The LFE Tutorial

Atoms

Atoms are another data type in LFE. They are words, for example charles, centimetre, inch and ok. Atoms are similar to symbols in Lisp except that are simply names, nothing else. They are not like variables which can have a value.

Enter the next program (file: tut4.lfe) which could be useful for converting from inches to centimetres and vice versa:

(defmodule tut4
  (export (convert 2)))

(defun convert
  ((m 'inch) (/ m 2.54))
  ((n 'centimetre) (* n 2.54)))

Compile and test:

lfe> (c "tut4.lfe")
#(module tut4)
lfe> (tut4:convert 3 'inch)
1.1811023622047243
lfe> (tut4:convert 7 'centimetre)
17.78

Notice that atoms and variables look the same so we have to tell LFE when we want it to be an atom. We do this by quoting the atom with a ', for example 'inch and 'centimetre. We have to do this both when we use it as argument in a function definition and when we use it when calling a function, otherwise LFE will assume that it is a variable.

Also notice that we have introduced decimals (floating point numbers) without any explanation, but I guess you can cope with that.

See what happens if I enter something other than centimetre or inch in the convert function:

lfe> (tut4:convert 3 'miles)
exception error: function_clause
  in (: tut4 convert 3 miles)

The two parts of the convert function are called its clauses. Here we see that miles is not part of either of the clauses. The LFE system can't match either of the clauses so we get an error message function_clause. The shell formats the error message nicely, but to see the actual error tuple we can do:

lfe> (catch (tut4:convert 3 'miles))
#(EXIT
  #(function_clause
    (#(tut2 convert (3 miles) (#(file "./tut2.lfe") #(line 4)))
     #(lfe_eval eval_expr 2 (#(file "src/lfe_eval.erl") #(line 160)))
     #(lfe_shell eval_form_1 2 (#(file "src/lfe_shell.erl") #(line 268)))
     #(lists foldl 3 (#(file "lists.erl") #(line 1261)))
     #(lfe_shell server_loop 1 (#(file "src/lfe_shell.erl") #(line 101))))))