Internal Domain-specific Languages in

C#By Chad MyersLosTechies.com

Housekeeping Stuff Meet the Presenter ALT.NET LosTechies.com

Introduction

Lambda Functions◦ Inline and Multiline ◦ Block

Closures Type, List, and Dictionary Initializers Type Inferrence Basics of Expression Trees Static Reflection

Quick Primer on Advanced C# 3.0

What? – “Language within a Language” for targeted, specific purpose

Why? – To enhance the usability of an API designed and highly targeted for a specific purpose

Who? – Martin Fowler, Jeremy Miller, Ayende Rahien, Jay Fields

When? – In .NET, primarily since C# 3.0 (mini-DSLs in .NET have existed since 1.1)

How? – Attend this Workshop! Read Fowler, Miller, Rahien, Fields

Internal DSL Introduction

Bending an existing language and compiler Unconventional – but still disciplined – use

of language techniques (Language Oriented Programming)

Focus on Flow, Comprehension and Accelerated Use of Finished Product

Should Be (Outwardly…):◦ Expressive◦ Efficient/Productive◦ Conventional (in the context of the domain)◦ Discoverable, Flowing

Internal DSL Concepts

Small DSLs◦ Problem/Purpose Identification◦ Pattern Application◦ Refactoring

Large DSLs◦ Problem/Purpose Identification◦ Major component Identification and Minimal

Design◦ Pattern Application◦ Refactoring

Internal DSL Strategies

End Result◦ Generative◦ Non-Generative

Internal Structure◦ Semantic Model (Internal & External)◦ No Model

Internal DSL Form and Type

Before we get into the meat, any questions?

Questions Here

Building Block Concepts◦ Context Variables and State Management◦ Generic Type Specifiers

Building Block Patterns◦ Method Chaining◦ Nested Function◦ Nested Closure◦ Literal Type/Collection Expression (Initializers)◦ Dynamic Reception (Extension Methods)◦ Parse Tree Manipulation (Expression Trees)

Internal DSL Patterns

Structural Patterns◦ Expression Builder◦ Object Scoping◦ Delegation (Nested Closures Revisited)◦ Convention

Internal DSL Patterns

Start with Host Object Return Host Object from Each Call Modifies State or Performs an Action Purpose:

◦ Assembly/Population of an Object or Objects◦ Allows for in-line or multi-line statements◦ Provides clear path for consumers

Examples:◦ System.DateTime◦ System.String

Method Chaining Pattern

Small, Focused, Deterministic Encapsulates Frequent Tasks Helps Keep Flow Helps Enhance Language Examples:

◦ DateTime.Parse(), *.Parse()◦ TimeSpan.FromMinutes

Nested Function Pattern

Delegation Scoping Physical Separation of Tasks Deterministic Resolution of Task Solves “Rathole” Problem Examples:

◦ Registry.Scan (StructureMap)◦ PersistenceModel.ForEach (Fluent Nhibernate)

Nested Closure Pattern

Type/Collection/Dictionary Initializers Can Replace Method Chaining in Many

Circumstances Can be More Expressive, in Less Spaces Examples:

◦ MsSqlConifgurationTester (Fluent Nhibernate)

Literal Type Expression Pattern

In C# 3.0, this means Extension Methods In C# 4.0, much, much more Attaching Different Behavior to Existing API Extensibility Point for Existing DSLs Allows for Different Grammars for Same API Examples:

◦ XmlExtensions (Fluent Nhibernate)◦ Specification Extensions (Fluent Nhibernate)

Dynamic Reception Pattern

In C#, this means Expression Trees Useful for Static Reflection Can Compose Expressions Examples:

◦ ClassMap<T> (Fluent Nhibernate)◦ EntityQuery stuff (Fluent Nhibernate)

Parse Tree Manipulation Pattern

Any questions on building block patterns?

Questions

State Management Progressive Interfaces The “Rathole” Problem Examples:

◦ CascadeExpression (Fluent Nhibernate)◦ ManyToManyPart (Fluent Nhibernate)◦ ActionLinkExpression (Dovetail)◦ ExpressionBase (Dovetail)

Expression Builder Pattern

Can be used directly, or serves as base class

Contains starter methods Launching point for other patterns Examples:

◦ Registry (StructureMap)◦ PersistenceModel (Fluent Nhibernate)

Object Scoping Pattern

Can also be structural Examples:

◦ ObjectFactory.Initialize (StructureMap)

Delegation (Nested Closures)

Discovery of types Identification of interesting types Application Examples:

◦ ITypeScanner (StructureMap)◦ AutoPersistenceModel (Fluent Nhibernate)

Conventions

Any questions on structural patterns?

Questions

Build a model-based generative Internal DSL for writing a fairy tale story

End result: Print the story to the debug window

Demonstrate as many patterns as possible in one DSL

In Practice: Fairy Tale Builder

FairyTale object Story Parts (Intro, focus, plot, ending) Rendering

Fairy Tale Model

Delegation -- Nested Closure Expression Builder in closure Method Chaining with Generic Type

Specifiers Literal Type Expressions Parse Tree Manipulation Dynamic Reception

Fairy Tale Builder

Any questions on Fairy Tale Builder?

Questions

Chad Myerschad@chadmyers.com

http://chadmyers.lostechies.com

Martin Fowler’s DSL WIPhttp://martinfowler.com/dslwip

Closing