ML Lecture 6

The let...in Statement: Defining temporary identifiers

  1. Function parameters are one type of temporary identifier-value binding.
  2. Another is the let..in statement. It allows you to create temporary identifier-value bindings for use in an expression. Its form is 
      3. let
         val id1 = value;
         val id2 = value;
      in
         some expression
      end;

    The identifiers id1, id2, ... are bound to the values specified (these can be expressions, of course), and then the expression after in is evaluated, and the resulting value of this expression is the resulting value of the let...in statement. After that expression is evaluated the bindings created for id1, id2, ... are erased.

The let statement allows you to break apart a computation into separate steps

  1. The examples used used in class are in here, the file let.sml, and the ML session is here.
  2. We used functions to break apart the computation as well, but then the expressions became complex because of the inside-to-out order of evaluation. The first step would be a function innermost in the expression, then the next outermost part of the expression, and on until the whole expression was evaluated. This is shown in the example function circle_area1 in the examples.
  3. The let statement lets us put each of the computation steps in a separate expression, and assign that intermediate step's value to a new identifier. So it is more like how we think when we solve the problem, and it prevents individual expressions from getting too complex. The example function circle_area2 in the examples uses the let statement to make this computation clearer.
  4. the final function in the examples is rect_area, and just shows another use of the let statement

Slides in class: function definitions, and ML sessions