Following the ML principle,

programming is done by defining an expression that represents the desired value.

conditional computation is represented as an expression. A conditional expression

if $E{}_{\mathrm{1}}$ then $E{}_{\mathrm{2}}$ else $E{}_{\mathrm{3}}$

is an expression that computes a value in the following steps.

1. 1.

   Evaluate $E1$ and obtain a value.

2. 2.

   If the value is true then evaluate $E2$ and return its value.

3. 3.

   If the value of $E1$ is false then evaluate $E3$ return its value.

true, false are two constants of type bool. Expressions of type bool contains comparison expressions and logical operations.

# 1 < 2;
val it = true : bool
# 1 < 2 andalso 1 > 2;
val it = false : bool
# 1 < 2 orelse 1 > 2;
val it = true : bool

Conditional expressions are made up with expressions of type bool.

# if 1 < 2 then 1 else 2;
val it = 1 : int

Since this is an expression, it can be combined with other expressions as show below.

# (if 1 < 2 then 1 else 2) * 10;
val it = 10 : int