Erlang atomic type

Atomic type

The atomic type is another data type in the Erlang language. A ll atomic types begin with lowercase letters (see Atomic Types). F or example, charles, centimeter, inch, etc. A tomic types are just names, with no other meaning. T hey are different from variables, which have values, whereus atomic types do not.

Enter the following program into file tut2.erl. T his procedure completes the conversion between inches and centimeters:

-module(tut2).
-export([convert/2]).

convert(M, inch) ->
    M / 2.54;

convert(N, centimeter) ->
    N * 2.54.

Compile:

9> c(tut2).
{ok,tut2}

Test:

10> tut2:convert(3, inch).
1.1811023622047243
11> tut2:convert(7, centimeter).
17.78

Note that so far we haven't covered the number of points. I hope you'll find out for a little time.

Let's see what happens if the input parameters are neither centimeter nor inch:

12> tut2:convert(3, miles).
** exception error: no function clause matching tut2:convert(3,miles) (tut2.erl, line 4)

The two parts of the convert function are called the two clauses of the function. A s you can see, miles are not part of the clause. T he Erlang system could not find a matching clause, so an error message was returned function_clause. T he shell is responsible for the error message-friendly output, while the error group is stored in the shell's history list, which can be output using the v/1 command:

13> v(12).
{'EXIT',{function_clause,[{tut2,convert,
                                [3,miles],
                                [{file,"tut2.erl"},{line,4}]},
                          {erl_eval,do_apply,5,[{file,"erl_eval.erl"},{line,482}]},
                          {shell,exprs,7,[{file,"shell.erl"},{line,666}]},
                          {shell,eval_exprs,7,[{file,"shell.erl"},{line,621}]},
                          {shell,eval_loop,3,[{file,"shell.erl"},{line,606}]}]}}

The metagroup

The style of the previous tut2 program is not a good programming style. F or example:

tut2.convert(3,inch)  

Does this mean that 3 itself is already indicated by inches? O r convert 3 cm into inches? E rlang provides a mechanism for grouping certain elements and representing them in a more understandable way. I t's a metagroup. A metagroup is enclosed in parentheses.

So, the point is 3 inches, and the point of 5 centimeter, 5, is 5 centimeters. N ext, we'll rewrite the conversion program between centimeters and inches. E nter the following code into the file tut3.erl file:

-module(tut3).
-export([convert_length/1]).

convert_length({centimeter, X}) ->
    {inch, X / 2.54};
convert_length({inch, Y}) ->
    {centimeter, Y * 2.54}.

Compile and test:

14> c(tut3).
{ok,tut3}
15> tut3:convert_length({inch, 5}).
{centimeter,12.7}
16> tut3:convert_length(tut3:convert_length({inch, 5})).
{inch,5.0}

Note that line 16 code converts 5 inches to centimeters and then inches, so it gets the original value. T his also indicates that one function argument can be the return result of another function. T ake a closer look at how line 16 of code works. A fter passing the argument s inch, 5, to the convert_length function is matched convert_length({inch,5}) is matched. Y ou can also think of it as a success that the previous content of the .centimeter, X, did not match the .inch, 5' (the "-" A fter the first match fails, the program tries the second convert_length({inch,5}) The second statement matched successfully, so the Y value is 5.

There can be more elements in the group than just two parts, as described above. I n fact, you can use as many parts as you want in a metagroup, as long as each part is a legitimate Erlang item. F or example, represents the temperature values of different cities around the world:

{moscow, {c, -10}}
{cape_town, {f, 70}}
{paris, {f, 28}}

Each of these groups has a fixed number of items. E ach item in a metagroup is called an element. T he first element is moscow and the second element is . Where c represents Celsius and f represents Fahrenheit.