nosewheelie

Debasish Ghosh has another excellent Scala post, this one on External DSLs made easy with Scala Parser Combinators:

External DSLs are hard since implementing them involves reinventing most of the mechanisms found in a general purpose language. Designing internal DSLs are equally hard, more so in a statically typed language. Dynamically typed languages like Ruby offer strong meta-programming facilities, which help in implementing internal DSLs. But metaprogramming in Ruby is still considered elitist by many, and is not an art mastered by programmers at large.
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Here is a sample DSL (simplified for brevity) for accepting client orders to buy/sell equities ..

(buy 100 IBM shares at max USD 45, sell 50 CISCO shares at min USD 25, buy 100 Google shares at max USD 800) for trading_account "SSS1234"

The equivalent XML will be too verbose, too painful for the eyes, and will definitely need more extraneous infrastructure over native language support for meaningful processing.