Question

The task: I’m attempting to write a function with type signature minimum_recursive :: (a -> a -> Bool) -> [a] -> a . For its first parameter, it accepts a function I will call less that takes two parameters, and returns True if the first param is less than the second, False otherwise. minimum_recursive also accepts a list as its second parameter. Using explicit recursion, minimum_recursive should determine the smallest value in its input list [a].

My thinking: I was thinking to put the actual recursion in a helper function that also accepts an accumulator. I would call the helper function with the first item as the accumulator.

What I have so far: So far I have the following:

-- function as first parameter to min'
-- accepts two params, returns True if
-- first must come before second in sorted
-- order
less :: Ord a => a -> a -> Bool
less a b = a < b

-- Subpart B
minimum_recursive :: (a -> a -> Bool) -> [a] -> a
minimum_recursive func list = minimum_recursive_h func list []

I am having trouble figuring out how to even begin to write minimum_recursive_h.

Note: I know there probably is an easier way to accomplish this task, but I’m required to go about it as specified above.

Answer 1

You could do it like this:

minimum_recursive _ [] = error "no minimum of empty list"
minimum_recursive _ [x] = x
minimum_recursive f (x:xs) = let m = minimum_recursive f xs
                             in if f x m then x else m

Or, with an accumulator:

minimum_recursive _ [] = error "no minimum of empty list"
minimum_recursive f (x:xs) = helper f x xs
    where
      helper _ m [] = m
      helper f m (x:xs) 
          | f m x = helper f m xs
          | otherwise = helper f x xs