Question

here are some type definitions in my program FYI:

type BitString -> String

type Plateau -> [BitString]

I have a function called:

--Extract will take a list of lists, and return the inner list of the most items. Empty list should return ["00000"]


extract::[Plateau]->Plateau
extract _ = ["00000"]
extract (x:xs)
  |x==maximumBy(compare `on` length)xs=x  --thanks SOF
  |otherwise = extract (xs)

The problem is, no matter what i do, extract returns ["00000"]

here are some outputs from GHCI, that are working:

>plateau graycodes
[["01000"],["01010","11010","10010"],["00101"],["01101","01001"]]

this is expected, and is in the form of a [Plateau] since this is a list of lists of string.

>maximumBy(compare `on` length)(plateau graycodes)
["01010","11010","10010"]

>extract (plateau graycodes)
["00000"]

in this case, i know for sure that extract will be called with a not empty [Plateau]. But the _ part of the function is returning.

I have tried also:

extract (x:xs)
  |x==[]=["00000"]
  |x==[""]=["00000"]
  |x==maximumBy(compare `on` length)xs=x  --thanks SOF
  |otherwise = extract (xs)


error: List.maximumBy: Empty list

Answer 1

you are getting that error, because you are not passing in your list (x:xs) to maximumBy:

extract :: [Plateau] -> Plateau
extract (x:xs)
    |x == maximumBy (compare `on` length) (x:xs) = x
    |otherwise = extract (xs)
extract _ = ["00000"]

or, preferably,

extract :: [Plateau] -> Plateau
extract s@(x:xs)
    |x == maximumBy (compare `on` length) s = x
    |otherwise = extract (xs)
extract _ = ["00000"]

(this also adds a needed = after your otherwise)

EDIT:

I was not satisfied with my answer, or your acceptance of that answer.

I believe this is the code you are really after:

extract :: [Plateau] -> Plateau
extract (x:[]) = x
extract s@(x:xs) = maximumBy (compare `on` length) s
extract _ = ["00000"]