The Little Schemer by Daniel P. Friedman & Matthias Felleisen

• The Law of Car: The primitive car is defined only for non-empty lists.
• The Law of Cdr: The primitive cdr is defined only for non-empty lists. The cdr of any non-empty list is always another list.
• The Law of Cons: The primitive cons takes two arguments. The second argument to cons must be a list. The result is a list.
• The Law of Null?: The primitive “null?” is defined only for lists.
• The Law of Eq?: The primitive “eq?” takes two arguments. Each must be a non-numeric atoms.
• Lambda creates a function and define gives it a name.
• The First Commandment:
• When recurring on a list of atoms, lat, ask two questions about it: (null? lat) and else.
• When recurring on a number, n, ask two questions about it: (zero? n) and else.
• When recurring on a list of S-expressions, l, ask three questions about it: (null? l), (atom? (car l)), and else.
• The Second Commandment: Use cons to build lists.
• The Third Commandment: When building a list, describe the first typical element, and then cons it onto the natural recursion.
• The Fourth Commandment:
• Always change at least one argument while recurring.
• When recurring on a list of atoms, lat, use (cdr lat). When recurring on a number, n, use (sub1 n). And when recurring on a list of S-expressions, l, use (car l) and (cdr l) if neither (null? l) nor (atom? (car l)) are true.
• It must be changed to be closer to termination. The changing argument must be tested in the termination condition:
• When using cdr, test termination with “null?”
• When using sub1, test termination with “zero?”
• The Fifth Commandment:
• When building a value with ‘+’, always use 0 for the value of the terminating line, for adding 0 does not change the value of an addition.
• When building a value with ‘x’, always use 1 for the value of the terminating line, for multiplying by 1 does not change the value of a multiplication.
• When building a value with cons, always consider () for the value of the terminating line.
• The Sixth Commandment: Simplify only after the function is correct.
• The Seventh Commandment: Recur on the subparts that are of the same nature:
• On the sublists of a list.
• On the subexpressions of an arithmetic expression.
• The Eighth Commandment: Use help functions to abstract from representations.
• The Ninth Commandment: Abstract common patterns with a new function.
• The Tenth Commandment: Build functions to collect more than one value at a time.