Scopes for Bindings

Each binding is given a fixed scope in which the binding is considered legal or effective. The scope of a toplevel binding in a file starts from the point where the binding is introduced until the very end of the file. The bindings introduced in the following example between the keywords let and in are effective until the keyword end is reached:

val area = let
  val PI = 3.14 and radius = 10.0 in PI * radius * radius
end // end of [let]

Such bindings are referred to as local bindings, and the names such as PI and radius are referred to as local names. This example can also be written in the following style:

val area =
  PI * radius * radius where {
  val PI = 3.14 and radius = 10.0 // simultaneous bindings
} // end of [where] // end of [val]

The keyword where appearing immediately after an expression introduces bindings that are solely effective for evaluating names contained in the expression. Note that expressions formed using the keywords let and where are often referred to as let-expressions and where-expressions, respectively. The former can always be translated into the latter directly and vice versa. Which style is better? I have not formed my opinion yet. The answer seems to entirely depend on the taste of the programmer.

The following example demonstrates an alternative approach to introducing local bindings:

local

val PI = 3.14 and radius = 10.0

in (* in-of-local *)

val area = PI * radius * radius

end // end of [local]