[PhillyOnRails] Notes from DSL talk

[Aaron Blohowiak <aaron@aaronblohowiak.com>]

Below are my notes from Obie's excellent talk.

DSLs and Ruby – Obie Fernandez

CRm / CRP / Automating batch jobs

Rails just makes the ruby easier –  even if it is just a frunt end  
for more complex ruby scripts

DSL is a smaller part of a larger system –  the stuff that changes,  
that models the particular business processes.

Other stuff – boring backend stuff

Dynamic languages should cover the dynamic aspects of the language.


Words in your dsl relate to the language of the business that you’re  
modeling

Html – DSL for web, et cetera

Context – a well written API would seem as a DSL.

Some call rails a DSL for web apps.


Object model first, then applying the DSL causes friction – it can be  
hard to map DSL onto traditional OOP methods.

Write a script that you want to work, then make that work!!!!

The object model should not be up-front design… the object model  
should EMERGE FROM the script.


Change of mindset to escape above. Them, you must understand the domain.

You also have to have a language development expertise –

“DSLs become this thing you just do.”

You translate the human language into code.

Knowledge of implementation technology – ie: leveraging  
metaprogramming features.

External – write your own parser et al
Internal dsl – hosted in general purpose language like ruby, like rake

Often implemented las an API –EX:  fluent interfaces – changing  
method calls together

pay 5.dolars.times(number_of_transactions), explosives

An advantage of internal DSLs is that they preserve host language  
structures, methods and control

“How much ruby language do I want to use? How much ruby language do I  
not want to use?”

ie: pay 5.dollars * number_of_transactions, explosives

“You have a lot of freedom, so you have to learn how to use it.”

If English was executable, most of us would be out of work!

“Have you seen the writing skills of your average programmer?”

Code should read as closely as possible to what it does?

Who writes story cards or product specs? Customer or customer advocate.

If your code resembles the requirements, cool stuff happens (later)

Coding as a modeling excersize:
                        Taking real concepts and actions and  
expressing them in words

With a profusion of classes (like in java) the faithfulness of your  
code to real life can be obscured.


Sitting down with a non programmer, they should be able to understand  
your code.

STEPs
		
            1) DEFINE SCRIPT LANGUAGE
            2) Implement
            3) make scripting language faithful to business process
(about a month)

            4) lay GUI over it – code generation
                        (about another month)
5) Visual view – source view – taking the WYSIWYG paradigm from web  
sites to business processes


You could even, with simple heuristics, spit out human-readable code

For instance, instance_eval in different contexts to spit out  
different outputs
acts_as_state_machine – “best plugin ever”

* it is a DSL that represents a state machine

True bckend can be hardcore ruby programming – extending sql classes,  
et cetera,  “behind the scenes”, test-driven.

During the implementation, we write the tests and implement

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