little_schemer.rdoc

Primitives¶ ↑

atom¶ ↑

A string of letters, numbers, or special characters other than parentheses, similar to a string. Examples

atom
1982
$abc*

list¶ ↑

A collection of S-expressions, similar to an array Examples

(a b c)
(a (x y x) (b) c)

car¶ ↑

Returns the first S-expression form a list. Arguments a non-empty list Examples

car (a b c)  # => a

Ruby Equivalent

['a', 'b', 'c'].first  # => 'a'

cdr¶ ↑

Returns the passed list with the first S-expression removed. Arguments a non-empty list Example

cdr (a b c)  # => (b c)

Ruby Equivalent

['a', 'b', 'c'].unshift  # => ['b', 'c']

cons¶ ↑

Adds the first argument (an S-expression) to the beginning of the second argument (a list). Arguments takes two arguments (1) any S-expression and (2) any list. Example

cons z (a b c)  # => (z a b c)

Ruby Equivalent

['a', 'b', 'c'].unshift('z')  # => ['z', 'a', 'b', 'c']

null?¶ ↑

Checks if the passed argument is an empty list. Returns boolean. In practice, it returns false for everything except an empty list. Arguments a list Example

null? (a b c)  # => false

Ruby Equivalent

['a', 'b', 'c'].empty?  # => false

atom?¶ ↑

Check if the passed argument is an atom. Returns boolean. Arguments any S-expression Example

atom? abc  # => true

Ruby Equivalent

'abc'.is_a?(String)  # => true

eq?¶ ↑

Tests equality of two non-numeric atoms. Returns boolean. Arguments takes two arguments – both of them must be non-numeric atoms Example

eq? abc abc  # => true

Ruby Equivalent

'abc' == 'abc'  # => true

lat?¶ ↑

Checks if the passed list contains only atoms (no lists). Returns boolean. Arguments takes a list Example

lat? (a b c)  # => true

Ruby Equivalent

['a', 'b', 'c'].all? { |i| i.is_a?(String) }

cond¶ ↑

(cond . . . ) asks questions

lambda¶ ↑

(lambda . . . ) creates a function;

define¶ ↑

(define . . . ) gives it a name.

or¶ ↑

(or … ) asks two questions, one at a time. If the first one is true it stops and answers true. Otherwise it asks the second question and answers with whatever the second question answers. Example

(or (null? (a b c)) (atom? dummy) )  # => true

Ruby Equivalent

['a', 'b', 'c'].empty? || 'dummy'.is_a(String)   # => true

Problems¶ ↑

Write the function lat? using some, but not necessarily all, of the following functions: car cdr cons null? atom? and eq?

LISP

# sudo code
lambda lat?(array)
  return false unless (atom? (car array))
  (null? array) ? true : (lat? (cdr array))
end

# actual answer
(define lat?
  (lambda (l)
    (cond
      ((null? l) #t) 
      ((atom? (car l)) (lat? (cdr l))) 
      (else #f))))

Ruby

def lat?(array)
  return false unless array.shift.is_a?(String)
  array.empty? ? true : lat?(array)
end