[erlang-questions] Why does Erlang have control structures?

[黃耀賢 (Yau-Hsien Huang)]

On Tue, Aug 28, 2012 at 4:56 AM, Jayson Barley <jayson.barley@REDACTED>wrote:

> I am not sure I understand why we have them. For instance I can take the
> following code
>
> is_greater_than(X, Y) ->
>     if
>         X>Y ->
>             true;
>         true -> % works as an 'else' branch
>             false
>     end.
>
> And make it
>
> is_true(true) ->
>     true;
> is_true(false) ->
>     false.
>
> is_greater_than(X, Y) ->
>     is_true(X>Y).
>
> Your is_true/1 means that it may meet either true, false, or other values.
However, 'if' statement means that it would meet two cases definitely.

is_true/1 is an identity function (fun(X) -> X end) working on Boolean
values.
Then it may be is_true(X>Y) or is_true(is_true(X>Y)) or
is_true(is_true(is_true(is_true(is_true(is_true(X>Y)))))).
Eventually, It is (X > Y).


> I can do the same thing with case statements
>
> is_valid_signal(Signal) ->
>     case Signal of
>         {signal, _What, _From, _To} ->
>             true;
>         {signal, _What, _To} ->
>             true;
>         _Else ->
>             false
>     end.
>
> Becomes
>
> switch_signal({signal, _What, _From, _To}) ->
>     true;
> switch_signal({signal, _What, _To}) ->
>     true;
> switch_signal(_Else) ->
>     false.
>
> is_valid_signal(Signal) ->
>     switch_signal(Signal).
>
>
is_valid_signal(Signal) ->
    case Signal of
        {signal, _What, _From, _To} ->
            true;
        {signal, _What, _To} ->
            true;
        _Else ->
            false
    end.

The above is an alternative version of

is_valid_signal({signal, _What, _From, _To}) ->

    true;

is_valid_signal({signal, _What, _To}) ->

true;

s_valid_signal(_Else) ->

false.

Don't repeat it.



> I know that the control structures are a little bit faster, not much, but
> I find that the function form is more readable.
>
> _______________________________________________
> erlang-questions mailing list
> erlang-questions@REDACTED
> http://erlang.org/mailman/listinfo/erlang-questions
>
>

In my opinion, 'if' block limits the domain, and 'case' block provides
a way more writable than to write many function heads to pick its
matching patterns.


-- 

Best Regards.

--- Y-H. H.
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