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An internal DSL (often called an Embedded DSL) is a DomainSpecificLanguage that is written
inside an existing host language. It's a common way of thinking in
a number of programming language communities - particularly the Lisp
community. It's now gaining a lot of attention as DSLs are a common
way of thinking in the rapidly growing Ruby community. When people talk about internal DSLs I see two styles: internal
minilanguages and language enhancements. An internal minilanguage is really using an internal DSL to do
the same thing as you would with an external DSL. You consciously
decide to use a subset of the full GPL for a minilanguage section of
your program. It might look something like this (example from my
Language Workbench paper)
mapping('SVCL', ServiceCall) do
extract 4..18, 'customer_name'
extract 19..23, 'customer_ID'
extract 24..27, 'call_type_code'
extract 28..35, 'date_of_call_string'
end
mapping('USGE', Usage) do
extract 9..22, 'customer_name'
extract 4..8, 'customer_ID'
extract 30..30, 'cycle'
extract 31..36, 'read_date'
end
This is all valid ruby, but its use of a subset of ruby makes it
seem almost like a custom DSL. Unlike an external DSL you are limited by the syntax
and programming model of your host language, but you don't need to
bother with building a parser. You're also able to use the host
language features in complicated cases should you need to. The alternative way of using internal DSLs is quite different to
anything you might do with an external DSL. This is where you are
using DSL techniques to enhance the host language. A good example of
this is many of the facilities of Ruby on Rails. Look at these bits
of Rails validation
validates_numericality_of :age
validates_uniqueness_of :ssn
validates_format_of :length, :with => /^\d+(in|cm)/
Reading these bits of Rails's validation, it looks like we've
given the ruby language new keywords. Of course we haven't modified
ruby, this is all clever metaprogramming. But it feels like we've
enhanced the ruby language. These are both very useful techniques. As with any classification
there's a fuzzy line between them (Rake could be thought of either way.)
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