Ordered By

This post is literate Haskell; you can load the source into GHCi and play along.

Here’s a task I haven’t found a good pipeline for: showing that some lines in a file are in a prescribed order – but not necessarily alphabetical/numerical/asciibetical order. The main motivating case is checking the order of imports in a Haskell module, but another example is finding the first word in an alphabetized list that doesn’t appear in an alphabetized dictionary. Essentially, thinking of two files as lists of lines, find the longest suffix of one list that is not a sublist of the other.

module OrderedBy where

import System.Environment

This function is shaped like a fold on the “dictionary”.

orderedBy :: (Eq a) => [a] -> [a] -> [a]
orderedBy as bs = foldl checkHead as bs
  where
    checkHead :: (Eq a) => [a] -> a -> [a]
    checkHead []     x = []
    checkHead (y:ys) x = if x == y then ys else y:ys

Usage is simple: we pass the path to the “dictionary” as an argument, and look for the list to be checked on stdin.

main :: IO ()
main = do
  args <- getArgs

  case args of
    ["--dict",path] -> do
      dict <- fmap lines $ readFile path
      words <- fmap lines getContents
      mapM_ putStrLn $ orderedBy words dict

    _ -> do
      putStrLn "usage: orderedby --dict PATH"