groovy: switch statement and closure comprehension – nice for DSL

It is rather easy to extend groovy switch statement with our own DSL:


The trick here is that single-argument versions of IsGreaterThan, IsLessThan return closures. Switch-statement “understands” closures: it passes it’s argument (x in our case) as a parameter to the closure and expects boolean result being returned from the closure.Same thing can be done via function currying, but it looks not so nice, as with function overload.

Groovy DSL == thermonuclear way of writing XML

The code:

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import groovy.xml.MarkupBuilder String createEAD(Closure closure) { def writer = new StringWriter() def xml = new MarkupBuilder(writer) xml.mkp.xmlDeclaration(version: '1.0', encoding: 'UTF-8') xml.'ead:ead'('xmlns:ead': 'urn:isbn:1-931666-22-9') { closure.delegate = new Object() { def text(Map attrs, content) { def a = attrs.keySet().find { it in ['bold', 'italic', 'underline'] } if(a && attrs[a]) { xml.'ead:emph' render: a, { text attrs.findAll({ it.key != a }), content } } else text content } def text(content) { if(content instanceof String) xml.mkp.yield content else if(content instanceof Closure) content() } } closure() } return writer.toString() } println createEAD { text bold: true, italic: true, { text 'Hello, ' text underline: true, 'world!' } }

produces XML:

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<?xml version='1.0' encoding='UTF-8'?> <ead:ead xmlns:ead='urn:isbn:1-931666-22-9'> <ead:emph render='bold'> <ead:emph render='italic'>Hello, <ead:emph render='underline'>world!</ead:emph> </ead:emph> </ead:emph> </ead:ead>