Summary: PLAI Ch.1 — Introduction

• PLAI is structured as a conversation, not a top-down narrative; it includes deliberate mistakes to enforce active reading.
• The book's implementation language is plai-typed, a statically typed dialect of Racket. Advprog substitutes Common Lisp.
• define-type introduces algebraic data types with named variants; type-case pattern-matches over them. These are PLAI's primary structuring mechanisms.
• Type annotations are optional (inference fills in most cases) but preferred for discipline and readability.
• Tests are first-class via the test form; writing them before function definitions is encouraged.

Chapter 1 is a brief methodological introduction. Krishnamurthi explains the book's non-linear, conversation-like structure: the reader will encounter mistakes, corrections, and backtracking. This is intentional; passive reading is explicitly discouraged via "Do Now!" prompts.

The chapter introduces the implementation language plai-typed. Key constructs:

define-type

defines an algebraic datatype with named variants, each carrying typed fields. Analogous to an abstract class with concrete subclasses in Java, or defclass hierarchies in Common Lisp.

type-case

pattern-matches over a define-type value, binding field names in each branch. Analogous to ematch in Common Lisp.

test

asserts equality between an expression and an expected value.

Example from the chapter:

(define-type MisspelledAnimal
  [caml (humps : number)]
  [yacc (height : number)])

(define (good? [ma : MisspelledAnimal]) : boolean
  (type-case MisspelledAnimal ma
    [caml (humps) (>= humps 2)]
    [yacc (height) (> height 2.1)]))

The Common Lisp / tolk equivalent uses defclass hierarchies and trivia ematch for the same purpose.

• Interpreter Architecture
• Abstract Syntax Tree
• PLAI
• PLAI Ch.1 online