chain methods calls, so we don't have to type the dots anymore. This results in a DSL that looks and reads like a natural language.

Let 's see a little sample where we can see how the DSL maps to real methods with arguments:

// DSL:
take 3.apples from basket
// maps to:
take(3.apples).from(basket)

// DSL (odd number of elements):
calculate high risk
// maps to:
calculate(high).risk
// or:
calculate(high).getRisk()

// DSL:
// We must use () for last method call, because method doesn't have arguments.
talk to: 'mrhaki' loudly()  
// maps to:
talk(to: 'mrhaki').loudly()

Implementing the methods to support these kind of DSLs can be done using maps and closures. The following sample is a DSL to record the time spent on a task at different clients:

worked 2.hours on design at GroovyRoom
developed 3.hours at OfficeSpace
developed 1.hour at GroovyRoom
worked 4.hours on testing at GroovyRoom

We see how to implement the methods to support this DSL here:

// Constants for tasks and clients.
enum Task { design, testing, developing }
enum Client { GroovyRoom, OfficeSpace }

// Supporting class to save work item info.
class WorkItem {
    Task task
    Client client
    Integer hours
}

// Support syntax 1.hour, 3.hours and so on.
Integer.metaClass.getHour = { -> delegate }
Integer.metaClass.getHours = { -> delegate }

// Import enum values as constants.
import static Task.*
import static Client.*

// List to save hours spent on tasks at
// different clients.
workList = []
 
def worked(Integer hours) {
    ['on': { Task task ->
        ['at': { Client client ->
            workList << new WorkItem(task: task, client: client, hours: hours)
        }]
    }]
}

def developed(Integer hours) {
    ['at': { Client client ->
        workList << new WorkItem(task: developing, client: client, hours: hours)
    }]
}

// -----------------------------------
// DSL
// -----------------------------------
worked 2.hours on design at GroovyRoom
developed 3.hours at OfficeSpace
developed 1.hour at GroovyRoom
worked 4.hours on testing at GroovyRoom


// Test if workList is filled
// with correct data.
def total(condition) {
    workList.findAll(condition).sum { it.hours }
}

assert total({ it.client == GroovyRoom }).hours == 7
assert total({ it.client == OfficeSpace }).hours == 3
assert total({ it.task == developing }).hours == 4
assert total({ it.task == design }).hours == 2
assert total({ it.task == testing }).hours == 4

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