distinctG.hs

distinctG.hs

-- Link to youtube https://youtu.be/c4saFSUAmlg

-- | Remove all duplicate integers from a list. Produce a log as you go.
-- If there is an element above 100, then abort the entire computation and produce no result.
-- However, always keep a log. If you abort the computation, produce a log with the value,
-- "aborting > 100: " followed by the value that caused it.
-- If you see an even number, produce a log message, "even number: " followed by the even number.
-- Other numbers produce no log message.
--
-- /Tip:/ Use `filtering` and `StateT` over (`OptionalT` over `Logger` with a @Data.Set#Set@).
--
-- >>> distinctG $ listh [1,2,3,2,6]
-- Logger ["even number: 2","even number: 2","even number: 6"] (Full [1,2,3,6])
--
-- >>> distinctG $ listh [1,2,3,2,6,106]
-- Logger ["even number: 2","even number: 2","even number: 6","aborting > 100: 106"] Empty
distinctG ::
  (Integral a, Show a) =>
  List a
  -> Logger Chars (Optional (List a))
--filtering ::
--  Applicative k =>
--  (a -> k Bool) -- StateT Set OptionalT (Logger Chars Optional (Bool, Set))
--  -> List a
--  -> k (List a) -- StateT Set OptionalT (Logger Chars Optional ((List a), Set))

distinctG as =
  let
    rs = filtering
        (\a -> StateT $ \s -> OptionalT $
            if a > 100
            then log1 ("aborting > 100: " ++ show' a) Empty
            else
                if even a
                then
                    log1 ("even number: " ++ show' a) (if S.notMember a s then Full(True, S.insert a s) else Full(False, s))
                else
                    pure (if S.notMember a s then Full(True, S.insert a s) else Full(False, s))
        )
        as
    x = runOptionalT $ evalT rs S.empty
  in x