Re: list transformers and Prolog HOF type behaviours...

[Paulo Moura]

On 16/12/2013, at 17:24, Lindsey Spratt <address@hidden> wrote:

> Sean,
> Here's a solution:
> 
> map(_Map, [], []).
> map(Map, [HIN|TIN], [HOUT|TOUT]) :-
>       Map =..Terms,
>       append(Terms, [HIN,HOUT], TermsExtended),
>       MapExtended =.. TermsExtended,
>       call(MapExtended),
>       map(Map, TIN, TOUT).

The code can be simplified by using the built-in and standard call/N control 
construct:

map(_Map, [], []).
map(Map, [HIN|TIN], [HOUT|TOUT]) :-
        call(Map, HIN, HOUT),
        map(Map, TIN, TOUT).

Cheers,

Paulo

> This works 'both ways':
> map(lower_upper, [a,b], U) gives U=['A','B']
> map(lower_upper, L, ['A','B']) gives L=[a,b].
> 
> The 'Map' can be any term where it is meaningful to append two arguments to 
> the end of the term and evaluate the extended term.
> 
> Lindsey
> 
> On Dec 16, 2013, at 11:41 AM, emacstheviking <address@hidden> wrote:
> 
>> I decided to write a maplist type predicate that will be used to map a 
>> function over a list and return the list of transformed values. I have 
>> wanted this for a while now and I sat down and came up with this. It works 
>> BUT I am not happy with it for reasons outlined below so any suggestions 
>> etc. are welcome.
>> 
>> maplist2(In, X, Out) :-
>>      transform_list(In, X, [], Out).
>> 
>> transform_list([], _, Acc, Final) :-
>>      reverse(Acc, Final).
>> 
>> transform_list([E|Es], Xfn, Acc, Final) :-
>>      Xform =.. [Xfn, E, Out],
>>      call(Xform),
>>      transform_list(Es, Xfn, [Out|Acc], Final).
>> 
>> For example, if I wanted to convert a string from lower case to upper case I 
>> would have to use a list of atoms to satisfy lower_upper/2, that's not an 
>> issue. The assumption for my maplist2 is that the transformer function has 
>> arity 2 and the first paramter is the input and the second is the output.
>> 
>> What is the issue though is that lower_upper/2 works either way around and 
>> if I wanted to convert a->'A' or 'A'->a then how would that work?
>> 
>> Haskell has sections and "flip"..is there a Prolog way to say that I want to 
>> change the order of evaluation of a function if you catch my drift?
>> 
>> Thinking on my feet here, it would be the difference between currently 
>> passing in:
>> 
>>     lower_upper
>> 
>> and then in a cool world using
>> 
>>     lower_upper(X,_)
>> or
>>     lower_upper(_,X)
>> 
>> instead and "it" would know to replace "_" with the input parameter...I am 
>> going to see what I can come up with!
>> Thanks,
>> Sean.
>> 
>> 
>> 
>> _______________________________________________
>> Users-prolog mailing list
>> address@hidden
>> https://lists.gnu.org/mailman/listinfo/users-prolog
> 
> _______________________________________________
> Users-prolog mailing list
> address@hidden
> https://lists.gnu.org/mailman/listinfo/users-prolog

-----------------------------------------------------------------
Paulo Moura
Logtalk developer

Email: <mailto:address@hidden>
Web:   <http://logtalk.org/>
-----------------------------------------------------------------