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list transformers and Prolog HOF type behaviours...

From: emacstheviking
Subject: list transformers and Prolog HOF type behaviours...
Date: Mon, 16 Dec 2013 16:41:08 +0000

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],
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" 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:


and then in a cool world using


instead and "it" would know to replace "_" with the input parameter...I am going to see what I can come up with!

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