Your instinct is correct. It *is* just a transformation, though there are few subtleties.

This is how (all) language processing works. Here is a simplified explanation.

A string is transformed into a data structure. This process is called parsing. Various traversals can be performed on the resulting data structure(”abstract syntax tree”) to give different effects.

e.g: (pseudocode)
aString = ” 2 + 3 ”

ast = parse(aString)
// results in a datatructure like {operator:”+” , operand1= “2″
, operand2 =”3″ }

def print(anAst) { puts anAst.operand1, anAst.operator. ast.operand2}

def evaluate(ast) { return ast.operand1 + ast.operand2}

def generate_html(ast) …

def generate_sql(ast) ..
def whatever_you_want(ast) { … }

print(ast) => 2+3

evaluate(ast) => 5

etc.

So in Obie’s case , the “dsl” code is transformed into a data structure , either explicitly, or implicitly by the ruby parser.

Then various transformations (which are traversals on the data structure, either explicit or implicit) generate different results.

Yes, for any dsl (or any programming language) you can write an equivalent xml (representing the data structure) and xsl (representing the transform) . This might be a *lot* more work than just writing it in ruby (or lisp or whatever), but the concept is the same.

Having said that, John’s comment is right on the mark. Though there is nothing particulary new or tough about ruby dsl s, the ideas that “code == data” , and that given a powerful enough language, one could use the same language to represent code and data is strange to many developers.

Hope that helped.

This entry is mainly in response to Danny, who
is pondering my description of writing DSLs in Ruby …
He refers to some questions
he left me as comments , asking how writing DSLs in Ruby
different from using XML/XSLT. It’s really different ac…