Sandwich.hs

module EvaluateSandwich
    ( SandwichComponent
    , Sandwich
    , validateSandwich
    , indicesSeparated
    ) where

import Data.List (elemIndices)

-- | Components of a sandwich
--   Allows for arbitrary fillings through `Filling String`
data SandwichComponent
    = Bread
    | Meat
    | Cheese
    | Filling String
      deriving (Eq, Show, Read)

-- | A sandwich is a list of sandwich components
type Sandwich = [SandwichComponent]

-- | Validates that a sandwich has at least
--   one filling between two slices of bread.
--   Valid sandwiches can contain components
--   outside of these two slices of bread
--   (This includes other slices of bread)
validateSandwich :: Sandwich -> Bool
validateSandwich = indicesSeparated . elemIndices Bread

-- | Validates that at least two indices are
--   separated by at least one index
indicesSeparated :: (Integral a) => [a] -> Bool
indicesSeparated indices
    | length indices < 2 = False
    | otherwise =
        let first : second : rest = indices in
        second - first > 1 || indicesSeparated (second : rest)

Example usage:

validateSandwich [Bread, Cheese, Filling "Tomato", Meat, Bread]
  => True
validateSandwich [Bread, Filling "Lettuce", Meat, Cheese, Bread, Cheese]
  => True
validateSandwich [Cheese, Cheese, Cheese]
  => False (but appetizing)
validateSandwich [Bread, Bread, Cheese]
  => False