Programming Scala by Wampler Dean & Payne Alex

Author:Wampler, Dean & Payne, Alex [Wampler, Dean]
Language: eng
Format: mobi
Tags: COMPUTERS / Programming Languages / General
Publisher: O'Reilly Media
Published: 2009-09-09T16:00:00+00:00

Long-term maintenance

DSLs can require more maintenance over the long term to factor in domain changes. Also, new developers will require more time to learn how to use and maintain a DSL.

However, when a DSL is appropriate for an application—e.g., when it would be used frequently to implement and change functionality—a well-designed DSL can be a powerful tool for building flexible and robust applications.

From the implementation point of view, DSLs are often classified as internal and external.

An internal (sometimes called embedded) DSL is an idiomatic way of writing code in a general-purpose programming language, like Scala. No special-purpose parser is necessary for internal DSLs. Instead, they are parsed just like any other code written in the language. In contrast, an external DSL is a custom language with its own custom grammar and parser.

Internal DSLs are easier to create because they don’t require a special-purpose parser. On the other hand, the constraints of the underlying language limit the options for expressing domain concepts. External DSLs remove this constraint. You can design the language any way you want, as long as you can write a reliable parser for it. The downside of external DSLs is the requirement to write and use a custom parser.

DSLs have been around a long time. For example, internal DSLs written in Lisp are as old as Lisp itself. Interest in DSLs has surged recently, driven in part by the Ruby community, because they are very easy to implement in Ruby. As we’ll see, Scala provides excellent support for the creation of internal and external DSLs.

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