1. 600.429 FUNCTIONAL PROGRAMING AT WORK - HASKELL AND DOMAIN SPECIFIC LANGUAGES Dr. John Peterson Western State Colorado University

2. Homework 4 It’s up – we’ll discuss it soon! I even added it to Blackboard :-) Don’t delay in installing Euterpea – there seem to be Mac issues. You can work on the assignment even if it’s not installed. There will be a turn-in later this week to kick off the DSL implementation project. Come talk to me about projects. Talk to other students about teaming up.

3. Domain Specific Languages What is a DSL? How do we build them? How is a DSL different from an API? Why is Haskell a good vehicle for DSLs?

4. DSL Design Space Front end: how do we express programs in this language? Custom: write a parser / preprocessor Embedded: add vocabulary to an existing language. Use language extension constructs (special syntax, overloading, monads)

5. DSL Design Space How do we express the semantics of the language? Embedded: within the host language, often augmented by libraries Translated: generate code in a different language (usually source code)

6. DSL Correctness No correctness issues beyond those in the embedded part of the language. The type vocabulary is embedded in standard Haskell Custom type checking – the type system of the DSL requires a user-written type checker Extended types – the type system requires extensions to the Haskell type system

7. Haskell Evaluation and the DSL How does the lambda in Haskell relate to the DSL? It doesn’t – the language has a custom syntax and there is no Lambda in the language Integrated – the Haskell lambda calculus is part of the evaluation mechanism in the language Expanded – the language doesn’t have lambda in its semantics; the lambdas in the input are expanded away (lambda as “cpp”)

8. DSL Semantics How are semantic properties expressed or used? Embedded in Haskell semantics As rewrite rules – domain specific rewrites augment ordinary equational reasoning Custom – program semantics are not related to Haskell at all Custom Functional – as above, except that the underlying lambda calculus reasoning is valid

9. DSL Design Space Program Transformation None – the program is run “as written” Lambda calculus – transformations are possible in the “lambda” part of the language Tree rewrite – domain-specific rules are applied to the program in an AST (Abstract Syntax Tree) for optimization or code generation

10. Euterpea – A Music DSL See the reading for documentation. Note that this teaches Haskell as well as the music language. This language has two components: a note level and a sound level. We’ll work at the note level for now.

11. Overview Syntax: embedded in standard Haskell Code generation: programs are compiled to Midi files Semantics: the book discusses the difference between the Haskell level of the semantics and the music level but the implementation does not rely on these semantics Semantics: theses are “custom” - not related to Haskell Types: embedded in the Haskell type system Lambda: expanded away Transformation: programs are transformed to MIDI semantics during code generation

12. A Simple DSL Formula Start with domain-specific objects (musical notes) as data declarations Add domain-specific combinators (:+: and :=:) and transformations (transpose, instrument) Write utilities which make the language easier to use (note, rest) Add an interpretation function (play) which creates values in the underlying domain (midi files) – this makes use of transformations that are validated by domain-level semantics

13. Code Time Off to Haskell!