CSc 520 — Principles of Programming Languages37 : Scheme — Symbols and StructuresChristian CollbergDepartment of Computer ScienceUniversity of [email protected] 2008 Christian CollbergMay 2, 20081 Symbols• In addition to numbers, strings, and booleans, Scheme has a primitive data-type (atom) called symbol.• A symbol is a lot like a string. It is written:’identifier• Here are some examples:’apple’pear’automobile• (symbol? arg) checks if an atom is a symbol.• To compare two symbols for equality, use (eq? arg1 arg2). HTDP says to use (symbol=? arg1arg2) but DrScheme doesn’t seem to support this.12 Symbols. . .> (symbol? "hello")#f> (symbol? ’apple)#t> (eq? ’a ’a)#t> (eq? ’a ’b)#f> (display ’apple)apple> (string->symbol "apple")apple> (symbol->string ’apple)"apple"3 Symbols. . .(define (healthy? f)(case f[(sushi sashimi) ’hell-yeah][(coke) ’I-wish][(licorice) ’no-but-yummy][else ’nope]))> (healthy? ’sashimi)hell-yeah> (healthy? ’coke)i-wish> (healthy? ’licorice)no-but-yummy> (healthy? ’pepsi)nope4 Structures• Some versions of Scheme have structures. Select Advanced Student in DrScheme.• These are similar to C’s struct, and Java’s class (but without inheritance and methods).• Use define-struct to define a structure:(define-struct struct-name(f1 f2 ...))• define-struct will automatically define a constructor:(make-struct-name(f1 f2 ...))and field-selectors:2struct-name-f1struct-name-f25 Structures. . .(define-struct person (name sex date-of-birth))> (define bob (make-person "bob" ’male ’1978))> bob(make-person "bob" ’male ’1978)> (define alice (make-person "alice" ’female ’1979))> (person-sex bob)’male> (person-date-of-birth alice)’19796 Equivalence• Every language definition has to struggle with equivalence; i.e. what does it mean for two languageelements to be the same?• In Java, consider the following example:void M( S t r i n g s1 , S t r i n g s2 , int i 1 , i n t i 2 ) {i f ( i 1 == i2 ) . . . ;i f ( s1 == s2 ) . . . ;i f ( s1 . e q u a l s ( s2 ) ) . . . ;}Why can I use == to compare ints, but it is it usually wrong to use it to compare strings?7 Equivalence. . .• Scheme has three equivalence predicates eq?, eqv? and equal?.• eq? is the pickiest of the three, then comes eqv?, and last equal?.• In other words,– If (equal? a b) returns #t, then so will (eq? a b) and (eqv? a b).– If (eqv? a b) returns #t, then so will (eq? a b)..• (equal? a b) generally returns #t if a and b are structurally the same, i.e. print the same.8 Equivalence. . .(eqv? a b) returns #t if:• a and b are both #t or both #f.3• a and b are both symbols with the same name.• a and b are both the same number.• a and b are strings that denote the same locations in the store.> (define S "hello")> (eqv? S S)true> (eqv? "hello" "hello")false> (eqv? ’hello ’hello)true9 Equivalence. . .• (equal? a b) returns #t if a and b are strings that print the same.• This is known as structural equivalence.> (equal? "hello" "hello")true> (equal? alice bob)false> (define alice1 (make-person "alice" ’female ’1979))> (define alice2 (make-person "alice" ’female ’1979))> (equal? alice1
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