5.1 Variables are names for values

let [var] : [type] = [expre. def] in [expre. body]

• [expre.def] is an expression defining a value of given type and [expre.body] is an expression within which the variable can be used as the name for the defined value
• in other words, the construction binds the name [var] to the value [expre.def] for use in [expre.body]
• let is often referred to as a binding construct, the name being defined is often the definiendum, and the expression it names being the definien

let pi: float = 3.1416 in 
pi *. 2. *. 2.;;
-: float = 12.5664

5.2 The type of a let- bound variable can be inferred

<aside> 💡 the type information in the let construct is optional because OCaml is implicitly typed

</aside>

let pi = 3.1416 in
pi *. 2. *. 2. ;;

5.3 let expressions are expressions

• all expressions in OCaml have values, therefore even let expressions have values

# 3.1416 *. (let radius = 2.
# in radius *. radius) ;;
- : float = 12.5664

• let expressions can also be embedded in other let expressions to get the effect of defining multiple names

# let pi = 3.1416 in
# let radius = 4. in
# pi *. radius ** 2. ;;
- : float = 50.2656

5.4 Scope

• the name defined in the let expression is available only in the body of the expression and is therefore local to the body

# (let s = "hi ho " in
# s ^ s) ^ s ;;
Line 2, characters 9-10:
2 | s ^ s) ^ s ;;
^
Error: Unbound value s

• the body of the let expression ends at the closing parenthesis and so variable s is unbound
• the scope of a local let naming doesn’t include the definition itself (the expre.def part between the = and the in)

# let x = x + 1 in
# x * 2 ;;
Line 1, characters 8-9:
1 | let x = x + 1 in
						^
Error: Unbound value x