[newbie] refactor function?

[Gaspar Chilingarov]

Hi!


Carsten Schultz wrote:
> Hi!
> 
> On Tue, Feb 01, 2005 at 01:07:10PM +0400, Gaspar Chilingarov wrote:
> 
>>Hello All!
>>
>>what is a correct way of writing following function (initQuota/1) -- 
>>probably i got problems thinking in a functional way ;) ?
>>
>>i wish to get rid of last two cases.
>>
>>following function just adds to list of tuples with range descriptions 
>>initial zero value, which will be changed later in application.
>>
>>-cut-
>>testInitQuota() ->
>>  [{ single, "AAA", 0 }] = initQuota( [ { single, "AAA" } ] ),
>>  [{ range, "AAA", "BBB", 0 }] =
>>	initQuota( [ { range, "AAA", "BBB" } ] ),
>>  [{ single, "AAA", 0 },{ range, "AAA", "BBB", 0 }] =
>>	initQuota( [ { single, "AAA" },{ range,    "AAA", "BBB" } ] ).
>>
>>initQuota([Head | QuotaList]) ->
>>  lists:reverse(initQuota(Head, QuotaList, [])).
>>  
>>initQuota( { range, Value1, Value2 }, [Head | QuotaList], NewList ) ->
>>  initQuota( Head, QuotaList, [ { range, Value1, Value2, 0 } ] ++ 
>>NewList );
>>initQuota( { single, Value1 }, [Head | QuotaList], NewList ) ->
>>  initQuota( Head, QuotaList, [ { single, Value1, 0 } ] ++ NewList );
>>initQuota( { range, Value1, Value2 }, [], NewList ) ->
>>  [ { range, Value1, Value2, 0 } ] ++ NewList;
>>initQuota( { single, Value1 }, [], NewList ) ->
>>  [ { single, Value1, 0 } ] ++ NewList.
>>-cut-
> 
> 
> Well I would first write a specification of what the function should
> do, like
> 
> initQuota(X) ->
>     lists:map(fun({range, Value1, Value2}) ->
> 		      {range, Value1, Value2, 0};
> 		 ({single, Value1}) ->
> 		      {single, Value1}
> 	      end, X).
> 
> :-)
> 
> If you do not like funs like this, that would have been
> 
> initQuota(List) ->
>     lists:map(fun initQuotaElement/1, List).
> 
> initQuotaElement({range, Value1, Value2}) ->
>     {range, Value1, Value2, 0};
> initQuotaElement({single, Value1}) ->
>     {single, Value1}.
> 

	thats ok -- elegant and very short way to write this i need.


> BTW, the problem with your solution seems to be that you are doing to
> pattern matches at once, although they are independent of each other.
> You could have written:
> 
> initQuota(Val, List, NewList) ->
>     NewVal = case Val of
> 		 {range, Value1, Value2} ->
> 		     {range, Value1, Value2, 0};
> 		 {single, Value1} ->
> 		     {single, Value1, 0}
> 	     end,
>     case List of
> 	[] ->
> 	    [NewVal | NewList];
> 	[Head | Tail] ->
> 	    initQuota(Head, Tail, [NewVal | NewList])
>     end.
> 

this was the most useful answer for me --
i've missed at all that case statement could return any value ...


> However, there is no need for the first argument of initQuota.  A
> usual formulation along your lines would have been
> 
> initQuota(List) ->
>     initQuota(List, []).
> 
> initQuota([], Acc) ->
>     lists:reverse(Acc);
> initQuota([{single, Val}|Tail], Acc) ->
>     initQuota(Tail, [{single, Val, 0}|Acc]);
> initQuota([{range, Val1, Val2}|Tail], Acc) ->
>     initQuota(Tail, [{range, Val1, Val2, 0}|Acc]).
> 
> (Your formulation is equivalent to the above, but with
>  initQuota(List=[_|_]) ->
> 	initQuota(List, []).)
> 
> HTH,
> 
> Carsten
> 

in case of joining single element to list


[ Element | List ] is equal to [ Element ] ++ List
but [ List | Element ] creates nested list instead of
List ++ [ Element ]


why it works in this way -- so, are there any general rule which i can 
learn, or it's a special case?



-- 
Gaspar Chilingarov
System Administrator

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