[erlang-questions] Newbie question: avoiding massive 'case' statement

[Richard Carlsson]

You can do something like this:

request_type("ZABC") -> zabc;
request_type("Z123") -> z123;
request_type("YPQX") -> ypqx;
...
request_type(String) -> throw({unknown_transaction_type, String}).

that maps identifier strings to module names, and then

    M = request_type(TransactionTypeString),
    Result = M:process_request(Request)

Since atoms are quicker to compare, you probably want to do the
string-to-atom mapping quite early. If you want to separate atoms that
identify request types from the actual handler module names,  you could
use another function that performs id-as-atom-to-handler-module mapping:

    M = request_handler(request_type(TransactionTypeString)),
    ...

     /Richard


David Mitchell wrote:
> Hello everyone,
> 
> Newbie alert...
> 
> I've got a block of code that looks like:
> 
> case TransactionType of
>   "ZABC" ->
>     zabc:process_request(Request);
>   "Z123" ->
>     z123:process_request(Request);
>   "YPQX" ->
>     ypqx:process_request(Request);
> ...
>   TransactionType ->
>     error_logger:info_msg("Unknown transaction type")
> end
> 
> This is all fine and dandy, but I'm expecting the 'case' statement to
> grow to handle several hundred different values of TransactionType
> over time.  The code is going to get unwieldy in size, and the
> likelihood of mistakes in cut/paste updating of the 'case' block is
> going to get pretty high.
> 
> As an alternative to the above, is there a way to do something like
> 
> try
>   &&&TransactionType&&&:process_request(Request)
> catch
>   error_logger:info_msg("Unknown transaction type")
> end
> 
> where &&&TransactionType&&&:process_request(...) is some "magic code"
> that lets me call the process_request function in the module that has
> the name of the value of TransactionType  (hopefully that's clear, if
> not necessarily English)?
> 
> Alternately, is there some sort of refactoring I could do to avoid the
> massive 'case' statement?
> 
> There may be some obvious solution to do this, but I haven't been able
> to find it...
> 
> Thanks in advance
> 
> Dave Mitchell
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