# [erlang-questions] syntax errror on :

Roelof Wobben r.wobben@REDACTED
Sat Aug 8 20:19:11 CEST 2015

```Op 8-8-2015 om 18:59 schreef Roelof Wobben:
> Op 8-8-2015 om 18:24 schreef Joe Armstrong:
>> On Sat, Aug 8, 2015 at 5:44 PM, Roelof Wobben <r.wobben@REDACTED> wrote:
>>> Op 8-8-2015 om 17:39 schreef Joe Armstrong:
>>>
>>>> On Sat, Aug 8, 2015 at 1:03 PM, Roelof Wobben <r.wobben@REDACTED>
>>>> wrote:
>>>>> Hello,
>>>>>
>>>>> I try to find a solution for this etude :
>>>>> http://chimera.labs.oreilly.com/books/1234000000726/ch05.html#CH05-ET01
>>>>>
>>>>>
>>>>> So far I have this :
>>>>>
>>>>> %% @author Roelof Wobben <r.wobben@REDACTED>
>>>>> %% @doc Function to calculate the area of a rectangle
>>>>> %% @reference from <a href=
>>>>> "http://shop.oreilly.com/product/0636920025818.do" >Introducing
>>>>> Erlang</a>,
>>>>> %% O'Reilly Media, Inc., 2012.
>>>>> %% @copyright 2012 by R.WObben
>>>>> %% @version 0.1
>>>>>
>>>>> -module(geom51).
>>>>>
>>>>> -export([area/0]).
>>>>>
>>>>> %% @doc ask the user for which shape the area
>>>>> %% must be calculated.
>>>>> %% Choices are :
>>>>> %% c or a C for a circle
>>>>> %% e or a E for a ellipse
>>>>> %% t or a T for a triangle
>>>>>
>>>>> -spec(area() -> char() ).
>>>>>
>>>>> area() ->
>>>>>       io:format("Which shape must the area be calculated?~n"),
>>>>>       io:format(" R. Rectangle ~n"),
>>>>>       io:format(" T. Triangle~n"),
>>>>>       io:format(" E. Ellipse~n"),
>>>>>
>>>>>       char_to_shape(Value).
>>>>>
>>>>> %% @doc convert the choice to a Shape and send the
>>>>> %5 data to the dimension function
>>>>> %% when the choice is a  :
>>>>> %% r or a R then the shape will be a rectangle
>>>>> %% t or a T then the shape will be a triangle
>>>>> %% e or a E then the shape will be a ellipse
>>>>>
>>>>> -spec(char_to_shape(char() ) -> atom() ).
>>>>>
>>>>> char_to_shape(Char) ->
>>>>>       case Char of
>>>>>           \$R -> get_dimension(rectangle);
>>>>>           \$T -> triangle;
>>>>>           \$E -> ellipse;
>>>>>           \$r -> rectangle;
>>>>>           \$t -> triangle;
>>>>>           \$e -> ellipse;
>>>>>           _Else -> "Wrong Shape"
>>>>>       end.
>>>> Just a quick comment on this function:
>>>>
>>>> char_to_shape can return a string (ie "Wrong Shape") and NOT a shape
>>>> (atom)
>>>> so you violate your own spec.
>>>>
>>>> A better solution is to omit the final clause and write
>>>>
>>>> char_to_shape(Char) ->
>>>>       case Char of
>>>>           \$R -> get_dimension(rectangle);
>>>>           \$T -> triangle;
>>>>           \$E -> ellipse;
>>>>           \$r -> rectangle;
>>>>           \$t -> triangle;
>>>>           \$e -> ellipse
>>>>     end.
>>>>
>>>> Now the function will generate an exception if called with an
>>>> incorrect
>>>> argument. Returning a string is just about the worse thing you
>>>> could do
>>>> since some other function somewhere else will have to deal with this.
>>>>
>>>> This is an example of the "let it crash" philosophy. Crashing is no
>>>> big
>>>> deal in
>>>> Erlang - in single threaded languages crashing is a big deal - if you
>>>> crash
>>>> you lose *everything*. In Erlang you can have millions of processes,
>>>> if a few crash who cares? In Erlang we let processes watch each other,
>>>> if one process
>>>> crashes some other process is supposed to observe this and try to
>>>> fix the
>>>> error.
>>>>
>>>> In Erlang we assume we'll never get things right and that programs
>>>> will
>>>> crash
>>>> so we don't really bother to program defensively - but we do make sure
>>>> errors are detected and logged and faulty computations restarted.
>>>>
>>>> This leads to a very clean and simple programming style since you
>>>> don't
>>>> have to add the additional clauses to catch errors - just leave
>>>> them out.
>>>> This make life easy for the programmer since it's in writing these
>>>> extra
>>>> clause the programmer is "out of spec" ie what should I do in the
>>>> error
>>>> case.
>>>> Return a string? (as you did) print a message - but who will deal
>>>> with the
>>>> string, who will read the message?
>>>>
>>>> The refactored code is actually easier to read and write since
>>>> there is no
>>>> nasty edge case.
>>>>
>>>> So there you are - two new principles:
>>>>
>>>>       "Let it crash"
>>>>       "Let some other process fix the error"
>>>>
>>>> Cheers
>>>>
>>>> /Joe
>>>>
>>>>
>>>>> -spec(get_dimension( atom() ) -> number() ) .
>>>>>
>>>>> %% @doc Calculates the area of a shape, given the
>>>>> %% shape and two of the dimensions. Returns the product
>>>>> %% of its arguments for a rectangle, one half the
>>>>> %% product of the arguments for a triangle, and
>>>>> %% math:pi times the product of the arguments for
>>>>> %% an ellipse.
>>>>>
>>>>> get_dimension(Shape)->
>>>>>       case Shape of
>>>>>           rectangle ->
>>>>>               Answer1 = io:get_line("What is the length of the
>>>>> rectangle?
>>>>> "),
>>>>>                        case Test of
>>>>>                            error -> Test2 =
>>>>>                            case  Test2 of
>>>>>                                error -> io:write("the input
>>>>> schould be a
>>>>> number");
>>>>>                                _Else -> Test2 = Test
>>>>>                            end
>>>>>                        end
>>>>>       end
>>>>>
>>>>>
>>>>> but now I see these error message :
>>>>>
>>>>> geom51.erl:72: syntax error before:
>>>>> geom51.erl:42: function get_dimension/1 undefined
>>>>> geom51.erl:51: spec for undefined function geom51:get_dimension/1
>>>>>
>>>>> geom51.erl:72: syntax error before:
>>>>> geom51.erl:42: function get_dimension/1 undefined
>>>>> geom51.erl:51: spec for undefined function geom51:get_dimension/1
>>>>>
>>>>> anyone who knows how to solve this one ?
>>>>>
>>>>> Roelof
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> ---
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>>>>> _______________________________________________
>>>>> erlang-questions mailing list
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>>>
>>> Correct but the exercise I stated that let it chrash is not handy when
>>> dealing with user input.
>>> As I understand the exercise well I have to write error messages on
>>> wrong
>>> input even as it against the erlang principle let it crash
>> You can do both. You could write shape_to_char as I suggested and
>> enclose the call
>> in a catch or try statement
>>
>>      case (catch shape_to_char(X)) of
>>         {'EXIT', _} ->
>>               io:format("invalid input");
>>         Shape ->
>>               ...
>>      end
>>
>> or
>>
>>      try shape_to_char(X) of
>>          Shape ->
>>             ...
>>      catch
>>          exit:Why ->
>>              ...
>>      end.
>>
>> The problem with your code is the name of the function and its type
>> signature.
>>
>> What you wrote should have been called:
>>
>> shape_to_char_or_error(X) ->
>>
>> with type:
>>
>> -spec (shape_to_char_or_error(char()) -> atom() | string()
>>
>> which mixes error handling with normal behaviour. I think it's better
>> to write it with the simpler type signature and then enclose the code in
>> a try-catch or catch construct - it's just prettier that way.
>>
>> Have fun
>>
>> /Joe
>>
>>
>>
>>
>>
>>>
>>> Roelof
>>>
>>>
>>> ---
>>> Dit e-mailbericht is gecontroleerd op virussen met Avast
>>> antivirussoftware.
>>> https://www.avast.com/antivirus
>>>
>
> Thanks,
>
> Im at chapter 5 of introducing erlang and the try catch was not
> mentioned/explained.
> For testing if the input of the dimensions is a integer or a float can
> that be in one clause or schould I write two.
> The input is a string and I need the integer / float to calculate the
> area later. That function I already wrote in a earlier exercise.
>
> Roelof
>
>
> ---
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>
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>

I tried it yoiur way like this :

char_to_shape(Char) ->
try shape_to_char(X) of
Shape -> case Shape of
\$R -> rectangle;
\$T -> triangle;
\$E -> ellipse;
\$r -> rectangle;
\$t -> triangle;
\$e -> ellipse
end,
catch
exit:"Wrong Shape",
end.

but now I see this error message :

c(geom51). geom51.erl:52: syntax error before: 'end'
geom51.erl:29: function char_to_shape/1 undefined
geom51.erl:38: spec for undefined function geom51:char_to_shape/1

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```