CODEWITHSUNDEEP
haskell
Marco Montalto M.
Marco Montalto M.

Reputation: 351

Haskell parentheses

Could anyone help me correcting and understanding why the syntax of the following is not working? I suppose it's a problem of parentheses.

findMinMaxRec smallest largest myList
  | myList == [] = [smallest, largest]
  | head myList < smallest && head myList > largest = findMinMaxRec head myList head myList tail myList 
  | head myList < smallest = findMinMaxRec head myList largest tail myList 
  | head myList > largest = findMinMaxRec smallest head myList tail myList 
  | otherwise = findMinMaxRec smallest largest tail myList

findMinMax [] = []
findMinMax [x] = findMinMaxRec head [x] head [x] [x]

Thanks

Upvotes: 0

Views: 62

Answers (1)

Jon Purdy
Jon Purdy

Reputation: 55079

findMinMaxRec head myList head myList tail myList means “call findMinMaxRec with 6 arguments: head, myList, head, myList, tail, and myList”. You want:

findMinMaxRec (head myList) (head myList) (tail myList)

However, it would be best to avoid head and tail here with pattern matching—here’s a slight improvement:

findMinMaxRec smallest largest [] = [smallest, largest]
findMinMaxRec smallest largest (x:xs)
  | x < smallest && x > largest = findMinMaxRec x x xs
  | x < smallest = findMinMaxRec x largest xs
  | x > largest = findMinMaxRec smallest x xs
  | otherwise = findMinMaxRec smallest largest xs

Likewise, when you write:

findMinMax [] = []
findMinMax [x] = findMinMaxRec head [x] head [x] [x]

This means findMinMax is only defined on 0-element ([]) and 1-element ([x]) lists; and it has the same problem with parentheses. Another small adjustment:

findMinMax [] = []
findMinMax (x:xs) = findMinMaxRec x x xs

Finally, since findMinMaxRec always returns a two-element list, a tuple would be preferable for the return type; then findMinMax can return a Maybe.

Upvotes: 3

Related Questions