Lab1: Exercise 8

[BertLisser]

{--
accusesAll: A reuseable function that checks if a boy accuses all the boys in
the given list.
--}
accusesAll :: Boy -> [Boy] -> Bool
accusesAll x l = all (\b -> accuses x b) l

{--
Plus the list of boys who made honest (true) statements:
Result: [Matthew,Peter,Carl]
--}
honest :: [Boy]
honest = filter (\b -> accusesAll b guilty ) boys

Nice solution. The question which is left open:
Suppose a boy who accuses all the boys. Is he a honest boy? He certainly makes honest declarations.
The specification is not clear about this.

Well done!

[piotrkosytorz]

Dear Bart,

Thank you.

After processing the input data, we've build a grid based on logical statements given in the exercise:

As it is can be seen on the photograph, from the logical point of view, a boy who accuses everybody from the "guilty" list should be considered an honest person (Mathew, Peter, Carl), because by the conditions of the exercise, there are 3 boys who are always telling the truth. Therefore the two boys who don't accuse the guilty one(s), should be considered liars (Jack, Arnold).

By this logic, a boy who accuses everyone should be on the list of honest boys, because he is telling the truth about the guilty ones.
This is also directly stated in our solution:

honest :: [Boy]
honest = filter (\b -> accusesAll b guilty ) boys