Expressions
Primitive Expressions
Just like in Python, atomic, or primitive, expressions in Scheme take a single step to evaluate. These include numbers, booleans, symbols.
Note that ; starts a comment in Scheme, just like # in Python.
scm> 1234 ; integer
1234
scm> 123.4 ; real number
123.4
Symbols
Out of these, the symbol type is the only one we didn't encounter in Python. A symbol acts a lot like a Python name, but not exactly. Specifically, a symbol in Scheme is also a type of value. On the other hand, in Python, names only serve as expressions; a Python expression can never evaluate to a name.
scm> quotient ; A name bound to a built-in procedure
#[quotient]
scm> 'quotient ; An expression that evaluates to a symbol
quotient
scm> 'hello-world!
hello-world!
Booleans
In Scheme, all values except the special boolean value #f are interpreted as true values (unlike Python, where there are some false-y values like 0). Our particular version of the Scheme interpreter allows you to write True and False in place of #t and #f. This is not standard.
scm> #t
#t
scm> True
#t
scm> #f
#f
scm> False
#f
Call Expressions
Like Python, the operator in a Scheme call expression comes before all the operands. Unlike Python, the operator is included within the parentheses and the operands are separated by spaces rather than with commas. However, evaluation of a Scheme call expression follows the exact same rules as in Python:
- Evaluate the operator. It should evaluate to a procedure.
- Evaluate the operands, left to right.
- Apply the procedure to the evaluated operands.
Here are some examples using built-in procedures:
scm> (+ 1 2)
3
scm> (- 10 (/ 6 2))
7
scm> (modulo 35 4)
3
scm> (even? (quotient 45 2))
#t
Special Forms
The operator of a special form expression is a special form. What makes a special form "special" is that they do not follow the three rules of evaluation stated in the previous section. Instead, each special form follows its own special rules for execution, such as short-circuiting before evaluating all the operands.
Some examples of special forms that we'll study today are the if, cond, define, and lambda forms. Read their corresponding sections below to find out what their rules of evaluation are!