Example: Converting Temperature

Now for a larger example to consolidate what we have learnt so far. Assume we have a list of temperature readings from a number of cities in the world. Some of them are in Celsius (Centigrade) and some in Fahrenheit (as in the previous list). First let's convert them all to Celsius, then let's print out the data neatly. Save the following code to temp-convert.lfe:

(defmodule tut8
  (export (format-temps 1)))

;; Only this function is exported
(defun format-temps
  ((())
    ;; No output for an empty list
    'ok)
  (((cons city rest))
    (print-temp (f->c city))
    (format-temps rest)))

(defun f->c
  (((tuple name (tuple 'C temp)))
    ;; No conversion needed
    (tuple name (tuple 'C temp)))
  (((tuple name (tuple 'F temp)))
    ;; Do the conversion
    (tuple name (tuple 'C (/ (* (- temp 32) 5) 9)))))

(defun print-temp
  (((tuple name (tuple 'C temp)))
    (lfe_io:format "~-15w ~w C~n" (list name temp))))

> (c "tut8.lfe")
#(module tut8)
> (tut8:format-temps
    '(#(Moscow #(C 10))
      #(Cape-Town #(F 70))
      #(Stockholm #(C -4))
      #(Paris #(F 28))
      #(London #(F 36)))))
Moscow          10 C
Cape-Town       21.11111111111111 C
Stockholm       -4 C
Paris           -2.2222222222222223 C
London          2.2222222222222223 C
ok

Before we look at how this program works, notice that we have added a few comments to the code. A comment starts with a ; character and goes on to the end of the line. ¹ Note as well that the (export (format-temps 1)) line only includes the function format-temps/1, the other functions are local functions, i.e. they are not visible from outside the module temp-convert.

When we call format-temps/1 the first time, the city variable gets the value #(Moscow #(C-10)) and the remainder of the list is assigned to the rest variable. Next, the f->c/1 function is called inside the print-temp/1 function, with f->c/1 getting passed #(Moscow #(C-10)).

Note that when we see function calls nested like (print-temp (f->c ...)) -- in other words when one function call is passed as the argument to another function -- we execute (evaluate) them from the inside out. We first evaluate (f->c city) which gives the value #(Moscow #(C 10)) as the temperature is already in Celsius and then we evaluate (print-temp #(Moscow #(C 10))). Note that the f->c/1 function works in a similar way to the convert-length/1 function we wrote in a previous section.

Next, print-temp/1 simply calls lfe_io:format/2 in a similar way to what has been described above. Note that ~-15w says to print the "term" with a field length (width) of 15 and left justify it.²

Now we call (format-temps rest) with the remainder of the list as an argument. This way of doing things is similar to the loop constructs in other languages. (Yes, this is recursion, but don't let that worry you). So the same format-temps/1 function is called again, this time city gets the value #(Cape-Town #(F 70)) and we repeat the same procedure as before. We go on doing this until the list becomes empty, i.e. (), which causes the first clause (format-temps '()) to match. This simply "returns" or "results in" the atom ok, so the program ends.

¹ In LFE, the convention is that a comment starting with a single ; is reserved for comments at the end of a line of code; lines which start with a comment use two ;;, as above. There are also conventions for ;;; and ;;;; -- to learn more about these, see Program Development Using LFE - Rules and Conventions and LFE Style Guide.

² LFE's lfe_io:format differs from the Erlang standard library io:format in that lfe_io displays its results using LFE-formatted data structures in its Lisp syntax; io uses the standard Erlang syntax. You may use either from LFE. lfe_io takes the same formatting parameters as io, so there should be no surprises if you're coming from Erlang. For more information, be sure to read the io:format documentation.