Statements vs Expressions
Statements
A statement performs some action
Expressions
An expression evaluates to some value
Examples
print("Hello World")
sleep(1000)
return 55
if (done) exit()
throw SomeError()
Examples
"Hello World" 1000 5 + 3 a * 5 > b x_flag & mask
An expression can be part of a statement:
while (x < 5)
x += 15
print("i: " + i)A statement is never part of an expression.
…statements typically don't have a type. If they do, the type is typically something like Unit or void.
…all expressions have types. "Hello World" is a string, 5 + 7 is an integer and so on.
Details
Some expressions such as i++ (unary increment in C-like languages) have side-effects and could be viewed as statements embedded in expressions: while (i++ < 15) …
Some things can be used both as expressions and statements. A call to a function returning a value for example. In C-like languages they need to be terminated by a ; to form a complete statement.
myFunction(); // As statement
if (myFunction()) // As expression
print("Returned true!");In scripting languages it's also common to use short circuiting boolean expressions. In shell scripting you often see things like
compile_code || exit 1
or in PHP where the following is a common pattern
fopen($site, "r") or die("Unable to connect");