Software Tools in Haskell: import

splice contents of a file into stdin

Posted on 2016-03-02 by nbloomf
Tags: software-tools-in-haskell, literate-haskell

This page is part of a series on Software Tools in Haskell.

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

As usual, we start with some imports.

-- import: splice contents of a file into stdin
module Main where

import System.Exit (exitSuccess, exitFailure)
import System.Environment (getArgs, getProgName)
import Data.List (unfoldr)
import Data.Char (isSpace)
import System.IO (hPutStrLn, stderr)

The import tool takes lines one at a time, writing them back to stdout until it sees one of the form


These lines should instead be replaced by the contents of FILENAME (relative to the working directory). I will make two tweaks: first, this program implicitly imposes a format on its input. To avoid being too opinionated, I will make the import “keyword” a parameter, so that if the user runs

import --with "go-go-gadget"

then the program will instead look for lines of the form

go-go-gadget FILENAME

This is because the text being filtered may have some other implicit format, where the word “import” means something. The second tweak is to allow the user to import only part of a file. If an import command has the form

import FILENAME between OPEN and CLOSE

then only those lines from FILENAME which are between lines OPEN and CLOSE are spliced in.

First, we write a generic function called takeBetween which cuts out portions of a list.

takeBetween :: (Eq a) => (a,a) -> [a] -> [a]
takeBetween (u,v) = concat . unfoldr (firstCut (u,v))
    firstCut (u,v) ys = case dropWhile (/= u) ys of
      []     -> Nothing
      (_:zs) -> Just $ span (/= v) zs

We use a custom data type, Import, to represent the two kinds of import commands. The readCommand function tries to interpret a line of text as an import command, and the splice function processes a single line of text (from reading a command to splicing in text from an external file). Now the main program behaves very much like a line filter, which we recall takes a mapping String -> String and applies it to all lines on stdin. Because splice reads files and writes to stdout, it must take place in the IO monad; its signature is String -> IO (). We write a variant of lineFilter to handle programs of this type.

lineFilterIO :: (String -> IO ()) -> IO ()
lineFilterIO f = do
  xs <- fmap getLines getContents
  sequence_ $ map f xs

The program is then not terribly complicated:

-- We accept two kinds of import commands:
data Import
  = Whole   String
  | Between String String String

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

  keyword <- case args of
    []             -> return "import"
    ["--with",str] -> return str
    otherwise      -> argErr >> exitFailure

    -- see if a line is an import command
    readCommand :: String -> Maybe Import
    readCommand str = case getWords str of
      [x,file] -> if x == keyword
        then Just $ Whole file
        else Nothing
      [x,file,"between",open,"and",close] -> if x == keyword
        then Just $ Between file open close
        else Nothing
      otherwise -> Nothing

    -- process a single line
    splice :: String -> IO ()
    splice line = case readCommand line of
      Nothing -> do
        putStrLn line
      Just (Whole name) -> do
        input <- fmap getLines $ readFile name
        sequence_ $ map putStrLn input
      Just (Between name open close) -> do
        input <- fmap getLines $ readFile name
        sequence_ $ map putStrLn $ takeBetween (open,close) input

  lineFilterIO splice

argErr :: IO ()
argErr = reportErrorMsgs
  [ "usage"
  , "  import            : expand any 'import' lines on stdin"
  , "  import --with STR : same, using custom import keyword STR"
  , "this program recognizes two kinds of import commands:"
  , "  import FILENAME"
  , "    insert contents of FILENAME in place of this line"
  , "  import FILENAME from START to END"
  , "    same, but cut out all lines except those between START and END lines."

import is used to help produce this documentation. These pages contain lots of code snippets, which are taken from the actual program source code using import commands. This way we don’t have to worry about keeping (at least part of) the documentation and the code in sync by hand as the code changes (as it does, frequently).

This tool could be improved in a few ways. First, the import command, filename, and open and close lines must not include any spaces. This may prove to be too restrictive; we could allow for quoted arguments or escaped arguments. Second, we could do something a litte more informative when readFile fails because a file does not exist; this version of import knows nothing about where a given import command came from. In a large pipeline, or a small pipeline operating on lots of data, this may be a problem.

Old Stuff

-- split on \n
getLines :: String -> [String]
getLines = unfoldr firstLine
    firstLine :: String -> Maybe (String, String)
    firstLine xs = case break (== '\n') xs of
      ("","")   -> Nothing
      (as,"")   -> Just (as,"")
      (as,b:bs) -> Just (as,bs)

-- write list of messages to stderr
reportErrorMsgs :: [String] -> IO ()
reportErrorMsgs errs = do
  name <- getProgName
  sequence_ $ map (hPutStrLn stderr) $ ((name ++ " error"):errs)

-- split a string into words
getWords :: String -> [String]
getWords = unfoldr firstWord
    firstWord :: String -> Maybe (String, String)
    firstWord xs = case dropWhile isSpace xs of
      "" -> Nothing
      ys -> Just $ break isSpace ys