The LFE Tutorial

Tuples

Now the tut4 program is hardly good programming style. Consider:

(tut4:convert 3 'inch)

Does this mean that 3 is in inches? or that 3 is in centimetres and we want to convert it to inches? So LFE has a way to group things together to make things more understandable. We call these tuples. Tuples are constructed and matched using (tuple ...), with literal tuples being written with #( ... ).

So we can write #(inch 3) to denote 3 inches and #(centimeter 5) to denote 5 centimetres. Now let's write a new program which converts centimetres to inches and vice versa (file tut5.lfe).

(defmodule tut5
  (export (convert-length 1)))

(defun convert-length
  (((tuple 'centimeter x)) (tuple 'inch (/ x 2.54)))
  (((tuple 'inch y)) (tuple 'centimeter (* y 2.54))))

Compile and test:

(c "tut5.lfe")
#(module tut5)
lfe> (tut5:convert-length #(inch 5))
#(centimeter 12.7)
lfe> (tut5:convert-length (tut5:convert-length #(inch 5)))
#(inch 5.0)

Note that in the last call we convert 5 inches to centimetres and back again and reassuringly get back to the original value. I.e the argument to a function can be the result of another function. Pause for a moment and consider how that line (above) works. The argument we have given the function #(inch 5) is first matched against the first head clause of convert-length i.e. in ((tuple 'centimeter x)) where it can be seen that the pattern (tuple 'centimeter x) does not match #(inch 5) (the head is the first bit in the clause with a list of argument patterns). This having failed, we try the head of the next clause i.e. ((tuple 'inch y)), this pattern matches #(inch 5) and y gets the value 5.

We have shown tuples with two parts above, but tuples can have as many parts as we want and contain any valid LFE term. For example, to represent the temperature of various cities of the world we could write

#(Moscow #(C -10))
#(Cape-Town #(F 70))
#(Paris #(F 28))

Tuples have a fixed number of things in them. We call each thing in a tuple an element. So in the tuple #(Moscow #(C -10)), element 1 is Moscow and element 2 is #(C -10). We have chosen C meaning Celsius (or Centigrade) and F meaning Fahrenheit.