'&$%CSE 341:Programming LanguagesWinter 2006Lecture 13— Scheme Intro, Several Binding FormsCSE341 Winter 2006, Lecture 13 1'&$%Scheme• 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– Less “compile-time” checking– Accepts more perfectly reasonable programs• Some “advanced” features for decades– Programs as data, hygienec macros, continuationsCSE341 Winter 2006, Lecture 13 2'&$%Which Scheme?Scheme has a few dialects and many extensions.We will use “PLT → Pretty Big” for the language and DrScheme as aconvenient environment.Most of what we do will be “pure Scheme”.Exceptions are multiline comments, define-struct, and a brief forayinto the MzScheme module system (if we have time).CSE341 Winter 2006, Lecture 13 3'&$%Scheme 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)CSE341 Winter 2006, Lecture 13 4'&$%Some special forms• define• lambda• if, cond, and, or• let, let*, letrecCSE341 Winter 2006, Lecture 13 5'&$%Some predefined values• #t, #f• (), cons, car, cdr, null?, list• a “numeric tower” with math operations (e.g., +) defined on all ofthem• tons more (strings vs. symbols discussed later)Note: Prefix and variable-arity help make lots of things functions.CSE341 Winter 2006, Lecture 13 6'&$%Parens MatterEvery parenthesis you write has meaning – get used to that fast!(define (fact n) (if (= n 0) 1 (* n (fact (- n 1))))) ; correct(define (fact n) (if (= n 0) (1) (* n (fact (- n 1)))))(define (fact n) (if = n 0 (1) (* n (fact (- n 1)))))(define fact (n) (if (= n 0) 1 (* n (fact (- n 1)))))(define (fact n) (if (= n 0) 1 (* n fact (- n 1))))(define (fact n) (if (= n 0) 1 (* n ((fact) (- n 1)))))CSE341 Winter 2006, Lecture 13 7'&$%Local 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 code, but in practice:• people assume non-malicious clients• implementations provide access to “real primitives”For your homework, assume top-level definitions are immutable.CSE341 Winter 2006, Lecture 13
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