CSc 520 - Principles of Programming Languages
37 : Scheme -- Symbols and Structures

Christian Collberg

Department of Computer Science

University of Arizona

1 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:
  1#1
  
• Here are some examples:
  2#2
  
• (symbol? arg) checks if an atom is a symbol.
• To compare two symbols for equality, use (eq? arg1 arg2). HTDP says to use (symbol=? arg1 arg2) but DrScheme doesn't seem to support this.

2 Symbols...

3#3

3 Symbols...

4#4

4 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:
  5#5
  
• define-struct will automatically define a constructor:
  6#6
  and field-selectors:
  7#7
  

5 Structures...

8#8

6 Equivalence

• Every language definition has to struggle with equivalence; i.e. what does it mean for two language elements to be the same?
• In Java, consider the following example:
  9#9
  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.
• 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.

10#10

9 Equivalence...

• (equal? a b) returns #t if a and b are strings that print the same.
• This is known as structural equivalence.

11#11