CSE 341:Programming LanguagesAutumn 2005Lecture 18 — Scheme Intro, Several Binding FormsCSE 341 Autumn 2005, Lecture 18 1Scheme• Like ML, functional focus with imperative features– anonymous functions, function closures, etc.– but every binding is mutable• A really minimalist syntax/semantics– In the LISP tradition– Current standard is 50 pages• Dynamically typed, type safe– Less “compile-time” checking– Accepts more perfectly reasonable programs• Some “advanced” features for decades– Programs as data, hygienic macros, continuationsCSE 341 Autumn 2005, Lecture 18 2Which Scheme?Scheme has a few dialects and many extensions.We will use “PLT → Pretty Big” for the language and DrScheme as aconvenient environment. Available in the ugrad labs, or you candownload it for a personal machine.Most of what we do will be “pure Scheme”.Good documentation available online, including the entire text ofStructure and Interpretation of Computer Programs (linked from the341 page)CSE 341 Autumn 2005, Lecture 18 3Scheme syntaxSyntactically, a Scheme term is either an atom (identifier, number,symbol, string, ...) or a sequence of terms (t1 ... tn).Note: Scheme used to get (still gets?) “paren bashed”, which ishilarious in an XML world.Semantically, identifiers are resolved in an environment and otheratoms are values.The semantics of a sequence depends on t1:• certain character sequences are “special forms”• otherwise a sequence is a function application. Semantics same asML — evaluate them, then call function (call-by-value)CSE 341 Autumn 2005, Lecture 18 4Some special forms• define• lambda• if, cond, and, or• let, let*, letrecCSE 341 Autumn 2005, Lecture 18 5Some predefined values• #t, #f• (), cons, car, cdr, null?, list• eq?, equal?• a “numeric tower” (integer, rational, real, complex, number) withmath operations (e.g., +, =, <) defined on all of them• tons more (strings vs. symbols discussed later)Note: Prefix and variable-arity help make lots of things functions.CSE 341 Autumn 2005, Lecture 18 6Parens MatterEvery parenthesis you write has meaning – get used to that fast!(define (fact n) (if (= n 0) 1 (* n (fact (- n 1))))) ; correct(define (fact n) (if (= n 0) (1) (* n (fact (- n 1)))))(define (fact n) (if = n 0 (1) (* n (fact (- n 1)))))(define fact (n) (if (= n 0) 1 (* n (fact (- n 1)))))(define (fact n) (if (= n 0) 1 (* n fact (- n 1))))(define (fact n) (if (= n 0) 1 (* n ((fact) (- n 1)))))CSE 341 Autumn 2005, Lecture 18 7Local bindingsThere are 3 forms of local bindings with different semantics:• let• let*• letrecAlso, in function bodies, a sequence of definitions is equivalent toletrec.But at top-level redefinition is assignment!This makes it ghastly hard to encapsulate c ode, but in practice:• people assume non-malicious clients• implementations provide access to “real primitives”For your homework, assume top-level definitions are immutable.CSE 341 Autumn 2005, Lecture 18
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