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[#]: collector: (lujun9972)
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[#]: translator: (HankChow)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (What developers need to know about domain-specific languages)
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[#]: via: (https://opensource.com/article/20/2/domain-specific-languages)
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[#]: author: (Girish Managoli https://opensource.com/users/gammay)
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What developers need to know about domain-specific languages
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======
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DSLs are used for a specific context in a particular domain. Learn more
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about what they are and why you might want to use one.
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![Various programming languages in use][1]
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A [domain-specific language][2] (DSL) is a language meant for use in the context of a particular domain. A domain could be a business context (e.g., banking, insurance, etc.) or an application context (e.g., a web application, database, etc.) In contrast, a general-purpose language (GPL) can be used for a wide range of business problems and applications.
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A DSL does not attempt to please all. Instead, it is created for a limited sphere of applicability and use, but it's powerful enough to represent and address the problems and solutions in that sphere. A good example of a DSL is HTML. It is a language for the web application domain. It can't be used for, say, number crunching, but it is clear how widely used HTML is on the web.
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A GPL creator does not know where the language might be used or the problems the user intends to solve with it. So, a GPL is created with generic constructs that potentially are usable for any problem, solution, business, or need. Java is a GPL, as it's used on desktops and mobile devices, embedded in the web across banking, finance, insurance, manufacturing, etc., and more.
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### Classifying DSLs
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In the DSL world, there are two types of languages:
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* **Domain-specific language (DSL):** The language in which a DSL is written or presented
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* **Host language:** The language in which a DSL is executed or processed
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A DSL written in a distinct language and processed by another host language is called an **external** DSL.
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This is a DSL in SQL that can be processed in a host language:
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```
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SELECT account
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FROM accounts
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WHERE account = '123' AND branch = 'abc' AND amount >= 1000
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```
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For that matter, a DSL could be written in English with a defined vocabulary and form that can be processed in another host language using a parser generator like ANTLR:
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```
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`if smokes then increase premium by 10%`
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```
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If the DSL and host language are the same, then the DSL type is **internal**, where the DSL is written in the language's semantics and processed by it. These are also referred to as **embedded** DSLs. Here are two examples.
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* A Bash DSL that can be executed in a Bash engine: [code]`if today_is_christmas; then apply_christmas_discount; fi` [/code] This is valid Bash that is written like English.
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* A DSL written in a GPL like Java: [code] orderValue = orderValue
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.applyFestivalDiscount()
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.applyCustomerLoyalityDiscount()
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.applyCustomerAgeDiscount(); [/code] This uses a fluent style and is readable like English.
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Yes, the boundaries between DSL and GPL sometimes blur.
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### DSL examples
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Some languages used for DSLs include:
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* Web: HTML
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* Shell: sh, Bash, CSH, and the likes for *nix; MS-DOS, Windows Terminal, PowerShell for Windows
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* Markup languages: XML
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* Modeling: UML
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* Data management: SQL and its variants
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* Business rules: Drools
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* Hardware: Verilog, VHD
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* Build tools: Maven, Gradle
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* Numerical computation and simulation: MATLAB (commercial), GNU Octave, Scilab
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* Various types of parsers and generators: Lex, YACC, GNU Bison, ANTLR
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### Why DSL?
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The purpose of a DSL is to capture or document the requirements and behavior of one domain. A DSL's usage might be even narrower for particular aspects within the domain (e.g., commodities trading in finance). DSLs bring business and technical teams together. This does not imply a DSL is for business use alone. For example, designers and developers can use a DSL to represent or design an application.
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A DSL can also be used to generate source code for an addressed domain or problem. However, code generation from a DSL is not considered mandatory, as its primary purpose is domain knowledge. However, when it is used, code generation is a serious advantage in domain engineering.
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### DSL pros and cons
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On the plus side, DSLs are powerful for capturing a domain's attributes. Also, since DSLs are small, they are easy to learn and use. Finally, a DSL offers a language for domain experts and between domain experts and developers.
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On the downside, a DSL is narrowly used within the intended domain and purpose. Also, a DSL has a learning curve, although it may not be very high. Additionally, although there may be advantages to using tools for DSL capture, they are not essential, and the development or configuration of such tools is an added effort. Finally, DSL creators need domain knowledge as well as language-development knowledge, and individuals rarely have both.
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### DSL software options
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Open source DSL software options include:
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* **Xtext:** Xtext enables the development of DSLs and is integrated with Eclipse. It makes code generation possible and has been used by several open source and commercial products to provide specific functions. [MADS][3] (Multipurpose Agricultural Data System) is an interesting idea based on Xtext for "modeling and analysis of agricultural activities" (however, the project seems to be no longer active).
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* **JetBrains MPS:** JetBrains MPS is an integrated development environment (IDE) to create DSLs. It calls itself a projectional editor that stores a document as its underlying abstract tree structure. (This concept is also used by programs such as Microsoft Word.) JetBrains MPS also supports code generation to Java, C, JavaScript, or XML.
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### DSL best practices
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Want to use a DSL? Here are a few tips:
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* DSLs are not GPLs. Try to address limited ranges of problems in the definitive domain.
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* You do not need to define your own DSL. That would be tedious. Look for an existing DSL that solves your need on sites like [DSLFIN][4], which lists DSLs for the finance domain. If you are unable to find a suitable DSL, you could define your own.
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* It is better to make DSLs "like English" rather than too technical.
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* Code generation from a DSL is not mandatory, but it offers significant and productive advantages when it is done.
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* DSLs are called languages but, unlike GPLs, they need not be executable. Being executable is not the intent of a DSL.
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* DSLs can be written with word processors. However, using a DSL editor makes syntax and semantics checks easier.
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If you are using DSL now or plan to do so in the future, please share your experience in the comments.
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/20/2/domain-specific-languages
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作者：[Girish Managoli][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/gammay
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/programming_language_c.png?itok=mPwqDAD9 (Various programming languages in use)
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[2]: https://en.wikipedia.org/wiki/Domain-specific_language
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[3]: http://mads.sourceforge.net/
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[4]: http://www.dslfin.org/resources.html
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@ -0,0 +1,130 @@
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[#]: collector: (lujun9972)
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[#]: translator: (HankChow)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (What developers need to know about domain-specific languages)
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[#]: via: (https://opensource.com/article/20/2/domain-specific-languages)
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[#]: author: (Girish Managoli https://opensource.com/users/gammay)
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开发者需要了解的领域特定语言
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======
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> 领域特定语言是在特定领域下用于特定上下文的语言。作为开发者，很有必要了解领域特定语言的含义，以及为什么要使用特定领域语言。
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![Various programming languages in use][1]
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<ruby>领域特定语言<rt>domain-specific language</rt></ruby>（DSL）是在特定领域下用于特定上下文的语言。这里的领域是指某种商业上的（例如银行业、保险业等）上下文，也可以指某种应用程序的（例如 Web 应用、数据库等）上下文。与之相比的另一个概念是<ruby>通用语言<rt>general-purpose language</rt></ruby>（GPL），通用语言则可以广泛应用于各种商业或应用问题当中。
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DSL 并不具备很强的普适性，它是仅为某个适用的领域而设计的，但它也足以用于表示这个领域中的问题以及构建对应的解决方案。HTML 是 DSL 的一个典型，它是在 Web 应用上使用的语言，尽管 HTML 无法进行数字运算，但也不影响它在这方面的广泛应用。
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而 GPL 则没有特定针对的领域，GPL 的设计者不可能知道这种语言会在什么领域被使用，更不清楚用户打算解决的问题是什么，因此 GPL 会被设计成可用于解决任何一种问题、适合任何一种业务、满足任何一种需求。例如 Java 就属于 GPL，它可以在 PC 或移动设备上运行，嵌入到银行、金融、保险、制造业等各种行业的应用中去。
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### DSL 的类别
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从使用方式的角度，语言可以划分出以下两类：
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* DSL：使用 DSL 形式编写或表示的语言
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* <ruby>宿主语言<rt>host language</rt></ruby>：用于执行或处理 DSL 的语言
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当 DSL 以独有的形式表达，并由另一种宿主语言来处理时，这种 DSL 称为<ruby>外部<rt>external</rt></ruby> DSL。
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以下就是可以在宿主语言中处理的 SQL：
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```
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SELECT account
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FROM accounts
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WHERE account = '123' AND branch = 'abc' AND amount >= 1000
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```
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因此，只要在规定了词汇和语法的情况下，DSL 也可以直接使用英语来编写，并使用诸如 ANTLR 这样的<ruby>解析器生成器<rt>parser generator</rt></ruby>以另一种宿主语言来处理 DSL：
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```
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`if smokes then increase premium by 10%`
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```
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如果 DSL 和宿主语言是同一种语言，这种 DSL 称为<ruby>内部<rt>internal</rt></ruby> DSL，其中 DSL 由以同一种语义的宿主语言编写和处理，因此又称为<ruby>嵌入式<rt>embedded</rt></ruby> DSL。以下是两个例子：
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* Bash 形式的 DSL 可以由 Bash 解释器执行：`if today_is_christmas; then apply_christmas_discount; fi` 同时这也是一段看起来符合英语语法的 Bash。
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* 使用类似 Java 语法编写的 DSL：
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```
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orderValue = orderValue
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.applyFestivalDiscount()
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.applyCustomerLoyalityDiscount()
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.applyCustomerAgeDiscount();
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```
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这一段的可读性也相当强。
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实际上，DSL 和 GPL 之间并没有非常明确的界限。
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### DSL 家族
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以下这些语言都可以作为 DSL 使用：
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* Web 应用：HTML
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* Shell：用于类 Unix 系统的 sh、Bash、CSH 等；用于 Windows 系统的 MS-DOS、Windows Terminal、PowerShell 等
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* 标记语言：XML
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* 建模：UML
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* 数据处理：SQL 及其变体
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* 业务规则管理：Drools
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* 硬件：Verilog、VHD
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* 构建工具：Maven、Gradle
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* 数值计算和模拟：MATLAB（商业）、GNU Octave、Scilab
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* 解析器和生成器：Lex、YACC、GNU Bison、ANTLR
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### 为什么要使用 DSL？
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DSL 的目的是在某个领域中记录一些需求和行为，在某些方面（例如金融商品交易）中，DSL 的适用场景可能更加狭窄。业务团队和技术团队能通过 DSL 有效地协同工作，因此 DSL 除了在业务用途上有所发挥，还可以让设计人员和开发人员用于设计和开发应用程序。
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DSL 还可以用于生成一些用于解决特定问题的代码，但生成代码并不是 DSL 的重点并不在此，而是对专业领域知识的结合。当然，代码生成在领域工程中是一个巨大的优势。
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### DSL 的优点和缺点
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DSL 的优点是，它对于领域的特征捕捉得非常好，同时它不像 GPL 那样包罗万有，学习和使用起来相对比较简单。因此，它在专业人员之间、专业人员和开发人员之间都提供了一个沟通的桥梁。
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而 DSL 最显著的缺点就在于它只能用于一个特定的领域和目标。尽管学习起来不算太难，但学习成本仍然存在。如果使用到 DSL 相关的工具，即使对工作效率有所提升，但开发或配置这些工具也会增加一定的工作负担。另外，如果要设计一款 DSL，设计者必须具备专业领域知识和语言开发知识，而同时具备这两种知识的人却少之又少。
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### DSL 相关软件
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开源的 DSL 软件包括：
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* Xtext：Xtext 可以与 Eclipse 集成，并支持 DSL 开发。它能够实现代码生成，因此一些开源和商业产品都用它来提供特定的功能。用于农业活动建模分析的<ruby>多用途农业数据系统<rt>Multipurpose Agricultural Data System</rt></ruby>（MADS）就是基于 Xtext 实现的一个项目，可惜的是这个项目现在已经不太活跃了。
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* JetBrains MPS：JetBrains MPS 是一个可供开发 DSL 的<ruby>集成开发环境<rt>Integrated Development Environment</rt></ruby>，它将文档在底层存储为一个抽象树结构（Microsoft Word 也使用了这一概念），因此它也自称为一个<ruby>投影编辑器<rt>projectional editor</rt></ruby>。JetBrains MPS 支持 Java、C、JavaScript 和 XML 的代码生成。
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### DSL 的最佳实践
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如果你想使用 DSL，记住以下几点：
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* DSL 不同于 GPL，DSL 只能用于解决特定领域中有限范围内的问题。
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* 不必动辄建立自己的 DSL，可以首先尝试寻找已有的 DSL。例如 [DSLFIN][4] 这个网站就提供了很多金融方面的 DSL。在实在找不到合适的 DSL 的情况下，才需要建立自己的 DSL。
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* DSL 最好像平常的语言一样具有可读性。
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* 尽管代码生成不是一项必需的工作，但它确实会大大提高工作效率。
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* 虽然 DSL 被称为语言，但 DSL 不需要像 GPL 一样可以被执行，可执行性并不是 DSL 需要达到的目的。
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* DSL 可以使用文本编辑器编写，但专门的 DSL 编辑器可以更轻松地完成 DSL 的语法和语义检查。
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如果你正在使用或将要使用 DSL，欢迎在评论区留言。
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--------------------------------------------------------------------------------
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||||
|
||||
via: https://opensource.com/article/20/2/domain-specific-languages
|
||||
|
||||
作者：[Girish Managoli][a]
|
||||
选题：[lujun9972][b]
|
||||
译者：[HankChow](https://github.com/HankChow)
|
||||
校对：[校对者ID](https://github.com/校对者ID)
|
||||
|
||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
|
||||
|
||||
[a]: https://opensource.com/users/gammay
|
||||
[b]: https://github.com/lujun9972
|
||||
[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/programming_language_c.png?itok=mPwqDAD9 (Various programming languages in use)
|
||||
[2]: https://en.wikipedia.org/wiki/Domain-specific_language
|
||||
[3]: http://mads.sourceforge.net/
|
||||
[4]: http://www.dslfin.org/resources.html
|
||||
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